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Evaluation of the TG-43 formalism for dose calculations around curvilinear brachytherapy sources
Abstracts / Brachytherapy 7 (2008) 91e194 PO93 The dosimetric quality of mesh brachytherapy implant following wedge resection of early lung cancer Christopher J. Houser, M.S., Tarun K. Podder, Ph.D., Maria WernerWasik, M.D., Mitchell Machtay, M.D., Yan Yu, Ph.D. Radiation Oncology, Thomas Jefferson University Hospital, Philadelphia, PA. Purpose: To evaluate the quality of four lung mesh implants in terms of coverage of D90, V100 and placement with respect to the lung. Methods and Materials: Four patients that had wedge resection for lung tumors received an I-125 mesh created by the radiation oncologist which was sutured over the staple line by the thoracic surgeon. All lung meshes had 1-cm spacing between seeds and a range of 1.0 cm e 1.2 cm between strands based on activity of seeds. A total of 40 seeds were implanted in each patient for a prescribed dose of 100 Gy to 5 mm. One patient received 0.575 mCi seeds while the other three patients received 0.5 mCi seeds. To assess coverage, a CT scan was obtained on or near day 30. Using the CT, the staple line was contoured by the radiation oncologist, the seeds were manually located by the physicist and the D90, V100 coverage was recorded. Results: The D90 doses and V100 coverage for the four cases were 85, 97, 102, 169 Gy and 81, 90, 90 and 98%, respectively. Of the four cases, the first three meshes were placed on the lateral and medial of the right lung. The fourth mesh was placed on the apex of the left lung. The left apex placement led to a non uniform target D90 dose of 168 Gy and V100 coverage of 98%. Conclusions: Since single plane dosimetry (no curvature) is required for these implants, knowledge of mesh placement with respect to the lung before the procedure is crucial. Without this insight, one can easily overdose the target and lung. PO94 3D Image-guided brachytherapy Slobodan Devic, Ph.D.1 Te Vuong, M.D.2 Michael Evans, M.Sc.1 Nada Tomic, M.Sc.3 Ervin Podgorsak, Ph.D.1 1Medical Physics, Montreal General Hospital, Montreal, QC, Canada; 2Radiation Oncology, Montreal General Hospital, Montreal, QC, Canada; 3Radiation Oncology, Jewish General Hospital, Montreal, QC, Canada. Purpose: Image guided radiotherapy (IGRT) is on the forefront of current efforts to improve tumor control by increasing dose to target volume and decreasing dose to nearby critical healthy tissues. Main contribution of IGRT is toward minimizing positional uncertainty of the target due to day-to-day organ motion (inter-fractional motion) or during the course of dose delivery (intra-fractional organ motion). Most IGRT methods handle inter-fractional tumor position uncertainties. However, intra-fractional motion is either incorporated into the remaining PTV margin, or subsequently incorporated into forthcoming treatments based on intratreatment acquired cone beam CT (CBCT) images. Brachytherapy does not suffer from the intra-treatment target motion uncertainties because the radiation source moves together with the target during dose delivery. Hence, adding the image guidance to improve the inter-treatment target uncertainties makes the image guided brachytherapy (IGBT) one of the most sophisticated IGRT modalities. Till now, many centers employ treatment planning (2D or 3D) and image guidance on a daily basis using mainly radiographs available within brachytherapy suites making today’s image guided brachytherapy a 2D modality. The incorporation of a cone beam simulator CT (CBSCT) in the brachytherapy suite opens the door to a step forward, towards a 3D IGBT. Methods and Materials: We describe a 3D image guidance method using a CBSCT for fractionated HDR endorectal brachytherapy, based on 3D CT based treatment planning. Results: Before every treatment a CBCT image set is acquired with the applicator and/or catheters in its place and patient in treatment position. Planning CT images and CBCT images are imported into appropriate treatment planning software and co-registration of the two image sets is performed based on previously inserted radio-opaque clips and bony and reliable soft-tissue landmarks. Based on planning CT catheter positions and planning dwell positions, new dwell positions are determined and
