- Email: [email protected]
Intra-fractional Motions and Potential Margin Reduction in Accelerated Partial Breast Irradiation (APBI)
I. J. Radiation Oncology d Biology d Physics
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Volume 75, Number 3, Supplement, 2009
outside the radiation field. 3. Communication between sub-systems - Accuracy and fidelity of communication between the Calypso and the DMLC tracking sub-systems were tested for 15 continuous hours - an upper limit for clinical use. Results: 1. For geometric studies, average accuracy was: lung - sub-2mm and prostate - sub-1mm. For dosimetric studies, average gamma failure rates were: lung - 11% with tracking and 41% without tracking; and prostate -16% with tracking and 30% without tracking. 2. The system showed robust and reproducible response to anomalous conditions. In each case, a beam-hold was asserted but the MLC continued to track, to ensure prompt resumption of dose delivery. 3. Communication between the sub-systems showed zero data loss/errors over continuous 15-hour operation. Insights from these studies were used to develop commissioning and QA tests that were easy to perform while yielding the requisite system-diagnostics. Tests were classified as: comprehensive (for commissioning/ annual QA), medium-level (monthly QA) and simple (daily QA). Conclusions: By combining non-ionizing-radiation-based internal position monitoring and continuous DMLC-based beam adaptation, the integrated system represents a significant improvement over existing technologies for dose delivery to moving targets. Promising initial results and the developed QA protocols show a clear path toward clinical integration. Author Disclosure: A. Sawant, Research support form Calypso and Varian, C. Other Research Support; B. Cho, None; P. Poulsen, None; D. Ruan, None; J. Newell, Employee of Calypso Medical Technologies, A. Employment; J. Petersen, Employee of Calypso Medical Technologies, A. Employment; P. Keall, Research Support from Calypso and Varian, B. Research Grant.
2880
Assessment of Image Guidance Techniques for Tumor Bed Localization in Accelerated Partial Breast Irradiation
B. W. Schuller1,2, J. Wolfgang2, M. Truong1,2, A. E. Hirsch1,2, A. O. Nawaz3, J. D. Willins1,2, G. T. Y. Chen2, L. A. Kachnic1,2 1
Boston Medical Center, Boston, MA, 2Massachusetts General Hospital, Boston, MA, 3Paoli Memorial Hospital, Paoli, PA
Purpose/Objective(s): In accelerated partial breast irradiation (APBI), accurate localization of the tumor bed is important, given highly conformal margins and large fractional doses. Therefore, the investigation of novel image guidance techniques to improve inter-fractional tumor bed localization is warranted. As such, serial daily imaging with three techniques (kV radiographs, cone beam CT (CBCT) and skin surface topology) is being prospectively analyzed in patients with implanted surgical clips receiving APBI as part of an institutional IRB protocol. Patient setup errors were determined for surgical clip- and soft tissue-based alignment or skin surface fitting to assess the accuracy of tumor bed localization. Initial data based on 5 patients are presented here. Materials/Methods: Patient positioning to the treatment isocenter consists of initial laser alignment to skin tattoos, followed by orthogonal kV radiographs to visualize bony anatomy and surgical clips, and then by CBCT (once daily) to visualize the 3-dimensional positions of the surgical clips, soft tissue and skin surface. Setup error is defined as the residual error following tumor bed localization. The possibility of using the skin surface as a surrogate for the lumpectomy cavity was investigated post-treatment by first co-registering the CBCT datasets to the surgical clips. The breast surfaces were then matched, and the 3-dimensional clip misalignment was recorded. The setup error determined by skin surface alignment is defined as the magnitude of the subsurface clip misalignment. Results: The average laser setup errors (3-dimensional vector magnitude), as determined by orthogonal kV imaging, for patients 1, 2, 3, 4 and 5 were 3.7 mm, 6.1 mm, 6.8 mm, 9.4 mm and 5.2 mm, respectively. The average kV imaging setup errors, as determined by CBCT following kV imaging shifts, for patients 1, 2, 3, 4 and 5 were 4.9 mm, 1.6 mm, 6.7 mm, 2.3 mm and 4.4 mm, respectively. The average surface fitting setup errors for patients 1 and 2 were 4.9 mm and 1.6 mm, respectively. Conclusions: Image guidance with CBCT may improve tumor bed localization in APBI as several large residual errors following kV imaging were detected. Factors contributing to this (breast size or patient motion) are under current investigation. Skin surface alignment may also be suitable for target localization and is subject to ongoing analysis. Supported by a grant from the Department of Defense to LAK. Author Disclosure: B.W. Schuller, None; J. Wolfgang, None; M. Truong, None; A.E. Hirsch, None; A.O. Nawaz, None; J.D. Willins, None; G.T.Y. Chen, None; L.A. Kachnic, None.
