- Email: [email protected]
Utilization of Thermoluminescent Dosimetry in Total Skin Electron Beam Radiotherapy of Mycosis Fungoides
Int. J. Radiation Oncology Biol. Phys., Vol. 40, No. 1, pp. 101–108, 1998 Copyright © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/98 $19.00 1 .00
PII S0360-3016(97)00585-3
●
Clinical Investigation UTILIZATION OF THERMOLUMINESCENT DOSIMETRY IN TOTAL SKIN ELECTRON BEAM RADIOTHERAPY OF MYCOSIS FUNGOIDES JOHN A. ANTOLAK, PH.D.,* JACKSON H. CUNDIFF, B.S.,*
AND
CHUL S. HA, M.D.†
Departments of *Radiation Physics and †Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX Purpose: The purpose of this report is to discuss the utilization of thermoluminescent dosimetry (TLD) in total skin electron beam (TSEB) radiotherapy to: (a) compare patient dose distributions for similar techniques on different machines, (b) confirm beam calibration and monitor unit calculations, (c) provide data for making clinical decisions, and (d) study reasons for variations in individual dose readings. Methods and Materials: We report dosimetric results for 72 cases of mycosis fungoides, using similar irradiation techniques on two different linear accelerators. All patients were treated using a modified Stanford 6-field technique. In vivo TLD was done on all patients, and the data for all patients treated on both machines was collected into a database for analysis. Means and standard deviations (SDs) were computed for all locations. Scatter plots of doses vs. height, weight, and obesity index were generated, and correlation coefficients with these variables were computed. Results: The TLD results show that our current TSEB implementation is dosimetrically equivalent to the previous implementation, and that our beam calibration technique and monitor unit calculation is accurate. Correlations with obesity index were significant at several sites. Individual TLD results allow us to customize the boost treatment for each patient, in addition to revealing patient positioning problems and/or systematic variations in dose caused by patient variability. The data agree well with previously published TLD results for similar TSEB techniques. Conclusion: TLD is an important part of the treatment planning and quality assurance programs for TSEB, and routine use of TLD measurements for TSEB is recommended. © 1998 Elsevier Science Inc. Mycosis fungoides, Total skin electron beam radiotherapy, In vivo dosimetry, Thermoluminescent dosimetry.
INTRODUCTION
is the Stanford technique (14, 16, 23), can be found in AAPM Report #23 (1). Our institution developed a variation of the Stanford technique and began treating MF patients in 1984 (2, 11). We report here our experience with in vivo thermoluminescent dosimetry (TLD) measurements, using a similar TSEB technique on two different treatment machines. In vivo dosimetry is a routine procedure for many institutions that provide TSEB therapy. However, systematic evaluations of the dosimetric data have not been widely reported. We believe that comprehensive dosimetric data for different techniques should be available in the literature. This benefits those who want to compare their technique to other techniques. It also benefits those that do TSEB therapy infrequently and have difficulty determining the ‘‘normal’’ dose for a given anatomic site. The purpose of this report is to describe the utilization of a systematic TLD quality assurance protocol to: (a) confirm that patient dose distributions can be made similar when moving the treatment technique from one linear accelerator
External beam radiotherapy was first used to treat mycosis fungoides (MF) in 1902 (24). Treatments were done mostly with low voltage X-rays before the advent of electron beams, which provided unique dose fall-off characteristics that were ideal for treatment of the dermatological lesions of MF. The use of total skin electron beam (TSEB) radiotherapy for the treatment of MF was first described in 1952 (25). Though there have been some controversies regarding the role of TSEB radiotherapy in the management of MF, a certain portion of early stage MF patients are considered to be cured by this treatment alone (13, 15, 17). TSEB has also been valuable for palliation of more advanced stage disease. The tolerance to TSEB has been extensively documented previously (5, 6, 15). The modern technique of irradiating the entire skin surface with multiple electron fields from linear accelerators has been used by different institutions since the 1960s. An overview of various techniques, the most common of which Reprint requests to: John A. Antolak, Ph.D., Department of Radiation Physics, Box 94, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030.
Acknowledgement—The authors thank Linda G. McCallister, BSN, for helping collect data on patients’ heights and weights. Accepted for publication 22 July 1997. 101
102
I. J. Radiation Oncology
●
Biology
●
Physics
Fig. 1. Schematic view of treatment geometry. Patient anterior surface is located 2 m from isocenter and is irradiated by beams at gantry angles of 67° and 113° for each of six positions.
or room to another, (b) ensure that beam calibration and monitor unit calculations are accurate, (c) provide data for making clinical decisions regarding patient shielding and boost therapy, and (d) study reasons for variation in individual dose readings. The data for this report are a continuation of the TLD results of Almond (2), which were initially reported by Hogstrom et al. (11). In 1994, the TSEB technique was moved to a different linear accelerator. The goal was to reproduce the original implementation as closely as possible because treatment results had been very satisfactory (6). METHODS AND MATERIALS Our TSEB technique, as reported by Hogstrom and colleagues (10, 11), is a modification of the well-known Stanford technique (23), which has two beams for each of six patient positions. The modification was the addition of a scatter plate, as seen in the schematic of the patient-beam geometry in Fig. 1. Holt and Perry (12) showed that the patient dose uniformity for six positions was much better than for four positions and only slightly worse than that for eight positions. They also showed that incorporation of a scatter plate close to the patient could improve dose uniformity on the patient’s skin surface by increasing the angular dispersion of the incident electrons for a given amount of lateral dispersion. This allows the electrons to more fully irradiate oblique skin surfaces and areas that would otherwise be greatly self-shielded. Hence, our technique uses a 1.3-cm (0.5-inch) acrylic scatter plate located 25 cm in front of the patient treatment plane. From 1984 to 1994, total skin irradiation was delivered using a Siemens Mevatron 801 that was modified to deliver a high dose rate beam (11). A total of 53 patients were treated using this machine. In 1994, the treatment technique was moved to a different treatment room having a Varian Clinac 2100C.2 As of October 1996, 19 patients have been treated using this machine under the supervision of a single physicist. Our objective in moving the technique was to match the previous technique as closely as possible. A 1
Siemens Medical Systems, Inc., Concord, CA.
Volume 40, Number 1, 1998
summary of the beam characteristics for our current TSEB beam vs. our previous TSEB beam is shown in Table 1. The beam profile in the horizontal (right-left) direction is approximately Gaussian-shaped, with a 90% width of approximately 60 cm. In the vertical (superior-inferior) direction, the dual-beam profile is 100% at isocenter height, increasing to 110% at approximately 6 60 cm and decreasing to 90% at approximately 6 100 cm from isocenter height (3). Measurements on an anthropomorphic phantom, using film, have shown that the depth of maximum dose for the combined 12-field treatment is within 0.1 cm of the skin surface, and the therapeutic depth (80% dose) is between 0.5 and 0.8 cm, depending on where the depth dose-curve is measured. The standard prescribed dose is 32 Gy to the skin surface at the umbilicus. The patient is treated 2 days per week for 8 weeks, and the weekly dose to the skin surface is 4 Gy. In the first week of treatment, the patient is irradiated in all six positions on each of the 2 days. This accommodates performing in vivo TLD dosimetry in one treatment session on the second day. In vivo dosimetry consists of TLD flat-packs being taped to the patient’s skin surface in 22 locations. Exposing the TLD on a single day eliminates possible errors due to dosimeter repositioning. In the second and subsequent weeks of treatment, the patient is treated in three positions each day, doubling the number of monitor units per field. This results in the same amount of irradiation time daily, but only half the setup time and, hence, a decrease in overall treatment time. TLD measurement positions have changed over the years, depending on physician preference, and the current standard TLD locations are shown in Table
Table 1. Comparison of beam parameters for the Clinac 2100C TSEB technique to the previous Mevatron 80 TSEB technique
Ep,0 (MeV, at patient) Single beam characteristics R80 (cm H2O) R50 (cm H2O) Rp (cm H2O) Dual beam characteristics Gantry angles 90% width right-left (cm) 90% width sup-inf (cm) SSDeff (cm) Dose calibration (cGy/MU) (single beam, 90° 6 23°) Treatment ‘‘beam-on’’ time (2 Gy) Daily MU settings Week 1: 12 fields/fx Weeks 2–8: 6 fields/fx 2
Current technique Varian Clinac 2100C
Previous technique Siemens Mevatron 80
4.41
4.29
1.12 1.53 2.11
1.05 1.45 2.05
90° 6 23° 61 '200 '190
90° 6 26° 58 '190 '200
0.100
N/A
'4.8 min
'2 min
357 714
N/A N/A
Varian Associates, Inc., Palo Alto, CA.