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dose recalculated and superimposed on the planning CT slices, together with the planning dose distribution. If the daily dose distribution is not to the satisfaction of radiation oncologist, additional dose distribution may be requested, after which the patient treatment can commence. Conclusions: The full three-dimensional patient anatomy data with respect to brachytherapy catheters prior to treatment provide additional information to further decrease uncertainty in daily target position with respect to delivered dose distribution during HDR brachytherapy treatments. Cone beam simulator CT may provide the ability to move from 2D to 3D image-guided brachytherapy. PO95 A novel calibration procedure of MOSFET dosimeters for use to measure in-vivo doses during HDR 192Ir brachytherapy Albert Sabbas, Ph.D., Fridon Kulidzhanov, Ph.D., Samuel Trichter, M.Sc., Lucy Nedialkova, Ph.D., Priti Patel, M.D., Dattatreyudu Nori, M.D. Stich Radiation Center, New York Presbyterian Hospital, New York, NY. Purpose: HDR brachytherapy as a radiation delivery modality to treat various cancers has received a boost in recent years, in particular in the case of breast (APBI). In this work, we describe a novel procedure to obtain calibration factors for five MOSFET dosimeters to be used to measure doses at various skin sites during MammoSite balloon irradiation of the breast or surface mould treatment of facial skin lesionsÒ. Methods and Materials: The exact positioning of the MOSFET dosimeters relative to the HDR dwell positions is crucial as small variations can result in large dose differences to the dosimeters. Our geometry consisted of taping the five dosimeters around the circumference of a vaginal 4-cm diameter cylinder. A 2-cm thick bolus was wrapped all around the dosimeters to provide full scatter. This large diameter cylinder was chosen to provide the least dose gradient in the radial direction. The treatment length chosen was 10-cm and the spacing of successive dwell positions was 0.5 cm. The MOSFETs were placed opposite to the central dwell position. The brachytherapy calculations were optimized to deliver a uniform dose of 5 Gy to the surface of the cylinder across a 10-cm length. This resulted in a very small dose gradient, less than 1.5%, within 2 cm from the location of the MOSFETS. The calibration factors in mV/cGy were individually calculated for each of the five dosimeters based on their response and the dose delivered as per the brachytherapy plan calculation and then stored for subsequent clinical use. Results: The calibration results were checked first using the same setup as the calibration measurement. All five dosimeters yielded doses well within 1% from the target dose of 500 cGy. Then the dosimeters were used during an actual mammosite case of a left breast of a patient with a pacemaker. There was good agreement, within 5%, between the calculated and measured skin doses to the medial and lateral aspects of the ipsilateral breast, medial aspect of the contralateral breast and the pacemaker. Conclusions: MOSFET dosimetry is a convenient and accurate method to assess in-vivo doses to the skin of patients receiving HDR brachytherapy. In particular, the skin doses to the ipsilateral and contralateral breasts during Mammosite treatments and the doses to the eyes during surface mould treatments of the face are of clinical importance. Unlike TLD’s, MOSFETs provide an immediate dose readout following treatment and their small size is an advantage over diodes for brachytherapy applications. PO96 Evaluation of the TG-43 formalism for dose calculations around curvilinear brachytherapy sources Prakash Aryal, M.S., Shahid B. Awan, M.S., Sharifeh A. Dini, M.S., Ali S. Meigooni, Ph.D., Robert D. Zwicker, Ph.D. Radiation Medicine, University of Kentucky, Lexington, KY. Purpose: Task Group 43 (TG-43) recommendations are designed mainly for linear with no curvatures. However, in a practical clinical procedure, the shape of the source is being dictated by the geometry and type of the tissue that is being implanted. In this project, the impact of utilizing the non-curved dosimetric parameters for dose calculation around a curvilinear brachytherapy source has been evaluated.
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Abstracts / Brachytherapy 7 (2008) 91e194
Methods and Materials: Monte Carlo simulation technique has been utilized for dose calculations around elongated (O1 cm in length) brachytherapy sources. The calculated dose profiles inside the curved segment of the source have been compared to the values outside the segment as well that the data from a source with no curvature. The results of these calculations have been compared with calculated data using the linear segmented source model (LLS) introduced by Awan et al (Med. Phys. 33(11) 2006 4271e4279). These evaluations have been performed as a function of the source length and the degree of the curvature. Results: The results of these investigations indicated that the inner dose profiles at 5 mm distance from a 5 cm long RadioCoilÔ Pd-103 source, with 180 degree curvature (half circle), is more than 20% than the outer dose profiles. The calculated data using the LLS model is based on the TG-43 parameters with a polar coordinate systems and it was with 7% of the Monte Carlo simulated values. Conclusions: Dose calculations for elongated curvilinear sources have been evaluated which was omitted in TG-43 dose calculation algorithms. The results of these investigations indicate that the degree of the curvature should be considered to achieve accurate dose calculations in a clinical practice.
