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Feasible measurement errors when undertaking in vivo dosimetry during external beam radiotherapy of the breast
Medical Dosimetry, Vol. 28, No. 1, pp. 45– 48, 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)00241-8
FEASIBLE MEASUREMENT ERRORS WHEN UNDERTAKING IN VIVO DOSIMETRY DURING EXTERNAL BEAM RADIOTHERAPY OF THE BREAST CLARE E. HERBERT, B.A. (SC), MARTIN A. EBERT, PH.D., and DAVID J. JOSEPH, M.B.B.S., F.R.A.N.Z.C.R. Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, Western Australia; and Department of Physics, University of Western Australia, Western Australia (Accepted 1 June 2002)
Abstract—In vivo dosimetry is a proven reliable method of checking overall treatment accuracy, allowing verification of dosimetry and dose calculation as well as patient treatment setup. We conducted a pilot study to assess the clinical utility of in vivo dosimetry in our department. Diodes (calibrated for typical treatment conditions) were used to record entrance dose measurements on 62 patients representing a variety of treatment sites. Measurements were compared with predictions from the planning system, with results found to be in tolerance for the majority of treatment sites. However, large discrepancies were encountered for measurements performed during breast irradiation (up to 16% for lateral tangential fields). The sensitivity of the recorded entrance dose to the positioning error of the diode placement was examined. The sensitivity of diode signal to small changes in position were compared with feasible variations in other parameters (e.g., dosimetry, FSD at setup). For the breast irradiation technique considered, wedges are used for the majority of fields. It was found that a proportion of error was predominantly due to the use of wedges and the presence of significantly nonuniform patient contours. In combination with diode placement errors, this resulted in increased measurement error. Correct diode placement is critical to ensure accurate data collection. The results of this study indicate the importance of separating errors due to measurement technique from actual treatment/setup errors. © 2003 American Association of Medical Dosimetrists. Key Words:
In vivo dosimetry, Radiotherapy, Breast.
patient’s surface in highly modulated fields. Difficulties can be encountered when performing in vivo dosimetry under conditions of oblique incidence and in the presence of beam modulators (e.g., wedges) due to inaccuracy in detector placement. Mixed results have previously been reported for in vivo dosimetry during breast treatments12,13 due to the use of tangential fields and the frequent use of wedged fields. This investigation examined specifically the issues associated with in vivo dosimetry in breast irradiation and the significance of error in placement of the detector.
INTRODUCTION In vivo dosimetry (dose measurements on patients during therapeutic exposure) has been established as a reliable method for the verification of many of the dosimetric aspects associated with external beam radiotherapy.1–7 An in vivo dosimetry program is highly recommended for quality assurance of machine calibration, planning dosimetry and dose calculation, patient setup, and the influence of beam modifying components. Semiconductor detectors (diodes)8 –10 are well suited to in vivo dosimetry, as they are small, robust, do not require bias voltages (unlike ionization chambers), and provide immediate readout of results (unlike thermoluminescent dosimeters). It is necessary that the full set of conditions determining diode response (field size, source-to-diode distance, surface temperature, dose rate, beam obliquity) be known and appropriate signal correction factors10,11 applied correctly. For an in vivo dosimetry program using diodes to be effective, it is essential that reproducible methods are established for the application of the detection system, especially for the placement of diode detectors on the
BREAST IRRADIATION AND IN VIVO DOSIMETRY A pilot study of in vivo dosimetry for verification of entrance dose was undertaken within our department to establish a baseline for suitable dose accuracy limits. The breast irradiation technique requires the patient in a supine position on a MammoRx breast board (DIACOR, Salt Lake City, UT) with 1 arm by the patient’s side, and the other placed into the arm and wrist support. The 6-MV photon fields are used with tangential fields applied with a wedge to compensate for patient contour.14 To create a coplanar match plane at the superior edge of the field, (for matching to supraclavicular fields at the time of treatment or for future treatment) the couch is
Reprint requests to: Clare E. Herbert, Department of Radiation Oncology, Sir Charles Gairdner Hospital, Hospital Avenue, Nedlands, Western Australia, Australia 6009. E-mail: Clare.Herbert@health. wa.gov.au 45
46