TLD measurement for TSEB radiotherapy
Table 2. List of current standard TLD positions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Scalp (top rear) Scalp vertex Top of right shoulder Top of left shoulder Right elbow Left elbow Right hand (mid dorsum) Left hand (mid dorsum) Right axilla Left axilla Upper thorax
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Upper back Anterior abdomen Lower back Right abdomen Left abdomen Right upper medial thigh Left upper medial thigh Right buttock Left buttock Right foot (mid dorsum) Left foot (mid dorsum)
2 and Fig. 2. When applicable, dosimeters are placed symmetrically. This provides us with two data-points for most locations and allows us to spot any patient positioning asymmetries. For female patients, extra dosimeters are usually placed in the inframammary folds to determine if that area needs to be treated with a boost field. Each patient’s TLD results are entered into a database and compared with aggregate arithmetic means and sample standard deviations (SDs) for each measurement site. Each dosimeter (flat-pack) consists of approximately 0.027 g of LiF TLD-100 powder, sandwiched between two thin, heat-sealed polyethylene sheets that form a small pouch, approximately 0.5-cm square by 0.055 cm thick. The TLD is taped to the patient’s skin at the specified locations; each piece of tape is numbered for later identification. In some cases, body hair is removed so that the TLD can be in good contact with the skin. Photographs showing the locations of the dosimeters are placed in the patient’s treatment record. For calibration purposes, reference dosimeters (usually three flat-packs) are exposed to 2 Gy at the depth of maximum dose in a plastic phantom at 100 cm SSD, using 9-MeV electrons (Ep,0 5 9.34 MeV) and a 10-cm2 cone. The energy at the depth of maximum dose is approximately the same as the energy of the electrons incident at the surface of the patient for the total skin irradiation. This allows the TLD dosimetry to provide an independent check of the calibration of the high dose rate mode of the linear accelerator used for total skin treatment. Our previous technique exposed the reference dosimeters using the TSE beam under calibration conditions, which is the current American Association of Physicists in Medicine recommendation (1). The TLD were read 1 or 2 days after exposure, and all results were reported as a percentage of the reference TLD dose. The powder was massed by an analytical balance and then evenly spread across the 1.3-cm diameter active surface of a rhodium-plated circular silver planchette. The powder was heated to approximately 300°C, and the integrated light reading was read by a modified Radiation Detection model 1100 TLD reader.3 The dose to the TLD sample was computed by multiplying the reference dose by the ratio of the light output per unit mass for the sample to the average light output per unit mass for the reference TLD. A sensitivity correction for su3
Radiation Detection, Sunnyvale, CA.
● J. A. ANTOLAK et al.
103
pralinearity, ranging from 1.035 at 0.2 Gy to 1.000 at 2.0 Gy and 0.981 at 3.0 Gy, was made. The estimated uncertainty (standard error) in the relative TLD dose is approximately 2%. Combined with the uncertainty in the calibrated dose rate of the linear accelerator (less than 2%), the total dose uncertainty is less than 3%. The data for all patients treated on both machines was collected in a database for analysis. Arithmetic means, sample SDs, and ranges were computed for both series of patients. It is reasonable to expect that there would be some dependence of the TLD dose on variables such as the patient’s height, weight, or both. To investigate this, we generated scatter plots of the TLD dose vs. weight, height, and an obesity index [ratio of the patient’s weight to their ideal weight based on height (22)]. We also calculated correlation coefficients vs. weight, height, and obesity index for the previous and current series of patients and the combined series of patients, where applicable.
RESULTS Dosimetric equivalence Table 3 summarizes the dosimetric data for the 53 patients treated with our previous technique on the Mevatron 80, and Table 4 summarizes the dosimetric data for the 19 patients treated with the current technique on a Clinac 2100C. The dose is given as a percentage of the prescribed dose. All of the patients in the current series were treated under the supervision of a single physicist, which may explain the slightly smaller sample SDs for the upper thorax, anterior abdomen, and lateral abdomen. The data for scalp vertex, shoulder, axilla, hand (mid dorsum), anterior abdomen, and foot (mid dorsum) is the same for both
Fig. 2. Standard TLD positions for TSEB radiotherapy, numbered as in Table 2.
104
I. J. Radiation Oncology
●
Biology
●
Physics
Volume 40, Number 1, 1998
Table 3. TLD results for 53 patients treated with previous total skin irradiation technique on Siemens Mevatron 80 Site
No. of measurements
Mean dose (% of prescribed dose)
Range (% of prescribed dose)
Scalp vertex Forehead Posterior mid-neck Submental Top of shoulder Lateral shoulder Axilla Hand (mid dorsum) Mid-medial finger Upper thorax Upper back Anterior abdomen Lower back Lateral abdomen Mid perineum Upper-medial thigh Mid-medial thigh Mid-anterior thigh Foot (mid dorsum) Foot (posterior) Big toe (dorsum)
43 27 20 29 103 75 108 85 62 33 16 54 52 104 34 89 88 48 89 66 59
76 6 21 96 6 8 103 6 6 101 6 6 67 6 15 100 6 12 59 6 19 88 6 7 123 6 27 95 6 9 101 6 6 100 6 9 96 6 7 98 6 9 25 6 21 54 6 25 96 6 12 100 6 9 124 6 9 117 6 10 141 6 10
38–122 82–111 91–116 84–110 34–111 50–120 8–102 65–103 63–178 61–104 88–110 64–109 79–118 72–119 1–82 2–107 34–119 71–116 104–145 90–146 110–162
The dose error is reported as 1 sample SD.
techniques (within 1 SD). These data support our hypothesis that the patient dosimetry is the same for both techniques. Beam calibration and monitor units In Tables 3 and 4, the mean dose to the anterior abdomen (umbilicus) is 100% of the prescribed dose for both the current and previous total skin irradiation techniques. As stated earlier, the current TLD protocol irradiates the standard dosimeters using a ‘‘normal’’ electron mode of the accelerator. Hence, these data confirm that the absolute calibration of the TSE beam is accurate and that the monitor unit calculation is correct. Clinical decision making To be most effective, TSEB requires meticulous treatment planning to ensure delivery of therapeutic doses as homogeneously as possible. There are great individual variations in dosimetry, as shown by our data. Kumar and colleagues (19, 21) have reported recurrences for the scalp vertex, axilla, palm of the hand, soles of feet, ventral surface of the penis, perineum, eyelids, and the area underneath the breast. The recurrences were caused by self-shielding in most cases (artificial shielding for the eyelids), and they recommended boost therapy. For our technique, the axilla, perineum, and soles of the feet are consistently underdosed and are therefore boosted with 6-MeV electrons and 1.5-cm bolus (water equivalent thickness) to reduce R80, the depth of the 80% dose, to 0.7 cm, which is similar to the therapeutic depth of the TSEB treatment. For some patients, the tops of the shoulders or scalp vertex may also need boost therapy. TLD has been instrumental in customizing supplementary boost treatments for such underdosed areas. TLD results are also used to verify the patient setup for
total skin treatments. After placement of TLD is done properly, comparing a patient’s TLD measurements with established mean dose and SD will aid the radiation oncologist and physicist in identifying any potential setup errors or individual variations in setups that would require special attention. Corrections in setups will be made as necessary and TLD measurements repeated to verify the corrections. For example, asymmetry in the measured doses often indicates a systematic error in the patient setup. Once the setups for the patient are established in a consistently reliable manner, boost volumes and doses can be decided. The boost volumes are drawn on the patient’s skin, taking into account the patient’s anatomy and angles of the incident beams during treatment. The boost fields are customized to compensate for the areas of irregular contour, self-shielding, or both. The amount of dose that needs to be made up to deliver a full dose (usually 32 Gy in the absence of thick plaques or tumors) is calculated from TLD measurements and delivered using a daily fraction size of approximately 2 Gy. For example, the mean dose to the axilla is approximately 60%, implying a dose deficit of 40% of 32 Gy or 12.8 Gy. The average patient, therefore, receives six 2-Gy fractions of 6-MeV electrons with 1.5-cm bolus. These boost (or supplementary) treatments are done on the days when TSEB is not done. Areas that are overdosed are blocked as needed. Usually patients begin to complain of tenderness, severe erythema, or early desquamation in such areas as fingers and the dorsum of the feet during the second half of the TSEB treatment. These areas are usually blocked with 0.15 cm of lead for 1 or 2 weeks during the 8-week course of treatment as long as there is no risk of underdosing adjacent gross disease. This is supported by the TLD results, which show that the fingers (mid medial) receive
TLD measurement for TSEB radiotherapy
● J. A. ANTOLAK et al.
