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Title: Evaluation of the accuracy of fetal dose estimates using TG-36 data

Abstract

The American Association of Physicists in Medicine Radiation Therapy Committee Task Group 36 report (TG-36) provides guidelines for managing radiation therapy of pregnant patients. Included in the report are data that can be used to estimate the dose to the fetus. The purpose of this study is to evaluate the accuracy of these fetal dose estimates as compared to clinically measured values. TG-36 calculations were performed and compared with measurements of the fetal dose made in vivo or in appropriately-designed phantoms. Calculation and measurement data was collected for eight pregnant patients who underwent radiation therapy at the MD Anderson Cancer Center as well as for several fetal dose studies in the literature. The maximum measured unshielded fetal dose was 47 cGy, which was 1.5% of the prescription dose. For all cases, TG-36 calculations and measured fetal doses differed by up to a factor of 3--the ratio of the calculated to measured dose ranged from 0.34 to 2.93. On average, TG-36 calculations underestimated the measured dose by 31%. No significant trends in the relationship between the calculated and measured fetal doses were found based on the distance from, or the size of, the treatment field.

Authors:
; ; ;  [1];  [2];  [2]
  1. Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20951140
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 34; Journal Issue: 4; Other Information: DOI: 10.1118/1.2710332; (c) 2007 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; ACCURACY; DOSIMETRY; DRUGS; EVALUATION; FETUSES; GYNECOLOGY; IN VIVO; NEOPLASMS; PATIENTS; PHANTOMS; RADIATION DOSES; RADIATION PROTECTION; RADIOTHERAPY; RECOMMENDATIONS