Medical Dosimetry
angled 5° and the collimator is adjusted to create even medial edges at the upper and lower levels of the treatment field. Lead shielding is used on the upper level border to shield the match line superiorly. The collimator and gantry angles of the lateral tangent are opposed and adjusted to provide the required coplanar match of the deep borders within the breast. Scanditronix EDP-10 diodes (Scanditronix Medical, Uppsala, Sweden) were used for in vivo measurements of entrance dose on the central axis of all fields. These diodes have a width (including diode and base plate) of approximately 1.2 cm. Entrance dose values were calculated by determining the peak dose along the central axis of each defined field using point-dose estimates from the planning system (FOCUS, CMS St. Louis, MO) (Entrance dose is here defined as the dose at a distance dmax (1.5 cm for the 6-MV beam) below the patient’s surface along the line of the beam.). For comparison with possible off-axis positioning of diodes, entrance dose values were also calculated at off-axis locations as described below. Our pilot study gave mean absolute discrepancy between diode-measured and calculated central-axis entrance dose of 4.3% (standard deviation [SD] 4.0%), with a maximum discrepancy of 13.4% over 184 measurements for tangential 6-MV breast fields on 46 patients. Prospective analysis of these treatments showed that all fields included a wedge and required the diode to be positioned on a surface that was significantly oblique to the beam. These factors suggested that diode positioning would have been important in determining recorded entrance dose. As a result of these findings, a subset of breast patients was chosen to aid in evaluation of the influence of diode placement on the resulting signal reading. It was anticipated that diode positioning errors would lead to changes in measured dose due to changes in SSD (source-to-surface distance) due to patient contour, changes in wedge transmission, and variation in response of the diodes due to changes in SSD and angle of beam incidence. METHODS Plans considered and evaluation of entrance doses A total of 20 isocentric breast treatments were considered. All treatment fields included lateral tangential fields using 6-MV photons and standard wedges (15°, 30°, 45°, 60°) for contour compensation. The field arrangement for a diode is shown in Fig. 1. The influence of diode misplacement was only considered in the sagittal plane shown on the plan. The intended location for the diode was at central axis. Points from ⫺2.0 cm to 2.0 cm from this position (following the patient outline) were considered, with entrance dose values determined at 0.5-cm intervals. Negative values are posterior, positive values are anterior. The contribution due to wedge transmission was determined from the wedge off-axis
Volume 28, Number 1, 2003
Fig. 1. Treatment plan showing definition of incidence angle, and typical locations of points (indicated with asterisks) where entrance dose was determined.
factor at each point. The effect of misalignment on diode signal was determined by evaluating the changes in the correction factors CSSD and CAng, for changes in SSD and beam obliquity (angle), respectively, at the various off-axis positions. Monte Carlo simulation of patient measurements The 20 plans were considered to form a random sample of breast patients treated with tangential fields. A Monte Carlo method of statistical analysis was developed and used to sample the results from the 20 plans to gauge the magnitude of dose-reading error with misplacement of the diodes. This would allow the SD in diode placement error to be estimated. Assuming that the diodes are placed with a mean position equal to the desired measurement point (i.e., assuming no systematic error in diode positioning), the expected results of dose measurements were calculated for increasing SDs in diode placement error. At each SD value, random diode positions were sampled and the data for the 20 plans used to determine the average value of the discrepancy between calculated and measured dose. Following this procedure, the expected diode positioning error in our measurement set could be estimated. This assumes that all other factors, including patient positioning, calculated entrance doses, etc. are exact, which is certainly not the case. In this way, the estimated SD in positioning error represents an upper limit. RESULTS Diode accuracy Figure 2 shows a frequency histogram of the percentage difference between the prescribed dose and the simulated measured dose when applying correction factors CSSD and CAng as the diode is moved off-axis. As can be seen, the difference is less than 0.5%. Mean surface angle with wedge In general, greater surface obliquity requires greater wedge compensation. Alteration in relative dose with
In vivo dosimetry of the breast ● C. E. HERBERT et al.
