Total skin electron irradiation: evaluation of dose uniformity throughout the skin surface

Medical Dosimetry, Vol. 28, No. 1, pp. 31–34, 2003 Copyright © 2003 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/03/$–see front matter

doi:10.1016/S0958-3947(02)00235-2

TOTAL SKIN ELECTRON IRRADIATION: EVALUATION OF DOSE UNIFORMITY THROUGHOUT THE SKIN SURFACE YAVUZ ANACAK, ZUMRE ARICAN, RAQUEL BAR-DEROMA, ADA TAMIR, and ABRAHAM KUTEN Department of Oncology, Rambam Medical Center, Haifa, Israel; The Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel; Department of Radiation Oncology, Ege University Faculty of Medicine, Izmir, Turkey; and Department of Radiation Oncology, Dokuz Eylul University Faculty of Medicine, Izmir, Turkey (Accepted 24 August 2002)

Abstract—In this study, in vivo dosimetic data of 67 total skin electron irradiation (TSEI) treatments were analyzed. Thermoluminescent dosimetry (TLD) measurements were made at 10 different body points for every patient. The results demonstrated that the dose inhomogeneity throughout the skin surface is around 15%. The homogeneity was better at the trunk than at the extratrunk points, and was worse when a degrader was used. There was minimal improvement of homogeneity in subsequent days of treatment. © 2003 American Association of Medical Dosimetrists. Key Words:

Mycosis fungoides, TSEI, Dosimetry, Homogeneity.

INTRODUCTION

PATIENTS AND METHODS

Total skin electron beam irradiation (TSEI) is one of the most sophisticated treatment techniques of modern radiation oncology practice. TSEI is used in cutaneous T-cell lymphoma, mycosis fungoides (MF), and Kaposi’s sarcoma, which limits this technique to only major radiotherapy centers. The objective of TSEI is to uniformly deliver a specified dose over the entire skin surface to a particular depth.1,2 However, considerable technical and dosimetric difficulties exist in achieving this goal due to (1) patient factors: variable thickness of the skin throughout the body and the irregularities of its surface; and (2) treatment machine factors: high output, large fields, and extended SSD. Therefore, a realistic goal of TSEI is to achieve the ideal situation as closely as possible, without harming the patients.3 TSEI was first described in 1952 and, since then, various techniques have been developed to improve dose homogeneity.4,5 Thermoluminescent dosimetry (TLD) measurements are usually performed to evaluate dose distribution throughout the body. Although this is a routine procedure in many institutions, very little data on these measurements are available in the literature.2,6 – 8 This paper presents the in vivo dosimetric data of 67 treatments on 58 patients with MF, treated with TSEI.

Between 1994 and 2000, 58 patients with MF were treated with TSEI at Rambam Medical Center. Routine in vivo TLD measurements were performed on all of them during the first fractions of the therapy. In 9 cases, a second set of dosimetry was also performed a few fractions later. Patients were treated by the Stanford technique9 on a Varian Clinac 1800 accelerator (Varian Medical Systems, Inc., Palo Alto, CA). In the Stanford technique, implemented at the Rambam Medical Center, the patient is irradiated standing in 6 positions in front of a 6-MeV electron beam at an extended source-to-surface distance (SSD) of 4 meters. Three anterior and 3 posterior stationary fields, each having a superior and inferior portal, with a gantry angulation of 16° or 17° are used; all fields are treated every day. In some patients, penetration depth of the electron beam is individually modified by using a 4- or 10-mm lucite degrader placed 20 cm in Table 1. Mean values of TLD readings in first measurements No. of Mean Dose Range Readings (% of prescription) (% of prescription) Reference Point Umbilicus Trunk Top of sternum Lateral abdomen Back of shoulder Extratrunk Vertex Top of shoulder Elbow Palm Knee Dorsum of foot

Reprint requests to: Prof. A. Kuten, Head, Oncology Department, Rambam Medical Center, POB 9602, Haifa 31096, Israel. E-mail: [email protected] 31

58

100

–

55 56 53

96 ⫾ 8 100 ⫾ 14 99 ⫾ 10

76–131 69–146 62–114

46 49 57 50 50 53

84 ⫾ 27 92 ⫾ 10 81 ⫾ 20 61 ⫾ 23 94 ⫾ 8 111 ⫾ 18

13–133 70–134 26–138 15–120 75–114 73–152

32

Medical Dosimetry

Volume 28, Number 1, 2003

Table 2. The effect of degrader on the skin surface dose distribution

Umbilicus Top of sternum Lateral of abdominal wall Back of shoulder Vertex Top of shoulder Elbow Palm of hand Middle of knee Dorsum of foot

0 mm No.

Mean Dose (% of prescription)

4 mm No.

Mean Dose (% of prescription)

10 mm No.