2881
Intra-fractional Motions and Potential Margin Reduction in Accelerated Partial Breast Irradiation (APBI)
N. J. Yue, S. Goyal, T. Kearney, L. Kirstein, V. Narra, A. J. Khan, J. Zhou, B. G. Haffty The Cancer Institute of New Jersey, UMDNJ-Robert Wood Johnson Medical School, New Brunswick, NJ Purpose/Objective(s): APBI distinguishes itself from the whole breast treatment in terms of conformal dose delivery to target volume (TV) that is highly related to the size of the surgical cavity in a hypofractionated setting. It demands not only accurate initial patient treatment positioning but also minimal subsequent intra-fractional TV motion to ensure treatment quality. Although adequate margin may account for this motion, knowledge of the motion magnitude is critical to the proper selection of the margin size. This study was designed to investigate the magnitude of the intra-fractional motion and dose coverage in APBI using sutured fiducial markers. Materials/Methods: An IRB-approved prospective study of external beam APBI using a fractionation scheme of 333 cGy/fraction over 15 fractions was initiated at our institution. At the time of lumpectomy, 4-6 gold fiducial markers were sutured to walls of the lumpectomy cavity and acted as seroma surrogate. TVs and planning were based on NSABP/RTOG B-39 guidelines. Weekly CBCT and fluoroscopy were conducted with each fluoroscopy covering 2-3 breathing cycles. Daily pre- and post-RT kV imaging was performed. The kV images were matched to DRRs based on bony anatomy and fiducial markers, respectively. The procedure was conducted for both the pre- and post-RT kV images to determine the motion magnitude over treatment. Correction of treatment setup was based on the bony alignment. The CTV were delineated on each CBCT and the dose distribution was computed from the CBCT based on the bony and marker matching, respectively. The TV motions were computationally simulated to evaluate margin size.
Proceedings of the 51st Annual ASTRO Meeting Results: Although lung exhibited significant motion, the markers showed limited motion on fluoroscopy. The average marker motion on the fluoroscopy was 0.4 mm (range 0-2), 1.6 mm (0-4), and 1.6 mm (0-3) in LR, AP, and SI, respectively. Over a typical 6-minute treatment time, in LR, AP and SI directions, the average motion (. 120 fractions) was 1.3 mm (0-17), 1.1 mm (0-6) and 1 mm (0-11) based on bony matching, and were 3.5 mm (0-25), 2 mm (0-11), and 2 mm (0-18) based on the markers. The CBCT dosimetry analysis showed the CTV was adequately covered with both matching methods, indicating the current margin size is sufficient. On the other hand, the computational motion simulation showed that marker based setup reduced the planning margin. Conclusions: Breathing has limited effect on the TV motion during APBI. Seroma exhibits larger motion than what is otherwise believed based on bony matching. A seroma specific marker based or image registration based approach holds the potential to reduce margin size. As more patients are enrolled into this study, more definitive conclusions can be drawn for the magnitudes of both intra-fractional motion and planning margin reduction. Author Disclosure: N.J. Yue, None; S. Goyal, None; T. Kearney, None; L. Kirstein, None; V. Narra, None; A.J. Khan, None; J. Zhou, None; B.G. Haffty, None.