105
Table 4. TLD results for 19 patients treated with current total skin irradiation technique on Varian Clinac 2100C Site
No. of measurements
Mean dose (% of prescribed dose)
Range (% of prescribed dose)
Scalp (top rear) Scalp vertex Top of shoulder Elbow Axilla Hand (mid dorsum) Upper thorax Upper back Anterior abdomen Lower back Lateral abdomen Upper-medial thigh Buttock Foot (mid dorsum)
17 17 36 35 43 36 18 18 21 19 40 19 19 36
105 6 8 87 6 20 74 6 8 90 6 13 60 6 25 85 6 6 93 6 4 93 6 7 100 6 4 91 6 7 98 6 6 59 6 23 58 6 14 117 6 7
89–120 49–114 59–88 60–115 3–105 71–97 86–100 76–105 91–106 79–106 84–113 7–93 26–80 102–132
The dose error is reported as 1 sample SD.
123 6 27% and the dorsum of foot receives 122 6 9% (combined results). Variations in dose readings The distributions of doses for the hand, abdomen, and foot have relatively small standard deviations, less than 10% of the prescribed dose. This is due to the reproducibility of positioning the dosimeter on each patient and the lack of self-shielding for the dosimeter position. For the elbow, the higher spread in the dose received is due to differences in how patients are able to hold their elbows, depending on their flexibility and strength. In the axilla, the dose received depends mainly on the patient’s ability to hold his or her arms up during treatment and secondarily on the patient’s axillary skin folds. The variability in the perineum doses (mid perineum in Table 3; inner thigh and buttock in Table 4) also depends on the patient’s inguinal and gluteal folds affecting the degree of self-shielding. However, another component of the spread is due to dosimeter placement variability from patient to patient. In this region, the dose gradient on the skin is generally large, implying that the dose depends on exactly where the dosimeter is placed for that patient. In many cases, systematic tendencies could be seen in the data when plotted vs. weight, height, or obesity index. In general, the best correlations were with obesity index, as shown for selected sites in Fig. 3. Table 5 summarizes all of the data vs. obesity. The previous and current series of patients were considered together for the purpose of calculating a best-fit line and linear correlation coefficient. The linear correlation coefficient was considered significant if the probability of getting a correlation coefficient with greater absolute value from an uncorrelated population was less than 5% (4). When calculating the linear correlation coefficient, we excluded obviously erroneous data-points. For example, two of the three plus signs in Fig. 3a were caused by errors in patient positioning for the oblique treatment fields. A total of five data-points were excluded for the summary in Table 5.
The dose to the anterior abdomen and lower back shows a slight dependence on the obesity of the patient, as seen in Fig. 3a and b. As the patient’s size increases, the SSD to the umbilicus increases for the anterior oblique patient positions, causing a small decrease in total dose to that point. The dose to the lower back shows a similar effect, but is systematically lower than the dose to the anterior abdomen. This is probably due to partial self-shielding by the paraspinal muscles. Figure 3c and d shows the dose to the top of the shoulder and mid-medial thigh vs. the obesity index. The slopes of the best-fit lines through the data-points is much larger than in Fig. 3a and b, implying a stronger dependence on the obesity index. The correlations are significant, but there is a large amount of scatter in the data. The large amount of scatter is probably due to other factors, such as differences in the patient’s treatment positioning and physical condition. Figure 3e and f shows the dose to the medial aspect of the middle finger and the dorsum of the big toe vs. the obesity index. The dose to the mid-medial finger is highly dependent on the obesity of the patient, which is consistent with the experimental observations of Wong and Kwa (28). To some extent, electrons are able to irradiate the medial aspect of the fingers for all fields, especially for thin fingers. It should be noted that our patients are treated with their fingers held together. Measurements on one patient showed that the dose to the medial aspect of the middle finger was reduced by approximately 30%, relative to fingers held apart. The dose to the fingers is, therefore, more homogeneous, without resorting to the plastic sheath used by Wong and Kwa (28). The dose to the big toe does not depend as much on the obesity of the patient. This is due to the larger diameter of the big toe and the fact that the TLD was placed on the top of the toe rather than between the toes. Limited numbers of measurements on the palmar aspect of the middle finger showed that the palmar aspect of the fingers receive a dose similar to the dorsum of the hand.
106
I. J. Radiation Oncology
●
Biology
●
Physics
Volume 40, Number 1, 1998
Fig. 3. Scatter plots of measured TLD dose vs. the ratio of the patient’s weight to their ideal weight. The crosses are from the first series of patients (Mevatron 80); the circles are from the current series of patients (Clinac 2100C); the line is a least squares fit through all of the data. The TLD sites are (a) anterior abdomen, (b) midline, lower back, (c) top of shoulder, (d) mid-medial thigh, (e) mid-medial finger, and (f) big toe. The plus symbols in (a) were excluded from the least squares fit.
DISCUSSION Other groups have published in vivo TLD results; however, four of these manuscripts give results for a ‘‘typical’’ patient using a rotational technique (18, 20), a variant of the Stanford technique (7), and a reclined patient-position technique (9). There are no data for the patient-to-patient variability in these reports. The data in the first three of these
reports shows similar dosimetric patterns to the data in Tables 3 and 4. The major exception is the axilla dose reported by Kumar et al. (20). In their technique, the patient holds both arms overhead for the duration of treatment, allowing the axilla to received 95% of the prescribed dose. However, it would be expected that the tops of the shoulders would probably receive a lower dose compared with our
TLD measurement for TSEB radiotherapy
● J. A. ANTOLAK et al.
107
Table 5. TLD results for all patients vs. obesity index Site and # of measurements
Best fit line (% of prescribed dose)
Correlation, r
Significant
Scalp (top rear), n 5 15 Scalp vertex, n 5 51 Forehead, n 5 23 Posterior mid-neck, n 5 16 Submental, n 5 23 Top of shoulder, n 5 116 Lateral shoulder, n 5 62 Elbow, n 5 32 Axilla, n 5 127 Hand (mid dorsum), n 5 102 Mid-medial finger, n 5 56 Upper thorax, n 5 42 Upper back, n 5 30 Anterior abdomen, n 5 59 Lower back, n 5 59 Lateral abdomen, n 5 120 Perineum, n 5 29 Upper-medial thigh, n 5 107 Mid-medial thigh, n 5 72 Mid-anterior thigh, n 5 44 Buttock, n 5 34 Foot (mid dorsum), n 5 105 Foot (posterior), n 5 60 Big toe (dorsum), n 5 54
D 5 117.0 2 9.2 I D 5 78.0 1 1.2 I D 5 94.8 1 1.2 I D 5 102.2 1 0.8 I D 5 99.8 1 0.9 I D 5 90.0 2 16.9 I D 5 124.1 2 20.0 I D 5 106.3 2 11.1 I D 5 88.1 2 24.2 I D 5 93.9 2 5.2 I D 5 216.7 2 76.5 I D 5 104.7 2 6.2 I D 5 104.7 2 6.2 I D 5 106.0 2 5.0 I D 5 108.2 2 5.5 I D 5 111.9 2 11.2 I D 5 53.5 2 26.9 I D 5 68.0 2 9.8 I D 5 120.9 2 20.4 I D 5 104.1 2 3.6 I D 5 85.6 2 21.2 I D 5 126.0 2 3.2 I D 5 140.6 2 19.8 I D 5 171.0 2 23.9 I
20.25 0.01 0.03 0.02 0.03 20.28 20.28 20.18 20.25 20.16 20.49 20.41 20.19 20.30 20.24 20.32 20.29 20.09 20.37 20.07 20.31 20.08 20.36 20.43
No No No No No Yes Yes No Yes No Yes Yes No Yes Yes Yes No No Yes No No No Yes Yes
The second column is the best fit line for the dose (D) vs. obesity index (I), and the third column is the linear correlation coefficient. If the last column is ‘‘Yes’’, then the probability of the dose and obesity index being not truly correlated is less than 5%.
technique. The reclined patient position technique of Gerbi et al. (9) is very different from our technique, making comparison difficult. The earliest report describing in vivo TLD results for multiple patients was by Fraas et al. (8) at the National Cancer Institute, using the Stanford technique. Their data for scalp vertex, posterior neck, axilla, abdomen, elbow, hand (mid dorsum), upper medial thigh, and foot (mid dorsum) shows good agreement (within 1 SD) with the data in Tables 3 and 4. However, it is interesting that their mean dose to the scalp vertex is 46%, compared with 76% and 87% for our two techniques. The increased dose for our technique is most probably due to the addition of the 1.3-cm acrylic scatter plate in front of the patient to increase the angular dispersion of the electrons at the patient plane. Desai et al. (5) presented limited data for 20 patients reporting mean doses to nine sites, but without any indication of dose variability. They used a technique similar to ours (i.e., Stanford technique with an acrylic screen). Their data for foot (mid dorsum) and palm [compared with our hand (mid dorsum)] shows good agreement with the data in Tables 3 and 4. However, their reported mean dose to the forehead was 1.4 SDs higher than in Table 3, and their reported mean dose to the axilla was approximately 25% higher than in Tables 3 and 4. Although the SDs for the axilla doses in Tables 3 and 4 are 19% and 25%, respectively, this most likely represents a significant dosimetric difference between the techniques. From their description of their TSEB technique, it is not clear what may have caused these discrepancies.