Citation Formats

Kry, Stephen F., Starkschall, George, Antolak, John A., Salehpour, Mohammad, Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905, and Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030. Evaluation of the accuracy of fetal dose estimates using TG-36 data. United States: N. p., 2007. Web. doi:10.1118/1.2710332.
Kry, Stephen F., Starkschall, George, Antolak, John A., Salehpour, Mohammad, Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905, & Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030. Evaluation of the accuracy of fetal dose estimates using TG-36 data. United States. doi:10.1118/1.2710332.
Kry, Stephen F., Starkschall, George, Antolak, John A., Salehpour, Mohammad, Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905, and Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030. Sun . "Evaluation of the accuracy of fetal dose estimates using TG-36 data". United States. doi:10.1118/1.2710332.
@article{osti_20951140,
title = {Evaluation of the accuracy of fetal dose estimates using TG-36 data},
author = {Kry, Stephen F. and Starkschall, George and Antolak, John A. and Salehpour, Mohammad and Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905 and Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030},
abstractNote = {The American Association of Physicists in Medicine Radiation Therapy Committee Task Group 36 report (TG-36) provides guidelines for managing radiation therapy of pregnant patients. Included in the report are data that can be used to estimate the dose to the fetus. The purpose of this study is to evaluate the accuracy of these fetal dose estimates as compared to clinically measured values. TG-36 calculations were performed and compared with measurements of the fetal dose made in vivo or in appropriately-designed phantoms. Calculation and measurement data was collected for eight pregnant patients who underwent radiation therapy at the MD Anderson Cancer Center as well as for several fetal dose studies in the literature. The maximum measured unshielded fetal dose was 47 cGy, which was 1.5% of the prescription dose. For all cases, TG-36 calculations and measured fetal doses differed by up to a factor of 3--the ratio of the calculated to measured dose ranged from 0.34 to 2.93. On average, TG-36 calculations underestimated the measured dose by 31%. No significant trends in the relationship between the calculated and measured fetal doses were found based on the distance from, or the size of, the treatment field.},
doi = {10.1118/1.2710332},
journal = {Medical Physics},
number = 4,
volume = 34,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}
  • Purpose: The current standard TG-43 dose calculation method for SAVI-based Accelerated Partial Breast Irradiation (APBI) assumes an ideal geometry of infinite homogeneous water. However, in SAVI treatments, the air cavity inside the device and the short source-to-skin distance raise concerns about the dose accuracy of the TG-43 method. This study is to evaluate TG-43 dose calculation accuracy in SAVI treatments using Monte Carlo (MC) simulations. Methods: We recalculated the dose distributions of 15 APBI patients treated with SAVI devices, including five cases with a size of 6–1, five with 8−1 and five with 10−1, using our in-house developed fast MCmore » dose package for HDR brachytherapy (gBMC). A phase-space file was used to model the Ir-192 HDR source. For each case, the patient CT was converted into a voxelized phantom and the dwell positions and times were extracted from treatment plans for MC dose calculations. Clinically relevant dosimetric parameters of the recalculated dose were compared to those computed via the TG-43 approach. Results: A systematic overestimation of doses was found for the 15 cases in TG-43 results, with D90, V150, and V200 for PTV-eval 2.8±1.8%, 2.0±2.2%, and 1.8±3.5% higher than MC results. TG-43 also overestimated the dose to skin with the maximum dose 4.4±8.4% higher on average. The relatively large standard deviation seen in the difference of maximum skin dose is partially ascribed to the statistical uncertainty of MC simulations when computing the maximum dose. It took gBMC ∼1 minute to compute dose for a SAVI plan. Conclusion: The high efficiency of our gBMC package facilitated the studies with a relatively large number of cases. An overestimation of TG-43 doses was found when using this MC package to recompute doses in SAVI cases. Clinical utilization of TG-43 dose calculation method in this scenario should be aware of this fact.« less
  • Purpose: A novel patient-specific intensity modulated radiation therapy (IMRT) QA system, 3DVH software and mapcheck 2, purports to be able to use diode array-measured beam doses and the patient's DICOM RT plan, structure set, and dose files to predict the delivered 3D dose distribution in the patient for comparison to the treatment planning system (TPS) calculated doses. In this study, the composite dose to an ion chamber and film in phantom predicted by the 3DVH and mapcheck 2 system is compared to the actual measured chamber and film doses. If validated in this context, then 3DVH can be used tomore » perform an equivalent dose analysis as that obtained with film dosimetry and ion chamber-based composite IMRT QA. This is important for those losing their ability to perform film dosimetry for true composite IMRT QA and provides a measure of confidence in the accuracy of 3DVH 3D dose calculations which may replace phantom-based IMRT QA. Methods: The dosimetric results from 15 consecutive patient-specific IMRT QA tests performed by composite field irradiation of ion chamber and EDR2 film in a solid water phantom were compared to the predicted doses for those virtual detectors based on the calculated 3D dose by the 3DVH software using mapcheck 2 measured doses of each beam within each plan. For each of the 15 cases, immediately after performing the ion chamber plus film measurements, the mapcheck 2 was used to measure the dose for each beam of the plan. The dose to the volume of the virtual ion chamber and the dose distribution in the plane of the virtual film calculated by the 3DVH software was extracted. The ratio of the measured to 3DVH or eclipse-predicted ion chamber doses was calculated. The same plane in the phantom measured using film and calculated with eclipse was exported from 3DVH and the 2D gamma metric was used to compare the relationship between the film doses and the eclipse or 3DVH predicted planar doses. Also, the 3D gamma value was calculated in the 3DVH software which compares the eclipse dose to the 3DVH predicted dose distribution. For the 2D and 3D gamma metrics, 2% dose and 2 mm distance to agreement (DTA) were used. In addition, a simple dose difference was performed using either a 2% or 3% dose difference tolerance. Results: The mean ratio {+-} standard deviation of the measured vs 3DVH or vs eclipse-predicted dose to the ion chamber was 1.013 {+-} 0.015 and 1.003 {+-} 0.012, respectively. For 3DVH vs eclipse, the mean percentage of pixels failing the 3D gamma metric was 1.2% {+-} 1.4% while the failure rate for the 2D gamma metric was 1.1% {+-} 0.9%. When either 3DVH or eclipse was compared to EDR2 film, the gamma failure rate was 2.3% {+-} 2.0% and 1.6% {+-} 1.7%, respectively. Mean dose difference failures were 9%-27% {+-} 5%-15% for 2 or 3% dose difference tolerances, depending on the combination of systems tested. No statistically significant differences were found for any of the planar dosimetric comparisons. Conclusions: 3DVH + mapcheck 2 predicts the same absolute dose, the percent of pixels failing the gamma metric, and the percent of pixels failing 2% or 3% dose difference tolerance tests as one would have obtained had one made measurements in solid water phantom using an ion chamber and coronal film instead of a diode array. This is also a necessary although not sufficient condition for validation of the accuracy of 3DVH predictions of the 3D dose using beam-by-beam measurements.« less