PO97 Development of the first inverse planning software able to incorporate multiple isotopes into a permanent implant brachytherapy plan J. Adam M Cunha, Ph.D., Barby Pickett, Ph.D., I.-Chow Hsu, M.D., Jean Pouliot, Ph.D. Radiation Oncology, University of California (UCSF), San Francisco, CA. Purpose: Recent studies have shown that, with the understanding of and the accounting for the biological equivalent dose delivered for each isotope, the use of two or more isotopes in a single plan may provide therapeutic gains in permanent prostate implant brachytherapy. The purpose of this work is to develop the software capability to allow an inverse planning algorithm to optimize a brachytherapy dose distribution that includes multiple isotopes in a single plan. Methods and Materials: A stand-alone research version of IPSA was modified to sample different isotopes at each source position at specific moments during the iteration process. The user specifies the isotope sources desired using the TG43 formalism. Each dose profile adds another dimension to the search space such that the search parameters now consist of a choice of isotope in addition to the rectangular 3-D grid of possible positions with needle constraints. The algorithm probes this space and evaluates the resulting dose distribution, compares it to the best distribution yet attained, keeps or rejects it, and finally continues to the next iteration. The user also has the ability to restrict the isotope type at any given source position. Results: We have developed an inverse planning tool that allows the user to incorporate multiple isotopes into any brachytherapy plan. Since the search space was expanded it was expected that the optimization time would be longer; however, it is only on the order of 10% longer for the prostate cases we have studied and is still less than one minute. The intent is not to claim biological effectiveness of multiple isotopes, but rather to present an inverse planning algorithm that includes multiple isotopes. Fast optimization using multiple isotopes will allow thorough investigations of new dose distributions not available when using a single isotope and will also allow the identification of the clinical implications of this new technology. Dose distributions using different strategies will be presented: i) Full target volume optimization using multiple isotopes; ii) Single (standard) isotope optimization augmented by multiple isotope optimization within boost areas. Conclusions: The potential gains achievable by using different isotopes has recently been explored in the literature. A literature search reveals no inverse planning algorithms that have the ability to incorporate multiple isotopes into the permanent implant optimization process. We have developed such a planning algorithm as an extension to IPSA. This work is supported by Nucletron B.V.
PO98 Differences in prostatic edema in patients treated with Cesium-131 and Palladium-103 for permanent prostate brachytherapy Jeffrey S. Musmacher, B.S., Richard T. Byrnes, M.D., Maged M. Ghaly, M.D., Kenneth Satchwill, M.S., Lawrence Rumpf. Radiation Oncology, North Shore Medical Accelerator, Smithtown, NY. Purpose: The effects of prostate edema following prostate brachytherapy can significantly affect the results of treatment. With the higher energy and shorter half-life of Cesium-131 (30.4 KeV and 9.7 days) when compared to Palladium-103 (20.8 KeV and 17 days), there is concern about this edema. This study reports on prostate edema following prostate brachytherapy in both Cesium-131 and Palladium-103 implants. Methods and Materials: All patients underwent prostate brachytherapy using a pre-planned Seattle technique. Prostate volume measurements were obtained at intervals to assess prostate edema. Ultrasound volumes were taken 2 weeks prior to implant, immediately prior to needle placement and following needle placement. CAT Scan volumes were obtained at 3 weeks post implant for Cesium-131 and 4 weeks post implant for Palladium-103. All contours were obtained using the Varian Variseed 7.2 TPS, and verified by an independent party. Results: The study group consisted of 32 consecutive patients, of which 16 received Palladium-103, and 16 received Cesium-131. Average difference between ultrasound volumes obtained 2 weeks prior to implant, and immediately prior to needle placement was -0.13% for Cesium-131 and 2.11% for Palladium-103. The average edema measured between preimplant and post implant on Day 1 was 2.38% for Cesium-131 and 1.13% for Palladium-103. The measured differences between pre-implant and post-implant on CAT scan was 25.86% for Cesium-131 (2.01 cc to 10.73 cc, median 7.01 cc) and 26.06% for Palladium-103 (3.62 cc to 10.92 cc, median 6.71 cc). Conclusions: There is significant edema with prostate brachytherapy causing an increase in prostate volume. This edema was not statistically significant on implant day as measured on ultrasound. The increased prostate volumes reported on CAT scan were significant, but not statistically different between Cesium-131 and Palladium-103. As a result of these findings, it is apparent that Cesium-131 does not cause more edema than Palladium-103. Further, post-implant dosimetry performed at 3 weeks for Cesium-131 and 4 weeks for Palladium-103 is a good method for assessing implant quality.