Data for three patients irradiated using a reclined patientposition technique were reported by Van Der Merwe (26). The data for each patient are reported separately, and SDs are given for each measurement. For the anterior abdomen and posterior neck, their data shows good agreement with the data in Tables 3 and 4. Their reported dose to the axilla is within 1 SD of our data for two of three patients, while the reported dose to the foot is much lower than in Tables 3 and 4, again for two of three patients. The dose to the ‘‘inner thigh’’ is more than 1 SD higher than for the mid-medial thigh in Tables 3 and 4. The dosimetric differences are most likely caused by the use of a different TSEB technique. More recently, Weaver et al. (27) reported dosimetric results for 22 patients treated at The University of Minnesota between 1983 and 1993. Their TSEB technique is quite similar to ours, using a 1.0-cm acrylic scatter plate 18 cm in front of the patient plane, compared with a 1.3-cm acrylic scatter plate 25 cm in front of the patient plane. Not surprisingly, their reported mean doses and SDs agree very well with the data in Tables 3 and 4. Their reported mean doses to the anterior abdomen, forehead, upper thorax, hand (mid dorsum), finger, scalp vertex, perineum, and axilla are much less than 1 SD different from our data. Their reported SDs are also similar to those in Tables 3 and 4, indicating that their TSEB technique is almost dosimetrically identical to our technique. Weaver et al. also commented that height was a key factor in determining the dose to the scalp vertex. However, our data indicate a correlation coefficient of 20.01 and 20.23 vs. height for our Mevatron 80 and
108
I. J. Radiation Oncology
●
Biology
●
Physics
Clinac 2100C techniques, respectively. Therefore, we suspect that the extent of our beam uniformity may be slightly greater. In conclusion, there is a wide individual variation in actual dosimetry of TSEB for MF. We have described a systematic QA program using TLD dosimetry that: (a) confirms that patient dose distributions can be made similar
Volume 40, Number 1, 1998
when moving the treatment technique from one linear accelerator or room to another, (b) ensures that beam calibration and monitor unit calculations are accurate, (c) provides data for making clinical decisions regarding patient shielding and boost therapy, and (d) allows us to study reasons for variation in individual dose readings. Therefore, routine use of TLD measurements for TSEB is recommended.
REFERENCES 1. AAPM. Total skin electron therapy: technique and dosimetry. Report #23. American Association of Physicists in Medicine; New York; 1987. 2. Almond, P. R. Total skin/electron irradiation technique and dosimetry. In: Kereiakes, J. G.; Elson, H. R.; Born, C. G., eds. Radiation oncology physics (Medical Physics monograph No. 15). New York: American Institute of Physics; 1987:296 –332. 3. Antolak, J. A.; Hogstrom, K. R. Multiple scattering theory for total skin electron beam design. Med. Phys. (submitted). 4. Bevington, P. R.; Robinson, D. K. Data reduction and error analysis for the physical sciences. New York: McGraw-Hill; 1992. 5. Desai, K. R.; Pezner, R. D.; Lipsett, J. A.; Vora, N. L.; Luk, K. H.; Wong, J. Y.; Chan, S. L.; Findley, D. O.; Hill, L. R.; Marin, L. A.; Archambeau, J. O. Total skin electron irradiation for mycosis fungoides: relationship between acute toxicities and measured dose at different anatomic sites [published erratum appears in Int. J. Radiat. Oncol. Biol. Phys. 16:A1; 1989.]. Int. J. Radiat. Oncol. Biol. Phys. 15:641– 645; 1988. 6. Duvic, M.; Lemak, N. A.; Redman, J. R.; Eifel, P. J.; Tucker, S. L.; Cabanillas, F. F.; Kurzrock, R. Combined modality therapy for cutaneous T-cell lymphoma. J. Am. Acad. Dermatol. 34:1022–1029; 1996. 7. el-Khatib, E.; Hussein, S.; Nikolic, M.; Voss, N. J.; Parsons, C. Variation of electron beam uniformity with beam angulation and scatterer position for total skin irradiation with the Stanford technique. Int. J. Radiat. Oncol. Biol. Phys. 33:469 – 474; 1995. 8. Fraass, B. A.; Roberson, P. L.; Glatstein, E. Whole-skin electron treatment: patient skin dose distribution. Radiology 146: 811– 814; 1983. 9. Gerbi, B. J.; Khan, F. M.; Deibel, F. C.; Kim, T. H. Total skin electron arc irradiation using a reclined patient position. Int. J. Radiat. Oncol. Biol. Phys. 17:397– 404; 1989. 10. Hogstrom, K. R.; Ames, J. C.; Cundiff, J. H. The use of the pencil beam algorithm in designing treatment beams for mycosis fungoides. Med. Phys. 10:525; 1983. 11. Hogstrom, K. R.; Ewton, J. R.; Cundiff, J. H.; Ames, J. C.; McNeese, M. D. Beam delivery system and dosimetry for total skin electron therapy at MDAH. Med. Phys. 11:389; 1984. 12. Holt, J. G.; Perry, D. J. Some physical considerations in whole skin electron beam therapy. Med. Phys. 9:769 –776; 1982. 13. Hoppe, R. T.; Cox, R. S.; Fuks, Z.; Price, N. M.; Bagshaw, M. A.; Farber, E. M. Electron-beam therapy for mycosis fungoides: the Stanford University experience. Cancer Treat. Rep. 63:691–700; 1979. 14. Hoppe, R. T.; Fuks, Z.; Bagshaw, M. A. Radiation therapy in the management of cutaneous T-cell lymphomas. Cancer Treat. Rep. 63:625– 632; 1979.
15. Jones, G. W.; Hoppe, R. T.; Glatstein, E. Electron beam treatment for cutaneous T-cell lymphoma. Hematol. Oncol. Clin. North Am. 9:1057–1076; 1995. 16. Karzmark, C. J.; Loevinger, R.; Steele, R. E.; Weissbluth, M. A technique for large-field, superficial electron therapy. Radiology 74:633– 644; 1960. 17. Kaye, F. J.; Bunn, P. A., Jr.; Steinberg, S. M.; Stocker, J. L.; Ihde, D. C.; Fischmann, A. B.; Glatstein, E. J.; Schechter, G. P.; Phelps, R. M.; Foss, F. M.; Parlette, H. L., III; Anderson, H. J.; Sausville, E. A. A randomized trial comparing combination electron-beam radiation and chemotherapy with topical therapy in the initial treatment of mycosis fungoides. N. Engl. J. Med. 321:1784 –1790; 1989. 18. Kim, T. H.; Pla, C.; Pla, M.; Podgorsak, E. B. Clinical aspects of a rotational total skin electron irradiation. Br. J. Radiol. 57:501–506; 1984. 19. Kumar, P. P.; Good, R. R.; Jones, E. O. Rotational total skin electron beam therapy. Strahlenther. Onkol. 164:73–78; 1988. 20. Kumar, P. P.; Henschke, K.; Mandal, K. P.; Nibhanupudy, J. R.; Patel, I. S. Early experience in using an 18 MeV linear accelerator for mycosis fungoides at Howard University Hospital. J. Natl. Med. Assoc. 69:223–226; 1977. 21. Kumar, P. P.; Henschke, U. K.; Nibhanupudy, J. R. Problems and solutions in achieving uniform dose distribution in superficial total body electron therapy. J. Natl. Med. Assoc. 69:645– 647; 1977. 22. Notari, R. E. Biopharmaceutics and clinical pharmacokinetics. New York: Marcel Dekker; 1987. 23. Page, V.; Gardner, A.; Karzmark, C. J. Patient dosimetry in the electron treatment of large superficial lesions. Radiology 94:635– 641; 1970. 24. Scholtz, W. Ueber den einfluss der Rontgenstrahlen auf die haut in gesundem und krankem zustande. Archiv Dermatol. Syphilis 59:421– 449; 1902. 25. Trump, J. G.; Wright, K. A.; Evans, W. W.; Anson, J. H.; Hare, H. F.; Fromer, J. L.; Jacque, G.; Horned, K. W. High energy electrons for the treatment of extensive superficial malignant lesions. Amer. J. Radiol. 69:623– 629; 1953. 26. Van Der Merwe, D. G. Total skin electron therapy: a technique which can be implemented on a conventional electron linear accelerator. Int. J. Radiat. Oncol. Biol. Phys. 27:391– 396; 1993. 27. Weaver, R. D.; Gerbi, B. J.; Dusenbery, K. E. Evaluation of dose variation during total skin electron irradiation using thermoluminescent dosimeters. Int. J. Radiat. Oncol. Biol. Phys. 33:475– 478; 1995. 28. Wong, F.; Kwa, W. Electron beam dosimetry study of the digits in total body surface irradiation. J. Can. Assoc. Radiol. 35:254 –256; 1984.