Fig. 2. Comparison of actual off-axis entrance dose with expected measured off-axis entrance dose. The difference in these two is a result of the change in diode calibration factors (CSSD and CAng) as the diode is moved off axis. The histogram shows that the change in correction factor will be less than 0.5% in all cases such that errors in diode placement will not adversely affect correction factors.
each of the 4 standard wedge compensators is shown in Fig. 3. It was found that larger wedge angles demonstrated larger discrepancies in relative dose, with movement off-axis resulting in relative dose discrepancies of approximately 8% for 15° wedge and 25% for 60° wedge (⫺2.0 cm to 2.0 cm off-axis). The effect of SSD and off-axis wedge contribution to the relative dose is shown in Fig. 4. The wedge contribution and SSD contribution curves are shown to be in the same direction, with off-axis wedge corrections contributing to a larger proportion of the relative dose than the SSD. Amount of measured error related to off-axis placement of the diode Figure 5 shows a graph simulating the effect of misplacing the diode at different intervals, with data generated using the Monte Carlo sampling method described above. Estimations of percentage dose discrepancies are plotted against the SD of the diode positioning
Fig. 3. Variations in relative entrance dose (relative to central axis entrance dose) with variation in off-axis position for the 20 plans considered. Symbols used in drawing curves have been selected to represent groups of plans which used the same wedge angle.
47
Fig. 4. SSD and off-axis wedge contribution to the variation in relative entrance dose.
error, where the mean is assumed to be the desired measurement point (central axis). DISCUSSION It is known that each step in the planning/treatment process can contribute to the total uncertainty in absorbed dose delivered to the patient, thus we place importance on finding errors due to planning calculations, algorithms, and treatment factors. Error in diode misplacement is a factor that has not as yet been adequately considered. Here, we have found factors that increase the error in the diode readings for an everyday technique. We set out to quantify the sensitivity of the recorded entrance dose to the diode placement. Variation in correction factors Figure 2 shows that, in the treatment situation, when a diode is placed incorrectly, though within 2 cm of the desired position, the accumulated correction factors will vary by less than 0.5% from those actually required to correct diode signal at central axis. Thus, results obtained with diode measurements where positioning error is possible may be directly related to actual entrance doses (to within 0.5%).
Fig. 5. Plot of percentage dose discrepancies against the standard deviation of error in positioning the diode (mean assumed to be central axis). Curves calculated using Monte Carlo sampling of data extracted from the 20 sample plans.
48
Medical Dosimetry
Effect of wedge angle Patient breast contour is directly related to wedge angle—an increase in mean surface obliquity resulted in a larger wedge angle required for tissue compensation. This leads to a compounding effect in error when performing diode measurements on oblique surfaces irradiated with wedged fields. As Fig. 3 shows (and which is quite obvious), changes in entrance dose with off-axis position increase with increasing wedge angle. Percentage relative dose increases with increased wedge angle and increasing distances off-axis posteriorly. This was as expected, as the positive movement off-axis is toward the “thick” edge of the wedge, with the larger wedges absorbing more dose. Figure 4 shows that the effect of change in wedge transmission and change in SSD complement one another and lead to even larger variations in entrance dose with error in diode placement. As shown in Fig. 4, when the diode is placed off-axis on a wedged field with changes in obliquity, percentage differences in relative dose can vary up to 7% over a 2-cm range, constituting a substantial amount of error before treatment and planning factors are taken into account. Variation in collimator angle at treatment should not influence central-axis entrance dose measurements for symmetric fields, as rotation of the wedge should not lead to central-axis dose variations. This would change if the diode is placed slightly off-axis. However, the contribution from this effect would be small. Even for a 10° rotation of the collimator with the diode placed 1.0 cm off central-axis in a 60° wedged field, the variation in entrance dose will be less than 0.1%. The ability to estimate the amount of error in the diode reading due to diode placement may be a useful tool when assessing overall accuracy of the data collection. The percentage error found can be used to estimate the SD of the distance the diode was placed off-axis using the results of the Monte Carlo study undertaken (see Fig. 5). Comparing the results of our pilot measurements on breast tangents in terms of differences between measured and expected entrance doses (mean absolute discrepancy 4.3%, SD 4.0%) to the results of Fig. 5 suggest an SD in diode placement error of the order of 1.8 cm. As there is no certainty regarding the patient setup, patient immobilization, or dosimetric accuracy during the pilot study, this value represents an upper limit to diode placement error. Given a diode width of approximately 1.2 cm, a placement error of 1.8 cm would not appear unreasonable when locating a diode on an uneven surface relative to a light field. However, more
Volume 28, Number 1, 2003
careful placement should be able to reduce this considerably. Diodes have been proven to be a satisfactory method of quality assurance in detecting the received entrance dose; however, highlighted is the importance of accurate diode placement on lateral tangential fields during isocentric breast irradiation. Due to the changing contour of the breast throughout the treatment volume (and thus the SSD), and the effect of the use of wedge compensators on the off-axis contribution, a substantial error is possible prior to other factors being considered.