PO99 Permanent lung implants: How many seeds? Gil’ad N. Cohen, M.S., DABMP1 Kenneth E. Rosenzweig, M.D.2 Michael J. Zelefsky, M.D.2 Kaled M. Alektiar, M.D.2 Karen D. Schupak, M.D.2 Yoshiya Yamada, M.D.2 Marco Zaider, Ph.D.1 1Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY; 2Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY. Purpose: Planar brachytherapy implants have been in use for a wide variety of clinical sites ranging in area from small brain implants to large soft tissue sarcoma implants. 125I has long been the isotope of choice for permanent implants (usually used for lung, chest wall, head & neck, and brain), and in the case of temporary implants, for reasons of tissue sparing, 125I (rather than 192Ir) is often used for pediatric cases and instances of implants within or adjacent to a previously treated area. Historically, the nomogram used for 125I permanent planar implants was a variation of the Memorial Planar Implant Guide for temporary 192Ir implants. Introduced by Marchese et al, the dosimetry upon which this nomogram is base predates TG-43 and assumes the inverse square law. As a result, the activity predicted by this nomogram is underestimated for large area implants. Here, we introduce an updated nomogram used at our hospital. Methods and Materials: 63 cases of permanent 125I were analyzed using current dosimetry protocols and calibration standards. The peripheral dose for each case was calculated and used to scale the total air kerma strength for standard dose prescription (150 Gy for permanent implants). A nomogram to relate air kerma strength to implant area was obtained using linear regression analysis.
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dose recalculated and superimposed on the planning CT slices, together with the planning dose distribution. If the daily dose distribution is not to the satisfaction of radiation oncologist, additional dose distribution may be requested, after which the patient treatment can commence. Conclusions: The full three-dimensional patient anatomy data with respect to brachytherapy catheters prior to treatment provide additional information to further decrease uncertainty in daily target position with respect to delivered dose distribution during HDR brachytherapy treatments. Cone beam simulator CT may provide the ability to move from 2D to 3D image-guided brachytherapy. PO95 A novel calibration procedure of MOSFET dosimeters for use to measure in-vivo doses during HDR 192Ir brachytherapy Albert Sabbas, Ph.D., Fridon Kulidzhanov, Ph.D., Samuel Trichter, M.Sc., Lucy Nedialkova, Ph.D., Priti Patel, M.D., Dattatreyudu Nori, M.D. Stich Radiation Center, New York Presbyterian Hospital, New York, NY. Purpose: HDR brachytherapy as a radiation delivery modality to treat various cancers has received a boost in recent years, in particular in the case of breast (APBI). In this work, we describe a novel procedure to obtain calibration factors for five MOSFET dosimeters to be used to measure doses at various skin sites during MammoSite balloon irradiation of the breast or surface mould treatment of facial skin lesionsÒ. Methods and Materials: The exact positioning of the MOSFET dosimeters relative to the HDR dwell positions is crucial as small variations can result in large dose differences to the dosimeters. Our geometry consisted of taping the five dosimeters around the circumference of a vaginal 4-cm diameter cylinder. A 2-cm thick bolus was wrapped all around the dosimeters to provide full scatter. This large diameter cylinder was chosen to provide the least dose gradient in the radial direction. The treatment length chosen was 10-cm and the spacing of successive dwell positions was 0.5 cm. The MOSFETs were placed opposite to the central dwell position. The brachytherapy calculations were optimized to deliver a uniform dose of 5 Gy to the surface of the cylinder across a 10-cm length. This resulted in a very small dose gradient, less than 1.5%, within 2 cm from the location of the MOSFETS. The calibration factors in mV/cGy were individually calculated for each of the five dosimeters based on their response and the dose delivered as per the brachytherapy plan calculation and then stored for subsequent clinical use. Results: The calibration results were checked first using the same setup as the calibration measurement. All five dosimeters yielded doses well within 1% from the target dose of 500 cGy. Then the dosimeters were used during an actual mammosite case of a left breast of a patient with a pacemaker. There was good agreement, within 5%, between the calculated and measured skin doses to the medial and lateral aspects of the ipsilateral breast, medial aspect of the contralateral breast and the pacemaker. Conclusions: MOSFET dosimetry is a convenient and accurate method to assess in-vivo doses to the skin of patients receiving HDR brachytherapy. In particular, the skin doses to the ipsilateral and contralateral breasts during Mammosite treatments and the doses to the eyes during surface mould treatments of the face are of clinical