PII S0360-3016(97)00585-3
●
Clinical Investigation UTILIZATION OF THERMOLUMINESCENT DOSIMETRY IN TOTAL SKIN ELECTRON BEAM RADIOTHERAPY OF MYCOSIS FUNGOIDES JOHN A. ANTOLAK, PH.D.,* JACKSON H. CUNDIFF, B.S.,*
AND
CHUL S. HA, M.D.†
Departments of *Radiation Physics and †Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX Purpose: The purpose of this report is to discuss the utilization of thermoluminescent dosimetry (TLD) in total skin electron beam (TSEB) radiotherapy to: (a) compare patient dose distributions for similar techniques on different machines, (b) confirm beam calibration and monitor unit calculations, (c) provide data for making clinical decisions, and (d) study reasons for variations in individual dose readings. Methods and Materials: We report dosimetric results for 72 cases of mycosis fungoides, using similar irradiation techniques on two different linear accelerators. All patients were treated using a modified Stanford 6-field technique. In vivo TLD was done on all patients, and the data for all patients treated on both machines was collected into a database for analysis. Means and standard deviations (SDs) were computed for all locations. Scatter plots of doses vs. height, weight, and obesity index were generated, and correlation coefficients with these variables were computed. Results: The TLD results show that our current TSEB implementation is dosimetrically equivalent to the previous implementation, and that our beam calibration technique and monitor unit calculation is accurate. Correlations with obesity index were significant at several sites. Individual TLD results allow us to customize the boost treatment for each patient, in addition to revealing patient positioning problems and/or systematic variations in dose caused by patient variability. The data agree well with previously published TLD results for similar TSEB techniques. Conclusion: TLD is an important part of the treatment planning and quality assurance programs for TSEB, and routine use of TLD measurements for TSEB is recommended. © 1998 Elsevier Science Inc. Mycosis fungoides, Total skin electron beam radiotherapy, In vivo dosimetry, Thermoluminescent dosimetry.
INTRODUCTION
is the Stanford technique (14, 16, 23), can be found in AAPM Report #23 (1). Our institution developed a variation of the Stanford technique and began treating MF patients in 1984 (2, 11). We report here our experience with in vivo thermoluminescent dosimetry (TLD) measurements, using a similar TSEB technique on two different treatment machines. In vivo dosimetry is a routine procedure for many institutions that provide TSEB therapy. However, systematic evaluations of the dosimetric data have not been widely reported. We believe that comprehensive dosimetric data for different techniques should be available in the literature. This benefits those who want to compare their technique to other techniques. It also benefits those that do TSEB therapy infrequently and have difficulty determining the ‘‘normal’’ dose for a given anatomic site. The purpose of this report is to describe the utilization of a systematic TLD quality assurance protocol to: (a) confirm that patient dose distributions can be made similar when moving the treatment technique from one linear accelerator
External beam radiotherapy was first used to treat mycosis fungoides (MF) in 1902 (24). Treatments were done mostly with low voltage X-rays before the advent of electron beams, which provided unique dose fall-off characteristics that were ideal for treatment of the dermatological lesions of MF. The use of total skin electron beam (TSEB) radiotherapy for the treatment of MF was first described in 1952 (25). Though there have been some controversies regarding the role of TSEB radiotherapy in the management of MF, a certain portion of early stage MF patients are considered to be cured by this treatment alone (13, 15, 17). TSEB has also been valuable for palliation of more advanced stage disease. The tolerance to TSEB has been extensively documented previously (5, 6, 15). The modern technique of irradiating the entire skin surface with multiple electron fields from linear accelerators has been used by different institutions since the 1960s. An overview of various techniques, the most common of which Reprint requests to: John A. Antolak, Ph.D., Department of Radiation Physics, Box 94, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030.
Acknowledgement—The authors thank Linda G. McCallister, BSN, for helping collect data on patients’ heights and weights. Accepted for publication 22 July 1997. 101
102
I. J. Radiation Oncology
●
Biology
●
Physics
Fig. 1. Schematic view of treatment geometry. Patient anterior surface is located 2 m from isocenter and is irradiated by beams at gantry angles of 67° and 113° for each of six positions.
or room to another, (b) ensure that beam calibration and monitor unit calculations are accurate, (c) provide data for making clinical decisions regarding patient shielding and boost therapy, and (d) study reasons for variation in individual dose readings. The data for this report are a continuation of the TLD results of Almond (2), which were initially reported by Hogstrom et al. (11). In 1994, the TSEB technique was moved to a different linear accelerator. The goal was to reproduce the original implementation as closely as possible because treatment results had been very satisfactory (6). METHODS AND MATERIALS Our TSEB technique, as reported by Hogstrom and colleagues (10, 11), is a modification of the well-known Stanford technique (23), which has two beams for each of six patient positions. The modification was the addition of a scatter plate, as seen in the schematic of the patient-beam geometry in Fig. 1. Holt and Perry (12) showed that the patient dose uniformity for six positions was much better than for four positions and only slightly worse than that for eight positions. They also showed that incorporation of a scatter plate close to the patient could improve dose uniformity on the patient’s skin surface by increasing the angular dispersion of the incident electrons for a given amount of lateral dispersion. This allows the electrons to more fully irradiate oblique skin surfaces and areas that would otherwise be greatly self-shielded. Hence, our technique uses a 1.3-cm (0.5-inch) acrylic scatter plate located 25 cm in front of the patient treatment plane. From 1984 to 1994, total skin irradiation was delivered using a Siemens Mevatron 801 that was modified to deliver a high dose rate beam (11). A total of 53 patients were treated using this machine. In 1994, the treatment technique was moved to a different treatment room having a Varian Clinac 2100C.2 As of October 1996, 19 patients have been treated using this machine under the supervision of a single physicist. Our objective in moving the technique was to match the previous technique as closely as possible. A 1
Siemens Medical Systems, Inc., Concord, CA.
Volume 40, Number 1, 1998
summary of the beam characteristics for our current TSEB beam vs. our previous TSEB beam is shown in Table 1. The beam profile in the horizontal (right-left) direction is approximately Gaussian-shaped, with a 90% width of approximately 60 cm. In the vertical (superior-inferior) direction, the dual-beam profile is 100% at isocenter height, increasing to 110% at approximately 6 60 cm and decreasing to 90% at approximately 6 100 cm from isocenter height (3). Measurements on an anthropomorphic phantom, using film, have shown that the depth of maximum dose for the combined 12-field treatment is within 0.1 cm of the skin surface, and the therapeutic depth (80% dose) is between 0.5 and 0.8 cm, depending on where the depth dose-curve is measured. The standard prescribed dose is 32 Gy to the skin surface at the umbilicus. The patient is treated 2 days per week for 8 weeks, and the weekly dose to the skin surface is 4 Gy. In the first week of treatment, the patient is irradiated in all six positions on each of the 2 days. This accommodates performing in vivo TLD dosimetry in one treatment session on the second day. In vivo dosimetry consists of TLD flat-packs being taped to the patient’s skin surface in 22 locations. Exposing the TLD on a single day eliminates possible errors due to dosimeter repositioning. In the second and subsequent weeks of treatment, the patient is treated in three positions each day, doubling the number of monitor units per field. This results in the same amount of irradiation time daily, but only half the setup time and, hence, a decrease in overall treatment time. TLD measurement positions have changed over the years, depending on physician preference, and the current standard TLD locations are shown in Table
Table 1. Comparison of beam parameters for the Clinac 2100C TSEB technique to the previous Mevatron 80 TSEB technique
Ep,0 (MeV, at patient) Single beam characteristics R80 (cm H2O) R50 (cm H2O) Rp (cm H2O) Dual beam characteristics Gantry angles 90% width right-left (cm) 90% width sup-inf (cm) SSDeff (cm) Dose calibration (cGy/MU) (single beam, 90° 6 23°) Treatment ‘‘beam-on’’ time (2 Gy) Daily MU settings Week 1: 12 fields/fx Weeks 2–8: 6 fields/fx 2
Current technique Varian Clinac 2100C
Previous technique Siemens Mevatron 80
4.41
4.29
1.12 1.53 2.11
1.05 1.45 2.05
90° 6 23° 61 '200 '190
90° 6 26° 58 '190 '200
0.100
N/A
'4.8 min
'2 min
357 714
N/A N/A
Varian Associates, Inc., Palo Alto, CA.