Acknowledgments—The authors thank Hwee Carter for participation in the measurements, and the radiation therapists from the Department of Radiation Oncology at the Sir Charles Gairdner Hospital.
REFERENCES 1. Essers, M.; Mijnheer, B.J. In vivo dosimetry during external photon beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 43:245– 59; 1999. 2. Leunens, G.; van Dam, J.; Dutreix, A.; van der Schueren, E. Quality assurance in radiotherapy by in vivo dosimetry. 1. Entrance dose measurements, a reliable procedure. Radiother. Oncol. 17:141–51; 1990. 3. Lee, P.; Sawicka, J.; Glasgow, G. Patient dosimetry quality assurance program with a commercial diode system. Int. J. Radiat. Oncol. Biol. Phys. 29:1175–82; 1994. 4. Li, C.; Lamel, L.; Tom, D. A patient dose verification program using diode detectors. Med. Dosim. 20:209 –14; 1995. 5. Essers, M.; Lanson, J.H.; Mijnheer, B.J. In vivo dosimetry during conformal therapy of prostatic cancer. Radiother. Oncol. 29:271–9; 1993. 6. van Dam, J.; Marinello, G. Methods for in vivo dosimetry in external radiotherapy. Physics for clinical radiotherapy. ESTRO Booklet No. 1. Belgium: Garant; 1994. 7. Heukelom, S.; Lanson, J.H.; Mijnheer, B.J. Comparison of entrance and exit dose measurements using ionization chambers and silicon diodes. Phys. Med. Biol. 36:47–59; 1991. 8. Nilsson, B.; Rude`n, B-I.; Sorcini, B. Characteristics of silicon diodes as patient dosimeters in external radiation therapy. Radiother. Oncol. 11:279 –88; 1988. 9. Rikner, G.; Grusell, E. General specification for silicon semiconductors for use in radiation dosimetry. Phys. Med. Biol. 32:1109 – 17; 1987. 10. Rikner, G.; Grusell, E. Patient dose measurements in photon fields by means of silicon semiconductor detectors. Med. Phys. 14: 870 –3; 1987. 11. Ding, W.; Patterson, W.; Tremethick, L.; Joseph, D. Calibration of entrance dose measurements for an in vivo dosimetry programme. Austr. Radiol. 39:369 –74; 1995. 12. Cozzi, L.; Fogliata-Cozzi, A. Quality assurance in radiation oncology. A study of feasability and impact on dose levels of an in vivo dosimetry program during breast cancer irradiation. Radiother. Oncol. 47:29 –36; 1998. 13. Fiorino, C.; Corletto, D.; Mangili, P.; et al. Quality assurance by systematic in vivo dosimetry: Results on a large cohort of patients. Radiother. Oncol. 56:85–95; 2000. 14. Kahn, F. The Physics of Radiation Therapy, 2nd ed. Baltimore: Williams & Wilkins; 1994.