2882
Quantification of a Baseline Shift of Tumor Motion Envelope for Stereotactic Body Radiation Therapy (SBRT) of Lung
H. Zhao, B. Wang, P. Rassiah-Szegedi, Y. Huang, Y. J. Hitchcock, K. E. Kokeny, B. J. Salter University of Utah, Salt Lake City, UT Purpose/Objective(s): To quantify daily tumor-motion-envelope shifts relative to bony anatomy for SBRT of lung patients. Materials/Methods: Our institution’s current SBRT workflow entails the acquisition of a treatment planning 4D CT data set and a 4D CT data set prior to each of the 3-5 fractions of a lung SBRT treatment. This data is used for treatment planning ITV generation and for image guided alignment to the targeted lesion on each treatment day. Here we analyze the 4D data sets of 20 patients, 30 total lesions, 104 total fractions treated by SBRT for lung in our clinic, and characterize the shift in tumor motion envelope relative to bony anatomy for each treatment fraction. Position of the ITV motion envelope was characterized by measuring the X, Y, Z position of the tumor centroid on the 50% phase of the 4D data set. The 50% phase was chosen because this is known to be a phase where the tumor changes direction at full exhale, thus pausing momentarily and offering an opportunity to acquire imaging data that is relatively un-aliased by motion. On the same CT data set, an easily visualized bony landmark was identified to be used as a reference point and the X, Y, Z coordinates of this bony landmark were collected. The location of the tumor centroid was then calculated relative to the bony landmark reference point. By similarly analyzing 4D control data sets acquired prior to each treatment fraction we were able to determine how the tumor motion-envelope was changing position for each treatment fraction as compared to simulation. Tumor X, Y, Z shifts from simulation location, along with magnitude vectors of these shifts were calculated for each treatment fraction 4D data set analyzed. Results: Of the 104 treatment fractions evaluated 28 (26.9%) were observed to experience ITV motion envelope shifts of . 5 mm in at least one principle direction. 4 patients saw such baseline shifts for all of their treatment fractions, thus indicating that the shift must have occurred between simulation and first treatment fraction. 11 of the 20 patients experienced at least one treatment fraction with an ITV shift of . 5 mm. Interestingly, one patient treated for 3 concurrent lesions saw no shifts . 5 mm for 2 of the lesions but saw shifts larger than 5 mm for all 3 fractions of the 3rd lesion. Conclusions: For the 20 lung SBRT patients studied here, baseline ITV shifts of 5 mm or greater in at least one principle direction were observed for at least one treatment fraction in 11 (55%) of the patients. This suggests that changes in lung filling dynamics which can lead to movement of the tumor motion envelope relative to bony anatomy occur relatively frequently and, thus, indicates that just as bony alignment is insufficient for prostate targeting, bony landmark alignment may also be insufficient for many SBRT of lung patients. Author Disclosure: H. Zhao, None; B. Wang, None; P. Rassiah-Szegedi, None; Y. Huang, None; Y.J. Hitchcock, None; K.E. Kokeny, None; B.J. Salter, None.
2883
A Novel Continuous Dosimeter for In-Vivo Absolute Dose Measurements in High Dose Rate Brachytherapy 1
M. Carrara , G. Gambarini2, I. Gambini3, S. Tomatis1, C. Fallai4, P. Olmi4, G. Zonca1 Medical Physics Unit, National Cancer Institute, Milan, Italy, 2Physics Department, Universita` degli Studi and INFN, Milan, Italy, 3Department of Energy, Polytechnic University, Milan, Italy, 4Radiotherapy Unit, National Cancer Institute, Milan, Italy 1
Purpose/Objective(s): Many steps are involved in a high dose rate (HDR) brachytherapy treatment, starting from applicators positioning to imaging, treatment planning and dose delivery. Consequently, radiation doses delivered to the patient are susceptible to many inaccuracies and may not accurately match the planned doses. Possible inaccuracies and errors might be due to applicators shift in the time lapse between imaging and treatment, artifacts in the CT images, failure in the digital reconstruction of the applicators during planning, limitations of the dose calculations algorithm, wrong calibration of the radiation source or wrong update of the source activity by the treatment console. In-vivo dosimetry is a reliable solution to compare planned and delivered dose distributions. In this study, novel dosimetric ‘‘catheters’’ (DC) have been developed and optimized to perform in-vivo measurements of HDR brachytherapy treatments. Materials/Methods: DC was produced injecting a radiochromic tissue-equivalent Fricke gel in a flexible plastic catheter with inner and outer diameter of 4F and 6F, respectively. The compound was obtained in laboratory combining a ferrous sulphate solution and the metal-ion indicator Xylenol-Orange with an Agarose gel matrix. Dose distribution measurements were performed in different irradiation configurations with ad hoc developed phantoms, both at room and human body temperature. Irradiations were achieved by means of a HDR remote afterloading device provided with an Ir-192 radioactive source as well as with a Co-60 unit. Results: Preliminary dose measurements have shown a lack of energy dependence of DC over much of the important energy range. A saturation effect of the dosimeters was observed at doses and dose rates higher than 2800cGy and 400cGy/min, respectively. To calibrate the dosimeters for absolute dosimetry, a double 400cGy pre-irradiation and analysis of the DC at the Co-60 unit resulted to be the optimal procedure. A correction factor was determined to convert the calibration factor depending from the experimental temperature. Finally, setting the DC evaluation system with the largest sensitivity range (20 to 2000cGy) and a resolution of 1mm3, accuracy of DC resulted to be within 3%.