importance. Unlike TLD’s, MOSFETs provide an immediate dose readout following treatment and their small size is an advantage over diodes for brachytherapy applications. PO96 Evaluation of the TG-43 formalism for dose calculations around curvilinear brachytherapy sources Prakash Aryal, M.S., Shahid B. Awan, M.S., Sharifeh A. Dini, M.S., Ali S. Meigooni, Ph.D., Robert D. Zwicker, Ph.D. Radiation Medicine, University of Kentucky, Lexington, KY. Purpose: Task Group 43 (TG-43) recommendations are designed mainly for linear with no curvatures. However, in a practical clinical procedure, the shape of the source is being dictated by the geometry and type of the tissue that is being implanted. In this project, the impact of utilizing the non-curved dosimetric parameters for dose calculation around a curvilinear brachytherapy source has been evaluated.
170
Abstracts / Brachytherapy 7 (2008) 91e194
Methods and Materials: Monte Carlo simulation technique has been utilized for dose calculations around elongated (O1 cm in length) brachytherapy sources. The calculated dose profiles inside the curved segment of the source have been compared to the values outside the segment as well that the data from a source with no curvature. The results of these calculations have been compared with calculated data using the linear segmented source model (LLS) introduced by Awan et al (Med. Phys. 33(11) 2006 4271e4279). These evaluations have been performed as a function of the source length and the degree of the curvature. Results: The results of these investigations indicated that the inner dose profiles at 5 mm distance from a 5 cm long RadioCoilÔ Pd-103 source, with 180 degree curvature (half circle), is more than 20% than the outer dose profiles. The calculated data using the LLS model is based on the TG-43 parameters with a polar coordinate systems and it was with 7% of the Monte Carlo simulated values. Conclusions: Dose calculations for elongated curvilinear sources have been evaluated which was omitted in TG-43 dose calculation algorithms. The results of these investigations indicate that the degree of the curvature should be considered to achieve accurate dose calculations in a clinical practice.
PO97 Development of the first inverse planning software able to incorporate multiple isotopes into a permanent implant brachytherapy plan J. Adam M Cunha, Ph.D., Barby Pickett, Ph.D., I.-Chow Hsu, M.D., Jean Pouliot, Ph.D. Radiation Oncology, University of California (UCSF), San Francisco, CA. Purpose: Recent studies have shown that, with the understanding of and the accounting for the biological equivalent dose delivered for each isotope, the use of two or more isotopes in a single plan may provide therapeutic gains in permanent prostate implant brachytherapy. The purpose of this work is to develop the software capability to allow an inverse planning algorithm to optimize a brachytherapy dose distribution that includes multiple isotopes in a single plan. Methods and Materials: A stand-alone research version of IPSA was modified to sample different isotopes at each source position at specific moments during the iteration process. The user specifies the isotope sources desired using the TG43 formalism. Each dose profile adds another dimension to the search space such that the search parameters now consist of a choice of isotope in addition to the rectangular 3-D grid of possible positions with needle constraints. The algorithm probes this space and evaluates the resulting dose distribution, compares it to the best distribution yet attained, keeps or rejects it, and finally continues to the next iteration. The user also has the ability to restrict the isotope type at any given source position. Results: We have developed an inverse planning tool that allows the user to incorporate multiple isotopes into any brachytherapy plan. Since the search space was expanded it was expected that the optimization time would be longer; however, it is only on the order of 10% longer for the prostate cases we have studied and is still less than one minute. The intent is not to claim biological effectiveness of multiple isotopes, but rather to present an inverse planning algorithm that includes multiple isotopes. Fast optimization using multiple isotopes will allow thorough investigations of new dose distributions not available when using a single isotope and will also allow the identification of the clinical implications of this new technology. Dose distributions using different strategies will be presented: i) Full target volume optimization using multiple isotopes; ii) Single (standard) isotope optimization augmented by multiple isotope optimization within boost areas. Conclusions: The potential gains achievable by using different isotopes has recently been explored in the literature. A literature search reveals no inverse planning algorithms that have the ability to incorporate multiple isotopes into the permanent implant optimization process. We have developed such a planning algorithm as an extension to IPSA. This work is supported by Nucletron B.V.