  • Purpose: An accurate dose estimate is necessary for effective patient management after a fetal exposure. In the case of a high-dose exposure, it is critical to use all resources available in order to make the most accurate assessment of the fetal dose. This work will demonstrate a methodology for accurate fetal dose estimation using tools that have recently become available in many clinics, and show examples of best practices for collecting data and performing the fetal dose calculation. Methods: A fetal dose estimate calculation was performed using modern data collection tools to determine parameters for the calculation. The reference pointmore » air kerma as displayed by the fluoroscopic system was checked for accuracy. A cumulative dose incidence map and DICOM header mining were used to determine the displayed reference point air kerma. Corrections for attenuation caused by the patient table and pad were measured and applied in order to determine the peak skin dose. The position and depth of the fetus was determined by ultrasound imaging and consultation with a radiologist. The data collected was used to determine a normalized uterus dose from Monte Carlo simulation data. Fetal dose values from this process were compared to other accepted calculation methods. Results: An accurate high-dose fetal dose estimate was made. Comparison to accepted legacy methods were were within 35% of estimated values. Conclusion: Modern data collection and reporting methods ease the process for estimation of fetal dose from interventional fluoroscopy exposures. Many aspects of the calculation can now be quantified rather than estimated, which should allow for a more accurate estimation of fetal dose.« less
  • Purpose: AAPM Task Group 204 described size specific dose estimates (SSDE) for body scans. The purpose of this work is to use a similar approach to develop patient-specific, scanner-independent organ dose estimates for head CT exams using an attenuation-based size metric. Methods: For eight patient models from the GSF family of voxelized phantoms, dose to brain and lens of the eye was estimated using Monte Carlo simulations of contiguous axial scans for 64-slice MDCT scanners from four major manufacturers. Organ doses were normalized by scannerspecific 16 cm CTDIvol values and averaged across all scanners to obtain scanner-independent CTDIvol-to-organ-dose conversion coefficientsmore » for each patient model. Head size was measured at the first slice superior to the eyes; patient perimeter and effective diameter (ED) were measured directly from the GSF data. Because the GSF models use organ identification codes instead of Hounsfield units, water equivalent diameter (WED) was estimated indirectly. Using the image data from 42 patients ranging from 2 weeks old to adult, the perimeter, ED and WED size metrics were obtained and correlations between each metric were established. Applying these correlations to the GSF perimeter and ED measurements, WED was calculated for each model. The relationship between the various patient size metrics and CTDIvol-to-organ-dose conversion coefficients was then described. Results: The analysis of patient images demonstrated the correlation between WED and ED across a wide range of patient sizes. When applied to the GSF patient models, an exponential relationship between CTDIvol-to-organ-dose conversion coefficients and the WED size metric was observed with correlation coefficients of 0.93 and 0.77 for the brain and lens of the eye, respectively. Conclusion: Strong correlation exists between CTDIvol normalized brain dose and WED. For the lens of the eye, a lower correlation is observed, primarily due to surface dose variations. Funding Support: Siemens-UCLA Radiology Master Research Agreement; Disclosures - Michael McNitt-Gray: Institutional Research Agreement, Siemens AG; Research Support, Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski.« less
  • Purpose: In order to facilitate a smooth transition for brachytherapy dose calculations from the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) formalism to model-based dose calculation algorithms (MBDCAs), treatment planning systems (TPSs) using a MBDCA require a set of well-defined test case plans characterized by Monte Carlo (MC) methods. This also permits direct dose comparison to TG-43 reference data. Such test case plans should be made available for use in the software commissioning process performed by clinical end users. To this end, a hypothetical, generic high-dose rate (HDR) {sup 192}Ir source and a virtual watermore » phantom were designed, which can be imported into a TPS. Methods: A hypothetical, generic HDR {sup 192}Ir source was designed based on commercially available sources as well as a virtual, cubic water phantom that can be imported into any TPS in DICOM format. The dose distribution of the generic {sup 192}Ir source when placed at the center of the cubic phantom, and away from the center under altered scatter conditions, was evaluated using two commercial MBDCAs [Oncentra{sup ®} Brachy with advanced collapsed-cone engine (ACE) and BrachyVision ACUROS{sup TM}]. Dose comparisons were performed using state-of-the-art MC codes for radiation transport, including ALGEBRA, BrachyDose, GEANT4, MCNP5, MCNP6, and PENELOPE2008. The methodologies adhered to recommendations in the AAPM TG-229 report on high-energy brachytherapy source dosimetry. TG-43 dosimetry parameters, an along-away dose-rate table, and primary and scatter separated (PSS) data were obtained. The virtual water phantom of (201){sup 3} voxels (1 mm sides) was used to evaluate the calculated dose distributions. Two test case plans involving a single position of the generic HDR {sup 192}Ir source in this phantom were prepared: (i) source centered in the phantom and (ii) source displaced 7 cm laterally from the center. Datasets were independently produced by different investigators. MC results were then compared against dose calculated using TG-43 and MBDCA methods. Results: TG-43 and PSS datasets were generated for the generic source, the PSS data for use with the ACE algorithm. The dose-rate constant values obtained from seven MC simulations, performed independently using different codes, were in excellent agreement, yielding an average of 1.1109 ± 0.0004 cGy/(h U) (k = 1, Type A uncertainty). MC calculated dose-rate distributions for the two plans were also found to be in excellent agreement, with differences within type A uncertainties. Differences between commercial MBDCA and MC results were test, position, and calculation parameter dependent. On average, however, these differences were within 1% for ACUROS and 2% for ACE at clinically relevant distances. Conclusions: A hypothetical, generic HDR {sup 192}Ir source was designed and implemented in two commercially available TPSs employing different MBDCAs. Reference dose distributions for this source were benchmarked and used for the evaluation of MBDCA calculations employing a virtual, cubic water phantom in the form of a CT DICOM image series. The implementation of a generic source of identical design in all TPSs using MBDCAs is an important step toward supporting univocal commissioning procedures and direct comparisons between TPSs.« less