S577
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Volume 75, Number 3, Supplement, 2009
outside the radiation field. 3. Communication between sub-systems - Accuracy and fidelity of communication between the Calypso and the DMLC tracking sub-systems were tested for 15 continuous hours - an upper limit for clinical use. Results: 1. For geometric studies, average accuracy was: lung - sub-2mm and prostate - sub-1mm. For dosimetric studies, average gamma failure rates were: lung - 11% with tracking and 41% without tracking; and prostate -16% with tracking and 30% without tracking. 2. The system showed robust and reproducible response to anomalous conditions. In each case, a beam-hold was asserted but the MLC continued to track, to ensure prompt resumption of dose delivery. 3. Communication between the sub-systems showed zero data loss/errors over continuous 15-hour operation. Insights from these studies were used to develop commissioning and QA tests that were easy to perform while yielding the requisite system-diagnostics. Tests were classified as: comprehensive (for commissioning/ annual QA), medium-level (monthly QA) and simple (daily QA). Conclusions: By combining non-ionizing-radiation-based internal position monitoring and continuous DMLC-based beam adaptation, the integrated system represents a significant improvement over existing technologies for dose delivery to moving targets. Promising initial results and the developed QA protocols show a clear path toward clinical integration. Author Disclosure: A. Sawant, Research support form Calypso and Varian, C. Other Research Support; B. Cho, None; P. Poulsen, None; D. Ruan, None; J. Newell, Employee of Calypso Medical Technologies, A. Employment; J. Petersen, Employee of Calypso Medical Technologies, A. Employment; P. Keall, Research Support from Calypso and Varian, B. Research Grant.
2880
Assessment of Image Guidance Techniques for Tumor Bed Localization in Accelerated Partial Breast Irradiation
B. W. Schuller1,2, J. Wolfgang2, M. Truong1,2, A. E. Hirsch1,2, A. O. Nawaz3, J. D. Willins1,2, G. T. Y. Chen2, L. A. Kachnic1,2 1
Boston Medical Center, Boston, MA, 2Massachusetts General Hospital, Boston, MA, 3Paoli Memorial Hospital, Paoli, PA
Purpose/Objective(s): In accelerated partial breast irradiation (APBI), accurate localization of the tumor bed is important, given highly conformal margins and large fractional doses. Therefore, the investigation of novel image guidance techniques to improve inter-fractional tumor bed localization is warranted. As such, serial daily imaging with three techniques (kV radiographs, cone beam CT (CBCT) and skin surface topology) is being prospectively analyzed in patients with implanted surgical clips receiving APBI as part of an institutional IRB protocol. Patient setup errors were determined for surgical clip- and soft tissue-based alignment or skin surface fitting to assess the accuracy of tumor bed localization. Initial data based on 5 patients are presented here. Materials/Methods: Patient positioning to the treatment isocenter consists of initial laser alignment to skin tattoos, followed by orthogonal kV radiographs to visualize bony anatomy and surgical clips, and then by CBCT (once daily) to visualize the 3-dimensional positions of the surgical clips, soft tissue and skin surface. Setup error is defined as the residual error following tumor bed localization. The possibility of using the skin surface as a surrogate for the lumpectomy cavity was investigated post-treatment by first co-registering the CBCT datasets to the surgical clips. The breast surfaces were then matched, and the 3-dimensional clip misalignment was recorded. The setup error determined by skin surface alignment is defined as the magnitude of the subsurface clip misalignment. Results: The average laser setup errors (3-dimensional vector magnitude), as determined by orthogonal kV imaging, for patients 1, 2, 3, 4 and 5 were 3.7 mm, 6.1 mm, 6.8 mm, 9.4 mm and 5.2 mm, respectively. The average kV imaging setup errors, as determined by CBCT following kV imaging shifts, for patients 1, 2, 3, 4 and 5 were 4.9 mm, 1.6 mm, 6.7 mm, 2.3 mm and 4.4 mm, respectively. The average surface fitting setup errors for patients 1 and 2 were 4.9 mm and 1.6 mm, respectively. Conclusions: Image guidance with CBCT may improve tumor bed localization in APBI as several large residual errors following kV imaging were detected. Factors contributing to this (breast size or patient motion) are under current investigation. Skin surface alignment may also be suitable for target localization and is subject to ongoing analysis. Supported by a grant from the Department of Defense to LAK. Author Disclosure: B.W. Schuller, None; J. Wolfgang, None; M. Truong, None; A.E. Hirsch, None; A.O. Nawaz, None; J.D. Willins, None; G.T.Y. Chen, None; L.A. Kachnic, None.