doi:10.1016/S0958-3947(02)00241-8
FEASIBLE MEASUREMENT ERRORS WHEN UNDERTAKING IN VIVO DOSIMETRY DURING EXTERNAL BEAM RADIOTHERAPY OF THE BREAST CLARE E. HERBERT, B.A. (SC), MARTIN A. EBERT, PH.D., and DAVID J. JOSEPH, M.B.B.S., F.R.A.N.Z.C.R. Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, Western Australia; and Department of Physics, University of Western Australia, Western Australia (Accepted 1 June 2002)
Abstract—In vivo dosimetry is a proven reliable method of checking overall treatment accuracy, allowing verification of dosimetry and dose calculation as well as patient treatment setup. We conducted a pilot study to assess the clinical utility of in vivo dosimetry in our department. Diodes (calibrated for typical treatment conditions) were used to record entrance dose measurements on 62 patients representing a variety of treatment sites. Measurements were compared with predictions from the planning system, with results found to be in tolerance for the majority of treatment sites. However, large discrepancies were encountered for measurements performed during breast irradiation (up to 16% for lateral tangential fields). The sensitivity of the recorded entrance dose to the positioning error of the diode placement was examined. The sensitivity of diode signal to small changes in position were compared with feasible variations in other parameters (e.g., dosimetry, FSD at setup). For the breast irradiation technique considered, wedges are used for the majority of fields. It was found that a proportion of error was predominantly due to the use of wedges and the presence of significantly nonuniform patient contours. In combination with diode placement errors, this resulted in increased measurement error. Correct diode placement is critical to ensure accurate data collection. The results of this study indicate the importance of separating errors due to measurement technique from actual treatment/setup errors. © 2003 American Association of Medical Dosimetrists. Key Words:
In vivo dosimetry, Radiotherapy, Breast.
patient’s surface in highly modulated fields. Difficulties can be encountered when performing in vivo dosimetry under conditions of oblique incidence and in the presence of beam modulators (e.g., wedges) due to inaccuracy in detector placement. Mixed results have previously been reported for in vivo dosimetry during breast treatments12,13 due to the use of tangential fields and the frequent use of wedged fields. This investigation examined specifically the issues associated with in vivo dosimetry in breast irradiation and the significance of error in placement of the detector.
INTRODUCTION In vivo dosimetry (dose measurements on patients during therapeutic exposure) has been established as a reliable method for the verification of many of the dosimetric aspects associated with external beam radiotherapy.1–7 An in vivo dosimetry program is highly recommended for quality assurance of machine calibration, planning dosimetry and dose calculation, patient setup, and the influence of beam modifying components. Semiconductor detectors (diodes)8 –10 are well suited to in vivo dosimetry, as they are small, robust, do not require bias voltages (unlike ionization chambers), and provide immediate readout of results (unlike thermoluminescent dosimeters). It is necessary that the full set of conditions determining diode response (field size, source-to-diode distance, surface temperature, dose rate, beam obliquity) be known and appropriate signal correction factors10,11 applied correctly. For an in vivo dosimetry program using diodes to be effective, it is essential that reproducible methods are established for the application of the detection system, especially for the placement of diode detectors on the
BREAST IRRADIATION AND IN VIVO DOSIMETRY A pilot study of in vivo dosimetry for verification of entrance dose was undertaken within our department to establish a baseline for suitable dose accuracy limits. The breast irradiation technique requires the patient in a supine position on a MammoRx breast board (DIACOR, Salt Lake City, UT) with 1 arm by the patient’s side, and the other placed into the arm and wrist support. The 6-MV photon fields are used with tangential fields applied with a wedge to compensate for patient contour.14 To create a coplanar match plane at the superior edge of the field, (for matching to supraclavicular fields at the time of treatment or for future treatment) the couch is
Reprint requests to: Clare E. Herbert, Department of Radiation Oncology, Sir Charles Gairdner Hospital, Hospital Avenue, Nedlands, Western Australia, Australia 6009. E-mail: Clare.Herbert@health. wa.gov.au 45
46
Medical Dosimetry