TLD measurement for TSEB radiotherapy
Table 2. List of current standard TLD positions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Scalp (top rear) Scalp vertex Top of right shoulder Top of left shoulder Right elbow Left elbow Right hand (mid dorsum) Left hand (mid dorsum) Right axilla Left axilla Upper thorax
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Upper back Anterior abdomen Lower back Right abdomen Left abdomen Right upper medial thigh Left upper medial thigh Right buttock Left buttock Right foot (mid dorsum) Left foot (mid dorsum)
2 and Fig. 2. When applicable, dosimeters are placed symmetrically. This provides us with two data-points for most locations and allows us to spot any patient positioning asymmetries. For female patients, extra dosimeters are usually placed in the inframammary folds to determine if that area needs to be treated with a boost field. Each patient’s TLD results are entered into a database and compared with aggregate arithmetic means and sample standard deviations (SDs) for each measurement site. Each dosimeter (flat-pack) consists of approximately 0.027 g of LiF TLD-100 powder, sandwiched between two thin, heat-sealed polyethylene sheets that form a small pouch, approximately 0.5-cm square by 0.055 cm thick. The TLD is taped to the patient’s skin at the specified locations; each piece of tape is numbered for later identification. In some cases, body hair is removed so that the TLD can be in good contact with the skin. Photographs showing the locations of the dosimeters are placed in the patient’s treatment record. For calibration purposes, reference dosimeters (usually three flat-packs) are exposed to 2 Gy at the depth of maximum dose in a plastic phantom at 100 cm SSD, using 9-MeV electrons (Ep,0 5 9.34 MeV) and a 10-cm2 cone. The energy at the depth of maximum dose is approximately the same as the energy of the electrons incident at the surface of the patient for the total skin irradiation. This allows the TLD dosimetry to provide an independent check of the calibration of the high dose rate mode of the linear accelerator used for total skin treatment. Our previous technique exposed the reference dosimeters using the TSE beam under calibration conditions, which is the current American Association of Physicists in Medicine recommendation (1). The TLD were read 1 or 2 days after exposure, and all results were reported as a percentage of the reference TLD dose. The powder was massed by an analytical balance and then evenly spread across the 1.3-cm diameter active surface of a rhodium-plated circular silver planchette. The powder was heated to approximately 300°C, and the integrated light reading was read by a modified Radiation Detection model 1100 TLD reader.3 The dose to the TLD sample was computed by multiplying the reference dose by the ratio of the light output per unit mass for the sample to the average light output per unit mass for the reference TLD. A sensitivity correction for su3
Radiation Detection, Sunnyvale, CA.
● J. A. ANTOLAK et al.
103
pralinearity, ranging from 1.035 at 0.2 Gy to 1.000 at 2.0 Gy and 0.981 at 3.0 Gy, was made. The estimated uncertainty (standard error) in the relative TLD dose is approximately 2%. Combined with the uncertainty in the calibrated dose rate of the linear accelerator (less than 2%), the total dose uncertainty is less than 3%. The data for all patients treated on both machines was collected in a database for analysis. Arithmetic means, sample SDs, and ranges were computed for both series of patients. It is reasonable to expect that there would be some dependence of the TLD dose on variables such as the patient’s height, weight, or both. To investigate this, we generated scatter plots of the TLD dose vs. weight, height, and an obesity index [ratio of the patient’s weight to their ideal weight based on height (22)]. We also calculated correlation coefficients vs. weight, height, and obesity index for the previous and current series of patients and the combined series of patients, where applicable.
RESULTS Dosimetric equivalence Table 3 summarizes the dosimetric data for the 53 patients treated with our previous technique on the Mevatron 80, and Table 4 summarizes the dosimetric data for the 19 patients treated with the current technique on a Clinac 2100C. The dose is given as a percentage of the prescribed dose. All of the patients in the current series were treated under the supervision of a single physicist, which may explain the slightly smaller sample SDs for the upper thorax, anterior abdomen, and lateral abdomen. The data for scalp vertex, shoulder, axilla, hand (mid dorsum), anterior abdomen, and foot (mid dorsum) is the same for both
Fig. 2. Standard TLD positions for TSEB radiotherapy, numbered as in Table 2.
104
I. J. Radiation Oncology
●
Biology
●
Physics
Volume 40, Number 1, 1998
Table 3. TLD results for 53 patients treated with previous total skin irradiation technique on Siemens Mevatron 80 Site
No. of measurements
Mean dose (% of prescribed dose)
Range (% of prescribed dose)
Scalp vertex Forehead Posterior mid-neck Submental Top of shoulder Lateral shoulder Axilla Hand (mid dorsum) Mid-medial finger Upper thorax Upper back Anterior abdomen Lower back Lateral abdomen Mid perineum Upper-medial thigh Mid-medial thigh Mid-anterior thigh Foot (mid dorsum) Foot (posterior) Big toe (dorsum)
43 27 20 29 103 75 108 85 62 33 16 54 52 104 34 89 88 48 89 66 59
76 6 21 96 6 8 103 6 6 101 6 6 67 6 15 100 6 12 59 6 19 88 6 7 123 6 27 95 6 9 101 6 6 100 6 9 96 6 7 98 6 9 25 6 21 54 6 25 96 6 12 100 6 9 124 6 9 117 6 10 141 6 10
38–122 82–111 91–116 84–110 34–111 50–120 8–102 65–103 63–178 61–104 88–110 64–109 79–118 72–119 1–82 2–107 34–119 71–116 104–145 90–146 110–162
The dose error is reported as 1 sample SD.
techniques (within 1 SD). These data support our hypothesis that the patient dosimetry is the same for both techniques. Beam calibration and monitor units In Tables 3 and 4, the mean dose to the anterior abdomen (umbilicus) is 100% of the prescribed dose for both the current and previous total skin irradiation techniques. As stated earlier, the current TLD protocol irradiates the standard dosimeters using a ‘‘normal’’ electron mode of the accelerator. Hence, these data confirm that the absolute calibration of the TSE beam is accurate and that the monitor unit calculation is correct. Clinical decision making To be most effective, TSEB requires meticulous treatment planning to ensure delivery of therapeutic doses as homogeneously as possible. There are great individual variations in dosimetry, as shown by our data. Kumar and colleagues (19, 21) have reported recurrences for the scalp vertex, axilla, palm of the hand, soles of feet, ventral surface of the penis, perineum, eyelids, and the area underneath the breast. The recurrences were caused by self-shielding in most cases (artificial shielding for the eyelids), and they recommended boost therapy. For our technique, the axilla, perineum, and soles of the feet are consistently underdosed and are therefore boosted with 6-MeV electrons and 1.5-cm bolus (water equivalent thickness) to reduce R80, the depth of the 80% dose, to 0.7 cm, which is similar to the therapeutic depth of the TSEB treatment. For some patients, the tops of the shoulders or scalp vertex may also need boost therapy. TLD has been instrumental in customizing supplementary boost treatments for such underdosed areas. TLD results are also used to verify the patient setup for
total skin treatments. After placement of TLD is done properly, comparing a patient’s TLD measurements with established mean dose and SD will aid the radiation oncologist and physicist in identifying any potential setup errors or individual variations in setups that would require special attention. Corrections in setups will be made as necessary and TLD measurements repeated to verify the corrections. For example, asymmetry in the measured doses often indicates a systematic error in the patient setup. Once the setups for the patient are established in a consistently reliable manner, boost volumes and doses can be decided. The boost volumes are drawn on the patient’s skin, taking into account the patient’s anatomy and angles of the incident beams during treatment. The boost fields are customized to compensate for the areas of irregular contour, self-shielding, or both. The amount of dose that needs to be made up to deliver a full dose (usually 32 Gy in the absence of thick plaques or tumors) is calculated from TLD measurements and delivered using a daily fraction size of approximately 2 Gy. For example, the mean dose to the axilla is approximately 60%, implying a dose deficit of 40% of 32 Gy or 12.8 Gy. The average patient, therefore, receives six 2-Gy fractions of 6-MeV electrons with 1.5-cm bolus. These boost (or supplementary) treatments are done on the days when TSEB is not done. Areas that are overdosed are blocked as needed. Usually patients begin to complain of tenderness, severe erythema, or early desquamation in such areas as fingers and the dorsum of the feet during the second half of the TSEB treatment. These areas are usually blocked with 0.15 cm of lead for 1 or 2 weeks during the 8-week course of treatment as long as there is no risk of underdosing adjacent gross disease. This is supported by the TLD results, which show that the fingers (mid medial) receive
TLD measurement for TSEB radiotherapy
● J. A. ANTOLAK et al.
105
Table 4. TLD results for 19 patients treated with current total skin irradiation technique on Varian Clinac 2100C Site
No. of measurements
Mean dose (% of prescribed dose)
Range (% of prescribed dose)
Scalp (top rear) Scalp vertex Top of shoulder Elbow Axilla Hand (mid dorsum) Upper thorax Upper back Anterior abdomen Lower back Lateral abdomen Upper-medial thigh Buttock Foot (mid dorsum)
17 17 36 35 43 36 18 18 21 19 40 19 19 36
105 6 8 87 6 20 74 6 8 90 6 13 60 6 25 85 6 6 93 6 4 93 6 7 100 6 4 91 6 7 98 6 6 59 6 23 58 6 14 117 6 7
89–120 49–114 59–88 60–115 3–105 71–97 86–100 76–105 91–106 79–106 84–113 7–93 26–80 102–132
The dose error is reported as 1 sample SD.