2881
Intra-fractional Motions and Potential Margin Reduction in Accelerated Partial Breast Irradiation (APBI)
N. J. Yue, S. Goyal, T. Kearney, L. Kirstein, V. Narra, A. J. Khan, J. Zhou, B. G. Haffty The Cancer Institute of New Jersey, UMDNJ-Robert Wood Johnson Medical School, New Brunswick, NJ Purpose/Objective(s): APBI distinguishes itself from the whole breast treatment in terms of conformal dose delivery to target volume (TV) that is highly related to the size of the surgical cavity in a hypofractionated setting. It demands not only accurate initial patient treatment positioning but also minimal subsequent intra-fractional TV motion to ensure treatment quality. Although adequate margin may account for this motion, knowledge of the motion magnitude is critical to the proper selection of the margin size. This study was designed to investigate the magnitude of the intra-fractional motion and dose coverage in APBI using sutured fiducial markers. Materials/Methods: An IRB-approved prospective study of external beam APBI using a fractionation scheme of 333 cGy/fraction over 15 fractions was initiated at our institution. At the time of lumpectomy, 4-6 gold fiducial markers were sutured to walls of the lumpectomy cavity and acted as seroma surrogate. TVs and planning were based on NSABP/RTOG B-39 guidelines. Weekly CBCT and fluoroscopy were conducted with each fluoroscopy covering 2-3 breathing cycles. Daily pre- and post-RT kV imaging was performed. The kV images were matched to DRRs based on bony anatomy and fiducial markers, respectively. The procedure was conducted for both the pre- and post-RT kV images to determine the motion magnitude over treatment. Correction of treatment setup was based on the bony alignment. The CTV were delineated on each CBCT and the dose distribution was computed from the CBCT based on the bony and marker matching, respectively. The TV motions were computationally simulated to evaluate margin size.
Proceedings of the 51st Annual ASTRO Meeting Results: Although lung exhibited significant motion, the markers showed limited motion on fluoroscopy. The average marker motion on the fluoroscopy was 0.4 mm (range 0-2), 1.6 mm (0-4), and 1.6 mm (0-3) in LR, AP, and SI, respectively. Over a typical 6-minute treatment time, in LR, AP and SI directions, the average motion (. 120 fractions) was 1.3 mm (0-17), 1.1 mm (0-6) and 1 mm (0-11) based on bony matching, and were 3.5 mm (0-25), 2 mm (0-11), and 2 mm (0-18) based on the markers. The CBCT dosimetry analysis showed the CTV was adequately covered with both matching methods, indicating the current margin size is sufficient. On the other hand, the computational motion simulation showed that marker based setup reduced the planning margin. Conclusions: Breathing has limited effect on the TV motion during APBI. Seroma exhibits larger motion than what is otherwise believed based on bony matching. A seroma specific marker based or image registration based approach holds the potential to reduce margin size. As more patients are enrolled into this study, more definitive conclusions can be drawn for the magnitudes of both intra-fractional motion and planning margin reduction. Author Disclosure: N.J. Yue, None; S. Goyal, None; T. Kearney, None; L. Kirstein, None; V. Narra, None; A.J. Khan, None; J. Zhou, None; B.G. Haffty, None.