angled 5° and the collimator is adjusted to create even medial edges at the upper and lower levels of the treatment field. Lead shielding is used on the upper level border to shield the match line superiorly. The collimator and gantry angles of the lateral tangent are opposed and adjusted to provide the required coplanar match of the deep borders within the breast. Scanditronix EDP-10 diodes (Scanditronix Medical, Uppsala, Sweden) were used for in vivo measurements of entrance dose on the central axis of all fields. These diodes have a width (including diode and base plate) of approximately 1.2 cm. Entrance dose values were calculated by determining the peak dose along the central axis of each defined field using point-dose estimates from the planning system (FOCUS, CMS St. Louis, MO) (Entrance dose is here defined as the dose at a distance dmax (1.5 cm for the 6-MV beam) below the patient’s surface along the line of the beam.). For comparison with possible off-axis positioning of diodes, entrance dose values were also calculated at off-axis locations as described below. Our pilot study gave mean absolute discrepancy between diode-measured and calculated central-axis entrance dose of 4.3% (standard deviation [SD] 4.0%), with a maximum discrepancy of 13.4% over 184 measurements for tangential 6-MV breast fields on 46 patients. Prospective analysis of these treatments showed that all fields included a wedge and required the diode to be positioned on a surface that was significantly oblique to the beam. These factors suggested that diode positioning would have been important in determining recorded entrance dose. As a result of these findings, a subset of breast patients was chosen to aid in evaluation of the influence of diode placement on the resulting signal reading. It was anticipated that diode positioning errors would lead to changes in measured dose due to changes in SSD (source-to-surface distance) due to patient contour, changes in wedge transmission, and variation in response of the diodes due to changes in SSD and angle of beam incidence. METHODS Plans considered and evaluation of entrance doses A total of 20 isocentric breast treatments were considered. All treatment fields included lateral tangential fields using 6-MV photons and standard wedges (15°, 30°, 45°, 60°) for contour compensation. The field arrangement for a diode is shown in Fig. 1. The influence of diode misplacement was only considered in the sagittal plane shown on the plan. The intended location for the diode was at central axis. Points from ⫺2.0 cm to 2.0 cm from this position (following the patient outline) were considered, with entrance dose values determined at 0.5-cm intervals. Negative values are posterior, positive values are anterior. The contribution due to wedge transmission was determined from the wedge off-axis
Volume 28, Number 1, 2003
Fig. 1. Treatment plan showing definition of incidence angle, and typical locations of points (indicated with asterisks) where entrance dose was determined.
factor at each point. The effect of misalignment on diode signal was determined by evaluating the changes in the correction factors CSSD and CAng, for changes in SSD and beam obliquity (angle), respectively, at the various off-axis positions. Monte Carlo simulation of patient measurements The 20 plans were considered to form a random sample of breast patients treated with tangential fields. A Monte Carlo method of statistical analysis was developed and used to sample the results from the 20 plans to gauge the magnitude of dose-reading error with misplacement of the diodes. This would allow the SD in diode placement error to be estimated. Assuming that the diodes are placed with a mean position equal to the desired measurement point (i.e., assuming no systematic error in diode positioning), the expected results of dose measurements were calculated for increasing SDs in diode placement error. At each SD value, random diode positions were sampled and the data for the 20 plans used to determine the average value of the discrepancy between calculated and measured dose. Following this procedure, the expected diode positioning error in our measurement set could be estimated. This assumes that all other factors, including patient positioning, calculated entrance doses, etc. are exact, which is certainly not the case. In this way, the estimated SD in positioning error represents an upper limit. RESULTS Diode accuracy Figure 2 shows a frequency histogram of the percentage difference between the prescribed dose and the simulated measured dose when applying correction factors CSSD and CAng as the diode is moved off-axis. As can be seen, the difference is less than 0.5%. Mean surface angle with wedge In general, greater surface obliquity requires greater wedge compensation. Alteration in relative dose with
In vivo dosimetry of the breast ● C. E. HERBERT et al.