PO98 Differences in prostatic edema in patients treated with Cesium-131 and Palladium-103 for permanent prostate brachytherapy Jeffrey S. Musmacher, B.S., Richard T. Byrnes, M.D., Maged M. Ghaly, M.D., Kenneth Satchwill, M.S., Lawrence Rumpf. Radiation Oncology, North Shore Medical Accelerator, Smithtown, NY. Purpose: The effects of prostate edema following prostate brachytherapy can significantly affect the results of treatment. With the higher energy and shorter half-life of Cesium-131 (30.4 KeV and 9.7 days) when compared to Palladium-103 (20.8 KeV and 17 days), there is concern about this edema. This study reports on prostate edema following prostate brachytherapy in both Cesium-131 and Palladium-103 implants. Methods and Materials: All patients underwent prostate brachytherapy using a pre-planned Seattle technique. Prostate volume measurements were obtained at intervals to assess prostate edema. Ultrasound volumes were taken 2 weeks prior to implant, immediately prior to needle placement and following needle placement. CAT Scan volumes were obtained at 3 weeks post implant for Cesium-131 and 4 weeks post implant for Palladium-103. All contours were obtained using the Varian Variseed 7.2 TPS, and verified by an independent party. Results: The study group consisted of 32 consecutive patients, of which 16 received Palladium-103, and 16 received Cesium-131. Average difference between ultrasound volumes obtained 2 weeks prior to implant, and immediately prior to needle placement was -0.13% for Cesium-131 and 2.11% for Palladium-103. The average edema measured between preimplant and post implant on Day 1 was 2.38% for Cesium-131 and 1.13% for Palladium-103. The measured differences between pre-implant and post-implant on CAT scan was 25.86% for Cesium-131 (2.01 cc to 10.73 cc, median 7.01 cc) and 26.06% for Palladium-103 (3.62 cc to 10.92 cc, median 6.71 cc). Conclusions: There is significant edema with prostate brachytherapy causing an increase in prostate volume. This edema was not statistically significant on implant day as measured on ultrasound. The increased prostate volumes reported on CAT scan were significant, but not statistically different between Cesium-131 and Palladium-103. As a result of these findings, it is apparent that Cesium-131 does not cause more edema than Palladium-103. Further, post-implant dosimetry performed at 3 weeks for Cesium-131 and 4 weeks for Palladium-103 is a good method for assessing implant quality.
PO99 Permanent lung implants: How many seeds? Gil’ad N. Cohen, M.S., DABMP1 Kenneth E. Rosenzweig, M.D.2 Michael J. Zelefsky, M.D.2 Kaled M. Alektiar, M.D.2 Karen D. Schupak, M.D.2 Yoshiya Yamada, M.D.2 Marco Zaider, Ph.D.1 1Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY; 2Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY. Purpose: Planar brachytherapy implants have been in use for a wide variety of clinical sites ranging in area from small brain implants to large soft tissue sarcoma implants. 125I has long been the isotope of choice for permanent implants (usually used for lung, chest wall, head & neck, and brain), and in the case of temporary implants, for reasons of tissue sparing, 125I (rather than 192Ir) is often used for pediatric cases and instances of implants within or adjacent to a previously treated area. Historically, the nomogram used for 125I permanent planar implants was a variation of the Memorial Planar Implant Guide for temporary 192Ir implants. Introduced by Marchese et al, the dosimetry upon which this nomogram is base predates TG-43 and assumes the inverse square law. As a result, the activity predicted by this nomogram is underestimated for large area implants. Here, we introduce an updated nomogram used at our hospital. Methods and Materials: 63 cases of permanent 125I were analyzed using current dosimetry protocols and calibration standards. The peripheral dose for each case was calculated and used to scale the total air kerma strength for standard dose prescription (150 Gy for permanent implants). A nomogram to relate air kerma strength to implant area was obtained using linear regression analysis.