Mean Dose (% of prescription)

p-value

15 14 14 12 10 9 14 11 11 12

100 99 ⫾ 12 111 ⫾ 16 100 ⫾ 7 90 ⫾ 25 91 ⫾ 11 97 ⫾ 18 83 ⫾ 21 98 ⫾ 7 124 ⫾ 26

34 32 34 32 28 32 34 31 31 33

100 97 ⫾ 5 98 ⫾ 9 100 ⫾ 8 79 ⫾ 29 92 ⫾ 7 78 ⫾ 18 54 ⫾ 19 94 ⫾ 8 111 ⫾ 10

9 9 8 9 8 8 9 8 8 8

100 89 ⫾ 6 89 ⫾ 6 92 ⫾ 15 92 ⫾ 22 94 ⫾ 18 67 ⫾ 14 55 ⫾ 18 90 ⫾ 6 91 ⫾ 10

0.006 ⬍0.0001. N.S. N.S. N.S. 0.001 0.001 0.07 ⬍0.0001

front of the patient. In 15 patients, no degrader was used, while a degrader of 4-mm thickness was used in 34 patients and a degrader of 10-mm thickness was used in 9 patients. The eyes were always shielded by internal or external lead shields covered with wax; the hands and feet of some patients were also covered with 4-mm lead gloves during part of the treatment. The reference dose is prescribed at the umbilicus. Dosimetric measurements were performed with LiF TLD chips (Victoreen Inc., Cleveland, OH; Harshaw TLD, Bicron RMP, Solon, OH). Chips were taped to several points on the skin, in small plastic bags. These points were chosen to represent the different angulations of the incident beam on the skin. In this paper, we report the results of TLD measurements in 10 points: umbilicus, top of shoulder, back of shoulder, suprasternal notch, lateral abdominal wall, vertex, elbow, palm, knee, and dorsum of foot. Some of these points were initially expected to receive a homogenous dose, whereas others were expected to be overdosed or underdosed due to anatomic irregularities and patient positioning. No measurements were taken for the shielded body parts. TLD chips were read by a Victoreen 2800M TLD reader, 1 to 2 days after measurement. The objective of this study was to analyze the dose distribution throughout the skin surface, to evaluate the intrapatient dosimetric variations between 2 fractions,

and to see the effect of degrader on dose distribution in vivo. Statistics Statistical analysis was done by using the SPSS V.9.0 program. Differences between trunk– extratrunk points were analyzed by t-test and differences between 2 fractions by paired-Wilcoxon test. ANOVA test was used to analyze the effect of degrader thickness on the surface dose distribution. P ⫽ 0.05 was accepted as the level of significance for all tests. RESULTS The mean values of the TLD readings in relation to the prescription point (umbilicus), are presented in Tables 1–3. The overall deviations from the prescription are shown in Table 4. The mean deviation of all measurements was 15.4%. As initially expected, the deviations were less than 10% at the trunk points. However, higher deviations were demonstrated at extratrunk points, around extremities, and at the points where the electron beam was tangential (Table 1). The difference between the trunk and extratrunk points was significant (Table 4). The surface dose, as measured by TLD chips, was considerably less homogenous in most body points when

Table 3. Mean values of TLD readings in patients with 2 sets of measurements 1st Set of Readings

Umbilicus Top of sternum Lateral of abdominal wall Back of shoulder Vertex Top of shoulder Elbow Palm Knee Dorsum of foot

2nd Set of Readings

No. of Readings

Mean Dose (% of prescription)

No. of Readings

Mean Dose (% of prescription)

p-value*

9 9 8 8 7 4 9 7 7 8

100 98 ⫾ 15 103 ⫾ 22 98 ⫾ 10 97 ⫾ 13 86 ⫾ 11 91 ⫾ 18 66 ⫾ 28 95 ⫾ 10 110 ⫾ 25

9 7 7 5 5 3 6 6 6 6

100 101 ⫾ 9 100 ⫾ 12 102 ⫾ 4 104 ⫾ 13 98 ⫾ 4 95 ⫾ 6 85 ⫾ 20 100 ⫾ 17 123 ⫾ 21

N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S.

*The test was performed only for paired measurements.

Total skin electron irradiation ● Y. ANACAK et al.

Table 4. Overall absolute deviations from the dose at prescription point considering all measurements No. of Readings Mean (excluding umbilicus) Deviation (%) All patients Body parts Trunk Extratrunk Pts. with 2 measurements 1st measurement 2nd measurement Degraders 0 mm 4 mm 10 mm

514 183 331

p-value

15.4 7.7 ⫾ 7.4 ⬍ 0.0001 19.7 ⫾ 17.7

75 51

14.5 ⫾ 15.1 11.4 ⫾ 11.0

N.S.