Fig. 2. Comparison of actual off-axis entrance dose with expected measured off-axis entrance dose. The difference in these two is a result of the change in diode calibration factors (CSSD and CAng) as the diode is moved off axis. The histogram shows that the change in correction factor will be less than 0.5% in all cases such that errors in diode placement will not adversely affect correction factors.
each of the 4 standard wedge compensators is shown in Fig. 3. It was found that larger wedge angles demonstrated larger discrepancies in relative dose, with movement off-axis resulting in relative dose discrepancies of approximately 8% for 15° wedge and 25% for 60° wedge (⫺2.0 cm to 2.0 cm off-axis). The effect of SSD and off-axis wedge contribution to the relative dose is shown in Fig. 4. The wedge contribution and SSD contribution curves are shown to be in the same direction, with off-axis wedge corrections contributing to a larger proportion of the relative dose than the SSD. Amount of measured error related to off-axis placement of the diode Figure 5 shows a graph simulating the effect of misplacing the diode at different intervals, with data generated using the Monte Carlo sampling method described above. Estimations of percentage dose discrepancies are plotted against the SD of the diode positioning
Fig. 3. Variations in relative entrance dose (relative to central axis entrance dose) with variation in off-axis position for the 20 plans considered. Symbols used in drawing curves have been selected to represent groups of plans which used the same wedge angle.
47
Fig. 4. SSD and off-axis wedge contribution to the variation in relative entrance dose.
error, where the mean is assumed to be the desired measurement point (central axis). DISCUSSION It is known that each step in the planning/treatment process can contribute to the total uncertainty in absorbed dose delivered to the patient, thus we place importance on finding errors due to planning calculations, algorithms, and treatment factors. Error in diode misplacement is a factor that has not as yet been adequately considered. Here, we have found factors that increase the error in the diode readings for an everyday technique. We set out to quantify the sensitivity of the recorded entrance dose to the diode placement. Variation in correction factors Figure 2 shows that, in the treatment situation, when a diode is placed incorrectly, though within 2 cm of the desired position, the accumulated correction factors will vary by less than 0.5% from those actually required to correct diode signal at central axis. Thus, results obtained with diode measurements where positioning error is possible may be directly related to actual entrance doses (to within 0.5%).
Fig. 5. Plot of percentage dose discrepancies against the standard deviation of error in positioning the diode (mean assumed to be central axis). Curves calculated using Monte Carlo sampling of data extracted from the 20 sample plans.
48
Medical Dosimetry
Effect of wedge angle Patient breast contour is directly related to wedge angle—an increase in mean surface obliquity resulted in a larger wedge angle required for tissue compensation. This leads to a compounding effect in error when performing diode measurements on oblique surfaces irradiated with wedged fields. As Fig. 3 shows (and which is quite obvious), changes in entrance dose with off-axis position increase with increasing wedge angle. Percentage relative dose increases with increased wedge angle and increasing distances off-axis posteriorly. This was as expected, as the positive movement off-axis is toward the “thick” edge of the wedge, with the larger wedges absorbing more dose. Figure 4 shows that the effect of change in wedge transmission and change in SSD complement one another and lead to even larger variations in entrance dose with error in diode placement. As shown in Fig. 4, when the diode is placed off-axis on a wedged field with changes in obliquity, percentage differences in relative dose can vary up to 7% over a 2-cm range, constituting a substantial amount of error before treatment and planning factors are taken into account. Variation in collimator angle at treatment should not influence central-axis entrance dose measurements for symmetric fields, as rotation of the wedge should not lead to central-axis dose variations. This would change if the diode is placed slightly off-axis. However, the contribution from this effect would be small. Even for a 10° rotation of the collimator with the diode placed 1.0 cm off central-axis in a 60° wedged field, the variation in entrance dose will be less than 0.1%. The ability to estimate the amount of error in the diode reading due to diode placement may be a useful tool when assessing overall accuracy of the data collection. The percentage error found can be used to estimate the SD of the distance the diode was placed off-axis using the results of the Monte Carlo study undertaken (see Fig. 5). Comparing the results of our pilot measurements on breast tangents in terms of differences between measured and expected entrance doses (mean absolute discrepancy 4.3%, SD 4.0%) to the results of Fig. 5 suggest an SD in diode placement error of the order of 1.8 cm. As there is no certainty regarding the patient setup, patient immobilization, or dosimetric accuracy during the pilot study, this value represents an upper limit to diode placement error. Given a diode width of approximately 1.2 cm, a placement error of 1.8 cm would not appear unreasonable when locating a diode on an uneven surface relative to a light field. However, more
Volume 28, Number 1, 2003
careful placement should be able to reduce this considerably. Diodes have been proven to be a satisfactory method of quality assurance in detecting the received entrance dose; however, highlighted is the importance of accurate diode placement on lateral tangential fields during isocentric breast irradiation. Due to the changing contour of the breast throughout the treatment volume (and thus the SSD), and the effect of the use of wedge compensators on the off-axis contribution, a substantial error is possible prior to other factors being considered.