123 6 27% and the dorsum of foot receives 122 6 9% (combined results). Variations in dose readings The distributions of doses for the hand, abdomen, and foot have relatively small standard deviations, less than 10% of the prescribed dose. This is due to the reproducibility of positioning the dosimeter on each patient and the lack of self-shielding for the dosimeter position. For the elbow, the higher spread in the dose received is due to differences in how patients are able to hold their elbows, depending on their flexibility and strength. In the axilla, the dose received depends mainly on the patient’s ability to hold his or her arms up during treatment and secondarily on the patient’s axillary skin folds. The variability in the perineum doses (mid perineum in Table 3; inner thigh and buttock in Table 4) also depends on the patient’s inguinal and gluteal folds affecting the degree of self-shielding. However, another component of the spread is due to dosimeter placement variability from patient to patient. In this region, the dose gradient on the skin is generally large, implying that the dose depends on exactly where the dosimeter is placed for that patient. In many cases, systematic tendencies could be seen in the data when plotted vs. weight, height, or obesity index. In general, the best correlations were with obesity index, as shown for selected sites in Fig. 3. Table 5 summarizes all of the data vs. obesity. The previous and current series of patients were considered together for the purpose of calculating a best-fit line and linear correlation coefficient. The linear correlation coefficient was considered significant if the probability of getting a correlation coefficient with greater absolute value from an uncorrelated population was less than 5% (4). When calculating the linear correlation coefficient, we excluded obviously erroneous data-points. For example, two of the three plus signs in Fig. 3a were caused by errors in patient positioning for the oblique treatment fields. A total of five data-points were excluded for the summary in Table 5.
The dose to the anterior abdomen and lower back shows a slight dependence on the obesity of the patient, as seen in Fig. 3a and b. As the patient’s size increases, the SSD to the umbilicus increases for the anterior oblique patient positions, causing a small decrease in total dose to that point. The dose to the lower back shows a similar effect, but is systematically lower than the dose to the anterior abdomen. This is probably due to partial self-shielding by the paraspinal muscles. Figure 3c and d shows the dose to the top of the shoulder and mid-medial thigh vs. the obesity index. The slopes of the best-fit lines through the data-points is much larger than in Fig. 3a and b, implying a stronger dependence on the obesity index. The correlations are significant, but there is a large amount of scatter in the data. The large amount of scatter is probably due to other factors, such as differences in the patient’s treatment positioning and physical condition. Figure 3e and f shows the dose to the medial aspect of the middle finger and the dorsum of the big toe vs. the obesity index. The dose to the mid-medial finger is highly dependent on the obesity of the patient, which is consistent with the experimental observations of Wong and Kwa (28). To some extent, electrons are able to irradiate the medial aspect of the fingers for all fields, especially for thin fingers. It should be noted that our patients are treated with their fingers held together. Measurements on one patient showed that the dose to the medial aspect of the middle finger was reduced by approximately 30%, relative to fingers held apart. The dose to the fingers is, therefore, more homogeneous, without resorting to the plastic sheath used by Wong and Kwa (28). The dose to the big toe does not depend as much on the obesity of the patient. This is due to the larger diameter of the big toe and the fact that the TLD was placed on the top of the toe rather than between the toes. Limited numbers of measurements on the palmar aspect of the middle finger showed that the palmar aspect of the fingers receive a dose similar to the dorsum of the hand.
106
I. J. Radiation Oncology
●
Biology
●
Physics
Volume 40, Number 1, 1998
Fig. 3. Scatter plots of measured TLD dose vs. the ratio of the patient’s weight to their ideal weight. The crosses are from the first series of patients (Mevatron 80); the circles are from the current series of patients (Clinac 2100C); the line is a least squares fit through all of the data. The TLD sites are (a) anterior abdomen, (b) midline, lower back, (c) top of shoulder, (d) mid-medial thigh, (e) mid-medial finger, and (f) big toe. The plus symbols in (a) were excluded from the least squares fit.
DISCUSSION Other groups have published in vivo TLD results; however, four of these manuscripts give results for a ‘‘typical’’ patient using a rotational technique (18, 20), a variant of the Stanford technique (7), and a reclined patient-position technique (9). There are no data for the patient-to-patient variability in these reports. The data in the first three of these
reports shows similar dosimetric patterns to the data in Tables 3 and 4. The major exception is the axilla dose reported by Kumar et al. (20). In their technique, the patient holds both arms overhead for the duration of treatment, allowing the axilla to received 95% of the prescribed dose. However, it would be expected that the tops of the shoulders would probably receive a lower dose compared with our
TLD measurement for TSEB radiotherapy
● J. A. ANTOLAK et al.
107
Table 5. TLD results for all patients vs. obesity index Site and # of measurements
Best fit line (% of prescribed dose)
Correlation, r
Significant
Scalp (top rear), n 5 15 Scalp vertex, n 5 51 Forehead, n 5 23 Posterior mid-neck, n 5 16 Submental, n 5 23 Top of shoulder, n 5 116 Lateral shoulder, n 5 62 Elbow, n 5 32 Axilla, n 5 127 Hand (mid dorsum), n 5 102 Mid-medial finger, n 5 56 Upper thorax, n 5 42 Upper back, n 5 30 Anterior abdomen, n 5 59 Lower back, n 5 59 Lateral abdomen, n 5 120 Perineum, n 5 29 Upper-medial thigh, n 5 107 Mid-medial thigh, n 5 72 Mid-anterior thigh, n 5 44 Buttock, n 5 34 Foot (mid dorsum), n 5 105 Foot (posterior), n 5 60 Big toe (dorsum), n 5 54
D 5 117.0 2 9.2 I D 5 78.0 1 1.2 I D 5 94.8 1 1.2 I D 5 102.2 1 0.8 I D 5 99.8 1 0.9 I D 5 90.0 2 16.9 I D 5 124.1 2 20.0 I D 5 106.3 2 11.1 I D 5 88.1 2 24.2 I D 5 93.9 2 5.2 I D 5 216.7 2 76.5 I D 5 104.7 2 6.2 I D 5 104.7 2 6.2 I D 5 106.0 2 5.0 I D 5 108.2 2 5.5 I D 5 111.9 2 11.2 I D 5 53.5 2 26.9 I D 5 68.0 2 9.8 I D 5 120.9 2 20.4 I D 5 104.1 2 3.6 I D 5 85.6 2 21.2 I D 5 126.0 2 3.2 I D 5 140.6 2 19.8 I D 5 171.0 2 23.9 I
20.25 0.01 0.03 0.02 0.03 20.28 20.28 20.18 20.25 20.16 20.49 20.41 20.19 20.30 20.24 20.32 20.29 20.09 20.37 20.07 20.31 20.08 20.36 20.43
No No No No No Yes Yes No Yes No Yes Yes No Yes Yes Yes No No Yes No No No Yes Yes
The second column is the best fit line for the dose (D) vs. obesity index (I), and the third column is the linear correlation coefficient. If the last column is ‘‘Yes’’, then the probability of the dose and obesity index being not truly correlated is less than 5%.
technique. The reclined patient position technique of Gerbi et al. (9) is very different from our technique, making comparison difficult. The earliest report describing in vivo TLD results for multiple patients was by Fraas et al. (8) at the National Cancer Institute, using the Stanford technique. Their data for scalp vertex, posterior neck, axilla, abdomen, elbow, hand (mid dorsum), upper medial thigh, and foot (mid dorsum) shows good agreement (within 1 SD) with the data in Tables 3 and 4. However, it is interesting that their mean dose to the scalp vertex is 46%, compared with 76% and 87% for our two techniques. The increased dose for our technique is most probably due to the addition of the 1.3-cm acrylic scatter plate in front of the patient to increase the angular dispersion of the electrons at the patient plane. Desai et al. (5) presented limited data for 20 patients reporting mean doses to nine sites, but without any indication of dose variability. They used a technique similar to ours (i.e., Stanford technique with an acrylic screen). Their data for foot (mid dorsum) and palm [compared with our hand (mid dorsum)] shows good agreement with the data in Tables 3 and 4. However, their reported mean dose to the forehead was 1.4 SDs higher than in Table 3, and their reported mean dose to the axilla was approximately 25% higher than in Tables 3 and 4. Although the SDs for the axilla doses in Tables 3 and 4 are 19% and 25%, respectively, this most likely represents a significant dosimetric difference between the techniques. From their description of their TSEB technique, it is not clear what may have caused these discrepancies.