2882
Quantification of a Baseline Shift of Tumor Motion Envelope for Stereotactic Body Radiation Therapy (SBRT) of Lung
H. Zhao, B. Wang, P. Rassiah-Szegedi, Y. Huang, Y. J. Hitchcock, K. E. Kokeny, B. J. Salter University of Utah, Salt Lake City, UT Purpose/Objective(s): To quantify daily tumor-motion-envelope shifts relative to bony anatomy for SBRT of lung patients. Materials/Methods: Our institution’s current SBRT workflow entails the acquisition of a treatment planning 4D CT data set and a 4D CT data set prior to each of the 3-5 fractions of a lung SBRT treatment. This data is used for treatment planning ITV generation and for image guided alignment to the targeted lesion on each treatment day. Here we analyze the 4D data sets of 20 patients, 30 total lesions, 104 total fractions treated by SBRT for lung in our clinic, and characterize the shift in tumor motion envelope relative to bony anatomy for each treatment fraction. Position of the ITV motion envelope was characterized by measuring the X, Y, Z position of the tumor centroid on the 50% phase of the 4D data set. The 50% phase was chosen because this is known to be a phase where the tumor changes direction at full exhale, thus pausing momentarily and offering an opportunity to acquire imaging data that is relatively un-aliased by motion. On the same CT data set, an easily visualized bony landmark was identified to be used as a reference point and the X, Y, Z coordinates of this bony landmark were collected. The location of the tumor centroid was then calculated relative to the bony landmark reference point. By similarly analyzing 4D control data sets acquired prior to each treatment fraction we were able to determine how the tumor motion-envelope was changing position for each treatment fraction as compared to simulation. Tumor X, Y, Z shifts from simulation location, along with magnitude vectors of these shifts were calculated for each treatment fraction 4D data set analyzed. Results: Of the 104 treatment fractions evaluated 28 (26.9%) were observed to experience ITV motion envelope shifts of . 5 mm in at least one principle direction. 4 patients saw such baseline shifts for all of their treatment fractions, thus indicating that the shift must have occurred between simulation and first treatment fraction. 11 of the 20 patients experienced at least one treatment fraction with an ITV shift of . 5 mm. Interestingly, one patient treated for 3 concurrent lesions saw no shifts . 5 mm for 2 of the lesions but saw shifts larger than 5 mm for all 3 fractions of the 3rd lesion. Conclusions: For the 20 lung SBRT patients studied here, baseline ITV shifts of 5 mm or greater in at least one principle direction were observed for at least one treatment fraction in 11 (55%) of the patients. This suggests that changes in lung filling dynamics which can lead to movement of the tumor motion envelope relative to bony anatomy occur relatively frequently and, thus, indicates that just as bony alignment is insufficient for prostate targeting, bony landmark alignment may also be insufficient for many SBRT of lung patients. Author Disclosure: H. Zhao, None; B. Wang, None; P. Rassiah-Szegedi, None; Y. Huang, None; Y.J. Hitchcock, None; K.E. Kokeny, None; B.J. Salter, None.
2883
A Novel Continuous Dosimeter for In-Vivo Absolute Dose Measurements in High Dose Rate Brachytherapy 1
M. Carrara , G. Gambarini2, I. Gambini3, S. Tomatis1, C. Fallai4, P. Olmi4, G. Zonca1 Medical Physics Unit, National Cancer Institute, Milan, Italy, 2Physics Department, Universita` degli Studi and INFN, Milan, Italy, 3Department of Energy, Polytechnic University, Milan, Italy, 4Radiotherapy Unit, National Cancer Institute, Milan, Italy 1
Purpose/Objective(s): Many steps are involved in a high dose rate (HDR) brachytherapy treatment, starting from applicators positioning to imaging, treatment planning and dose delivery. Consequently, radiation doses delivered to the patient are susceptible to many inaccuracies and may not accurately match the planned doses. Possible inaccuracies and errors might be due to applicators shift in the time lapse between imaging and treatment, artifacts in the CT images, failure in the digital reconstruction of the applicators during planning, limitations of the dose calculations algorithm, wrong calibration of the radiation source or wrong update of the source activity by the treatment console. In-vivo dosimetry is a reliable solution to compare planned and delivered dose distributions. In this study, novel dosimetric ‘‘catheters’’ (DC) have been developed and optimized to perform in-vivo measurements of HDR brachytherapy treatments. Materials/Methods: DC was produced injecting a radiochromic tissue-equivalent Fricke gel in a flexible plastic catheter with inner and outer diameter of 4F and 6F, respectively. The compound was obtained in laboratory combining a ferrous sulphate solution and the metal-ion indicator Xylenol-Orange with an Agarose gel matrix. Dose distribution measurements were performed in different irradiation configurations with ad hoc developed phantoms, both at room and human body temperature. Irradiations were achieved by means of a HDR remote afterloading device provided with an Ir-192 radioactive source as well as with a Co-60 unit. Results: Preliminary dose measurements have shown a lack of energy dependence of DC over much of the important energy range. A saturation effect of the dosimeters was observed at doses and dose rates higher than 2800cGy and 400cGy/min, respectively. To calibrate the dosimeters for absolute dosimetry, a double 400cGy pre-irradiation and analysis of the DC at the Co-60 unit resulted to be the optimal procedure. A correction factor was determined to convert the calibration factor depending from the experimental temperature. Finally, setting the DC evaluation system with the largest sensitivity range (20 to 2000cGy) and a resolution of 1mm3, accuracy of DC resulted to be within 3%.
S577