126 302 86

13.5 ⫾ 13.1 15.6 ⫾ 17.1 17.7 ⫾ 15.1

N.S.

a degrader was used (Table 2). It was found that the thicker the degrader, the worse the homogeneity at the surface. However, this was not demonstrated when all points were considered together (Table 4). In 9 patients, measurements were repeated a few fractions later. A modest improvement of the dose homogeneity at the surface was noted at the 2nd measurements; however, this was not significant (Tables 3 and 4). DISCUSSION TSEI has been shown to be an effective method in the treatment of MF. It is potentially curative in the early stages and usually offers good palliation in patients with advanced disease. The most widely used TSEI method is the dual 6-field technique known as the Stanford method. Dosimetry of the Stanford method under idealized conditions, such as humanoid phantom measurements, demonstrated a uniformity of ⫾ 5% over most of the body surfaces. However, these results could not be repeated by the TLD measurements on the actual patients.1,2– 8 A dose uniformity of ⫾ 10% to 15% is reported to be acceptable;1,2,7 hence, we consider that the overall deviation of 15% in this study is satisfactory. Published reports on TLD measurements of patients treated by

33

TSEI show that inhomogeneity of the dose distribution occurs mostly around the mobile body parts— head, arms, legs, hands, and feet. Dose distribution at the trunk, chest, abdomen, and pelvis is much more homogenous.2,6 – 8 The results of the current study confirm these reports, as the deviation at the trunk was 7%, whereas it was much higher at extratrunk points. The more distant the point to the trunk, the higher the deviation. The Stanford technique requires the irradiation of the patient at 6 different positions, which are achieved mainly by moving the extremities (Fig. 1). These rather unusual positions may be quite uncomfortable for the patients, especially for the elderly. They have to stand on their own throughout the long treatment procedure, with their eyes covered, often causing loss of orientation. Thus, movement of at least some body parts during irradiation is inevitable in most cases. This may explain the relatively higher deviations through the extratrunk points, along with the more irregular anatomy of the skin at these body parts than at the trunk. The second set of measurements, a few fractions later, demonstrated minimally improved dose homogeneity: 14.5% deviation in the first measurements reduced to 11.4%. However, this improvement was not statistically significant and may possibly be due to the patient’s learning curve of the treatment positions. Previous measurements on humanoid phantom demonstrated that the homogeneity of the surface dose is within ⫾ 5% for all degrader thicknesses. However, this could not be translated to the actual patient measurements. Measurements on humanoid phantom reflect an idealized situation where all surface points are at similar distance to the degrader and there is no patient movement. Thus, it was not unexpected that the degrader displayed a negative effect on the surface dose homogeneity on actual patients, where anatomy and stabilization of the patients play an important role. CONCLUSIONS Data of the current report on TLD measurements of 67 TSEI treatments demonstrate that the overall dose

Fig. 1. a) Description of the 6 different patient positions. b) Six beam arrangement scheme.

34

Medical Dosimetry

homogeneity throughout the skin surface of the body is around ⫾ 15%. As expected and reported in other studies, homogeneity is better at the trunk than in the mobile parts of the body. The 6 uncomfortable positions that the patient has to sustain during the treatment are reflected in the results. Because the mobile parts of the body have to change position 6 times, it is almost impossible to maintain precisely the same posture during each session and in further sessions. Homogeneity seems to be worse when degraders are used. Based on these results, we believe that TLD is a useful tool to monitor actual skin doses and therefore should be routinely used in TSEI treatments. Acknowledgments—The authors acknowledge the support of the NCI/ Middle East Consortium, for the fellowship of Dr. Yavuz Anacak from the Ege University Medical School, Izmir, Turkey; and of Dr. Zumre Arican from the Eylul University, Izmir, at the Rambam Medical Center, Haifa, Israel.

Volume 28, Number 1, 2003

2.

3.

4.

5.

6.

7.

8.

REFERENCES 9. 1. 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 –74; 1995. 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. 33:475–8; 1995. Podgorsak, E.B.; Podgorsak, M.B. Special techniques in radiotherapy: Part B. Total skin electron irradiation. In: Van Dyk, J., editor. Modern Technology of Radiation Oncology. Madison, WI: Medical Physics Publishing; 1999: 664 –78. Tetenes, P.J.; Goodwin, P.N. Comparative study of superficial whole-body radiotherapeutic techniques using a 4-MeV nonangulated electron beam. Radiology 122:219 –26; 1977. Williams, P.C.; Hunter, R.D.; Jackson, S.M. Whole body electron therapy in mycosis fungoides—a successful translational technique achieved by modification of an established linear accelerator. Br. J. Radiol. 52:302–7; 1979. Antolak, J.A.; Cundiff, J.H.; Ha, C.S. Utilization of thermoluminescent dosimetry in total skin electron beam radiotherapy of mycosis fungoides. Int. J. Radiat. Oncol. Biol. Phys. 40:101–8; 1998. Desai, K.R.; Pezner, R.D.; Lipsett, J.A.; et al. Total skin electron irradiation for mycosis fungoides: Relationship between acute toxicities and measured dose at different anatomic sites. Int. J. Radiat. Oncol. Biol. Phys. 15:641–5; 1988. Fraass, B.A.; Roberson, P.L.; Glatstein, E. Whole-skin electron treatment: Patient skin dose distribution. Radiology 146:811–4; 1983. 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.