Acknowledgments—The authors thank Hwee Carter for participation in the measurements, and the radiation therapists from the Department of Radiation Oncology at the Sir Charles Gairdner Hospital.
REFERENCES 1. Essers, M.; Mijnheer, B.J. In vivo dosimetry during external photon beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 43:245– 59; 1999. 2. Leunens, G.; van Dam, J.; Dutreix, A.; van der Schueren, E. Quality assurance in radiotherapy by in vivo dosimetry. 1. Entrance dose measurements, a reliable procedure. Radiother. Oncol. 17:141–51; 1990. 3. Lee, P.; Sawicka, J.; Glasgow, G. Patient dosimetry quality assurance program with a commercial diode system. Int. J. Radiat. Oncol. Biol. Phys. 29:1175–82; 1994. 4. Li, C.; Lamel, L.; Tom, D. A patient dose verification program using diode detectors. Med. Dosim. 20:209 –14; 1995. 5. Essers, M.; Lanson, J.H.; Mijnheer, B.J. In vivo dosimetry during conformal therapy of prostatic cancer. Radiother. Oncol. 29:271–9; 1993. 6. van Dam, J.; Marinello, G. Methods for in vivo dosimetry in external radiotherapy. Physics for clinical radiotherapy. ESTRO Booklet No. 1. Belgium: Garant; 1994. 7. Heukelom, S.; Lanson, J.H.; Mijnheer, B.J. Comparison of entrance and exit dose measurements using ionization chambers and silicon diodes. Phys. Med. Biol. 36:47–59; 1991. 8. Nilsson, B.; Rude`n, B-I.; Sorcini, B. Characteristics of silicon diodes as patient dosimeters in external radiation therapy. Radiother. Oncol. 11:279 –88; 1988. 9. Rikner, G.; Grusell, E. General specification for silicon semiconductors for use in radiation dosimetry. Phys. Med. Biol. 32:1109 – 17; 1987. 10. Rikner, G.; Grusell, E. Patient dose measurements in photon fields by means of silicon semiconductor detectors. Med. Phys. 14: 870 –3; 1987. 11. Ding, W.; Patterson, W.; Tremethick, L.; Joseph, D. Calibration of entrance dose measurements for an in vivo dosimetry programme. Austr. Radiol. 39:369 –74; 1995. 12. Cozzi, L.; Fogliata-Cozzi, A. Quality assurance in radiation oncology. A study of feasability and impact on dose levels of an in vivo dosimetry program during breast cancer irradiation. Radiother. Oncol. 47:29 –36; 1998. 13. Fiorino, C.; Corletto, D.; Mangili, P.; et al. Quality assurance by systematic in vivo dosimetry: Results on a large cohort of patients. Radiother. Oncol. 56:85–95; 2000. 14. Kahn, F. The Physics of Radiation Therapy, 2nd ed. Baltimore: Williams & Wilkins; 1994.