Data for three patients irradiated using a reclined patientposition technique were reported by Van Der Merwe (26). The data for each patient are reported separately, and SDs are given for each measurement. For the anterior abdomen and posterior neck, their data shows good agreement with the data in Tables 3 and 4. Their reported dose to the axilla is within 1 SD of our data for two of three patients, while the reported dose to the foot is much lower than in Tables 3 and 4, again for two of three patients. The dose to the ‘‘inner thigh’’ is more than 1 SD higher than for the mid-medial thigh in Tables 3 and 4. The dosimetric differences are most likely caused by the use of a different TSEB technique. More recently, Weaver et al. (27) reported dosimetric results for 22 patients treated at The University of Minnesota between 1983 and 1993. Their TSEB technique is quite similar to ours, using a 1.0-cm acrylic scatter plate 18 cm in front of the patient plane, compared with a 1.3-cm acrylic scatter plate 25 cm in front of the patient plane. Not surprisingly, their reported mean doses and SDs agree very well with the data in Tables 3 and 4. Their reported mean doses to the anterior abdomen, forehead, upper thorax, hand (mid dorsum), finger, scalp vertex, perineum, and axilla are much less than 1 SD different from our data. Their reported SDs are also similar to those in Tables 3 and 4, indicating that their TSEB technique is almost dosimetrically identical to our technique. Weaver et al. also commented that height was a key factor in determining the dose to the scalp vertex. However, our data indicate a correlation coefficient of 20.01 and 20.23 vs. height for our Mevatron 80 and
108
I. J. Radiation Oncology
●
Biology
●
Physics
Clinac 2100C techniques, respectively. Therefore, we suspect that the extent of our beam uniformity may be slightly greater. In conclusion, there is a wide individual variation in actual dosimetry of TSEB for MF. We have described a systematic QA program using TLD dosimetry that: (a) confirms that patient dose distributions can be made similar
Volume 40, Number 1, 1998
when moving the treatment technique from one linear accelerator or room to another, (b) ensures that beam calibration and monitor unit calculations are accurate, (c) provides data for making clinical decisions regarding patient shielding and boost therapy, and (d) allows us to study reasons for variation in individual dose readings. Therefore, routine use of TLD measurements for TSEB is recommended.
REFERENCES 1. AAPM. Total skin electron therapy: technique and dosimetry. Report #23. American Association of Physicists in Medicine; New York; 1987. 2. Almond, P. R. Total skin/electron irradiation technique and dosimetry. In: Kereiakes, J. G.; Elson, H. R.; Born, C. G., eds. Radiation oncology physics (Medical Physics monograph No. 15). New York: American Institute of Physics; 1987:296 –332. 3. Antolak, J. A.; Hogstrom, K. R. Multiple scattering theory for total skin electron beam design. Med. Phys. (submitted). 4. Bevington, P. R.; Robinson, D. K. Data reduction and error analysis for the physical sciences. New York: McGraw-Hill; 1992. 5. Desai, K. R.; Pezner, R. D.; Lipsett, J. A.; Vora, N. L.; Luk, K. H.; Wong, J. Y.; Chan, S. L.; Findley, D. O.; Hill, L. R.; Marin, L. A.; Archambeau, J. O. Total skin electron irradiation for mycosis fungoides: relationship between acute toxicities and measured dose at different anatomic sites [published erratum appears in Int. J. Radiat. Oncol. Biol. Phys. 16:A1; 1989.]. Int. J. Radiat. Oncol. Biol. Phys. 15:641– 645; 1988. 6. Duvic, M.; Lemak, N. A.; Redman, J. R.; Eifel, P. J.; Tucker, S. L.; Cabanillas, F. F.; Kurzrock, R. Combined modality therapy for cutaneous T-cell lymphoma. J. Am. Acad. Dermatol. 34:1022–1029; 1996. 7. el-Khatib, E.; Hussein, S.; Nikolic, M.; Voss, N. J.; Parsons, C. Variation of electron beam uniformity with beam angulation and scatterer position for total skin irradiation with the Stanford technique. Int. J. Radiat. Oncol. Biol. Phys. 33:469 – 474; 1995. 8. Fraass, B. A.; Roberson, P. L.; Glatstein, E. Whole-skin electron treatment: patient skin dose distribution. Radiology 146: 811– 814; 1983. 9. Gerbi, B. J.; Khan, F. M.; Deibel, F. C.; Kim, T. H. Total skin electron arc irradiation using a reclined patient position. Int. J. Radiat. Oncol. Biol. Phys. 17:397– 404; 1989. 10. Hogstrom, K. R.; Ames, J. C.; Cundiff, J. H. The use of the pencil beam algorithm in designing treatment beams for mycosis fungoides. Med. Phys. 10:525; 1983. 11. Hogstrom, K. R.; Ewton, J. R.; Cundiff, J. H.; Ames, J. C.; McNeese, M. D. Beam delivery system and dosimetry for total skin electron therapy at MDAH. Med. Phys. 11:389; 1984. 12. Holt, J. G.; Perry, D. J. Some physical considerations in whole skin electron beam therapy. Med. Phys. 9:769 –776; 1982. 13. Hoppe, R. T.; Cox, R. S.; Fuks, Z.; Price, N. M.; Bagshaw, M. A.; Farber, E. M. Electron-beam therapy for mycosis fungoides: the Stanford University experience. Cancer Treat. Rep. 63:691–700; 1979. 14. Hoppe, R. T.; Fuks, Z.; Bagshaw, M. A. Radiation therapy in the management of cutaneous T-cell lymphomas. Cancer Treat. Rep. 63:625– 632; 1979.
15. Jones, G. W.; Hoppe, R. T.; Glatstein, E. Electron beam treatment for cutaneous T-cell lymphoma. Hematol. Oncol. Clin. North Am. 9:1057–1076; 1995. 16. Karzmark, C. J.; Loevinger, R.; Steele, R. E.; Weissbluth, M. A technique for large-field, superficial electron therapy. Radiology 74:633– 644; 1960. 17. Kaye, F. J.; Bunn, P. A., Jr.; Steinberg, S. M.; Stocker, J. L.; Ihde, D. C.; Fischmann, A. B.; Glatstein, E. J.; Schechter, G. P.; Phelps, R. M.; Foss, F. M.; Parlette, H. L., III; Anderson, H. J.; Sausville, E. A. A randomized trial comparing combination electron-beam radiation and chemotherapy with topical therapy in the initial treatment of mycosis fungoides. N. Engl. J. Med. 321:1784 –1790; 1989. 18. Kim, T. H.; Pla, C.; Pla, M.; Podgorsak, E. B. Clinical aspects of a rotational total skin electron irradiation. Br. J. Radiol. 57:501–506; 1984. 19. Kumar, P. P.; Good, R. R.; Jones, E. O. Rotational total skin electron beam therapy. Strahlenther. Onkol. 164:73–78; 1988. 20. Kumar, P. P.; Henschke, K.; Mandal, K. P.; Nibhanupudy, J. R.; Patel, I. S. Early experience in using an 18 MeV linear accelerator for mycosis fungoides at Howard University Hospital. J. Natl. Med. Assoc. 69:223–226; 1977. 21. Kumar, P. P.; Henschke, U. K.; Nibhanupudy, J. R. Problems and solutions in achieving uniform dose distribution in superficial total body electron therapy. J. Natl. Med. Assoc. 69:645– 647; 1977. 22. Notari, R. E. Biopharmaceutics and clinical pharmacokinetics. New York: Marcel Dekker; 1987. 23. Page, V.; Gardner, A.; Karzmark, C. J. Patient dosimetry in the electron treatment of large superficial lesions. Radiology 94:635– 641; 1970. 24. Scholtz, W. Ueber den einfluss der Rontgenstrahlen auf die haut in gesundem und krankem zustande. Archiv Dermatol. Syphilis 59:421– 449; 1902. 25. Trump, J. G.; Wright, K. A.; Evans, W. W.; Anson, J. H.; Hare, H. F.; Fromer, J. L.; Jacque, G.; Horned, K. W. High energy electrons for the treatment of extensive superficial malignant lesions. Amer. J. Radiol. 69:623– 629; 1953. 26. Van Der Merwe, D. G. Total skin electron therapy: a technique which can be implemented on a conventional electron linear accelerator. Int. J. Radiat. Oncol. Biol. Phys. 27:391– 396; 1993. 27. Weaver, R. D.; Gerbi, B. J.; Dusenbery, K. E. Evaluation of dose variation during total skin electron irradiation using thermoluminescent dosimeters. Int. J. Radiat. Oncol. Biol. Phys. 33:475– 478; 1995. 28. Wong, F.; Kwa, W. Electron beam dosimetry study of the digits in total body surface irradiation. J. Can. Assoc. Radiol. 35:254 –256; 1984.