- Email: [email protected]
In Vivo Dosimetry in Mupit Applications - A Feasibility Study
Abstracts / Brachytherapy 15 (2016) S21eS204 91 Bq (source cable sample 1) and 5.0 0.3 Bq (source cable sample 2). No contamination was found neither on the brachytherapy suite nor on the personnel involved during the incident. Two CNSC inspectors were in place to take measurements and inquire about the events; all the contaminated material was kept in a constraint area and the brachytherapy suite returned to normal use. The contaminated source was prepared for shipping and returned to the provider 14 days later after the incident. All the contaminated equipment was completely replaced including the afterloader, transfer tubes and all equipment needed for a routinely source exchange. A complete commissioning of the new afterloader and spot checks of all the in house source dwell positions for ring applicators had to be completed. Conclusions: Leak testing of sources before and after a source exchange is a very important tool in verifying the integrity of a source and, in the unlikely event of contamination, in detecting the escape of radioactive material before serious contamination of facilities, personnel and equipment can occur.
PO132 Retrospective Review of Skin HDR Cases Using the Varian Catheter Flap Ileana Iftimia, PhD, Eileen T. Cirino, MS, Per H. Halvorsen, MS. Radiation Oncology, Lahey Clinic, Burlington, MA, USA. Purpose: This study is a retrospective review of skin HDR cases using the Varian catheter flap technique. Our previously published Standard Operating Procedure for the scalp/face was refined to include back/ extremity cases. Materials and Methods: Two simulation sessions are scheduled for each patient. During the first session an immobilization device and/or mask are created. The physician delineates a PTV over the skin (transcribed on the mask, when used). For the upper back cases the mask covers the shoulders. The required shape/ size of the flap are evaluated, and needles are subsequently placed inside the flap channels and labeled. The flap is then sutured over the mask with no air gap over the PTV region. For extremities a mask may not always be suitable. For such cases the required shape/size for the flap are evaluated based on the PTV contoured on the skin. For the second simulation session a wire is placed over the PTV contour on the inside part of the mask (when used) or directly on the skin for the other cases. The CT images are acquired with the mask/flap in place and dummy wires inserted in each channel. A careful evaluation of patient position and immobilization devices is performed prior to scanning. Thinner slices (!3 mm) are used for strongly curved treatment regions. The CT images are exported to the Brachytherapy TPS, where needles are identified and wire contoured. The PTV is defined with a thickness of 3 mm, and limited in the other two dimensions by the wire. Critical structures are contoured as needed. The prescription is 51 Gy in 17 fractions (2-4 times/week) for SCC/BCC patients, and 30 Gy in 15 fractions (5 times/week) for lymphomas. A plan is performed using a volume optimization approach with the following criteria: PTV D90% O 100%; goal PTV V100% O 95%; skin D 0.04cc ! 145%Rx; goal skin D 1cc ! 125%Rx. A 2nd check is performed using independent software. The checklist for verifying the plan includes prescription, needles (number/direction), total/maximum/minimum dwell time, accurate transfer to the HDR unit, and DVH criteria review. A dry test is performed prior to the treatment start to ensure that the needles have no obstruction and that their curvature is not problematic. Standardized treatment checklists are filled out before each treatment. Critical structures are shielded as deemed necessary. OSL dosimeters are used to measure the dose to organs at risk (e.g. eye lens). Results: To date we have treated 12 skin HDR patients following the procedure described above. Two patients were treated for lymphomas. Three cases were extremities, one upper back, and the rest scalp/face. The number of channels used for these patients was in the range 9-20 (average 15). For one case the physician decided to prescribe the dose to 4 mm depth, for all other cases the dose was prescribed to 3 mm depth. For all patients studied here the PTV D90% was O 100%Rx (range 101.5106.8%Rx; mean value 102.9%Rx) and the PTV V100% was in the range
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94.7%-99.9%Rx (mean value 96.9%Rx). For a single case PTV V100 was ! 95%Rx (our goal). The skin D 0.04cc was in the range 124.7-144.0% Rx (mean value 135.0%Rx) and the skin D 1cc was in the range 117.7131.7%Rx (mean value 124.6%Rx). For 5 cases the skin D 1cc was slightly higher than 125%Rx (our goal). The agreement for the calculation point dose was within 1%. The nominal total dwell time range was 124.5 - 693.9 sec (average 448.1 sec). The maximum dwell time per position was kept below 25 sec for the first 3 cases before we decided to lower this threshold to 10 sec to reduce hotspots. Eye shields were used and OSL measurements performed for two patients for whom the PTV was close to the eye. OSL measurements were also performed for the upper back patient to assess neck dose. Conclusions: Our scalp/face standardized procedure was updated for other anatomical locations of skin cancer and planning consistency was improved. The proposed DVH criteria were in general met, except for the skin D 1cc for which the values for 5 patients were slightly higher than our goal of 125%Rx. For future cases if the current DVH criteria for the skin are not met we will evaluate if skin D 0.1cc is ! 135%Rx. When possible, the skin dose should be further reduced while assessing if the PTV coverage is still acceptable. Care must be taken to evaluate the immobilization at the time of simulation and the fit for every fraction. Dose to anatomy in close proximity to the surface application should be evaluated. Distance can be optimized at the time of simulation with creative positioning and immobilization.
PO133 In Vivo Dosimetry in Mupit Applications - A Feasibility Study F.J.W.M. Dankers, MSc1, A. Loopstra, BSc2, L.H.M. Mestrom, BSc2, M.A.D. Haverkort, MD2, E.M. van der Steen - Banasik, MD2, M.P.R. Van Gellekom, PhD2. 1Radboudumc, Nijmegen, Netherlands; 2 Radiotherapiegroep, Arnhem, Netherlands. Purpose: To investigate the feasibility of in vivo dosimetry using microMOSFET dosimeters in patients treated with brachytherapy using the MUPIT applicator. Materials and Methods: In Radiotherapiegroep Arnhem, the MUPIT applicator (Martinez Universal Perineal Interstitial Template) is mainly used in patients with vaginal carcinoma or recurrence of endometrial carcinoma. Patients receive external beam radiotherapy, 23 fractions of 2 Gy to the pelvis with a possible boost of 10 Gy to the tumor or the lymph nodes, followed by brachytherapy, 8 to 9 fractions of 2,5 Gy (Ir-192 afterloader, source strength range 15-37 GBq). Treatment planning is performed on MRI, directly after the insertion of the applicator and flexible needles. Frequently, one or more needles are not loaded during treatment planning because adequate tumor coverage has already been achieved. In this study, we placed a microMOSFET dosimeter (TN502RDM, Best Medical) in an empty needle for independent verification of the treatment delivery. Measurements were performed in 4 patients, with 1 to 3 empty needles, and repeated for 2 to 4 fractions, resulting in 24 in vivo measurements. Phantom measurements were used to determine characteristics of the microMOSFETs and improve measurement accuracy. Results: Phantom measurements showed a linear relationship between dose and microMOSFET threshold voltage increase, and were used to determine the calibration coefficient (mV/cGy). The threshold voltage increase was distance (i.e. energy) dependent and microMOSFET sensitivity to radiation dose decreased by 4% over its lifetime (0-20.000 mV). Also, angle dependencies were measured; e.g. a 10% difference was measured between anterior and posterior irradiation of the microMOSFET. Reproducibility of repeated 50 cGy irradiations was 2% (1 SD). Based on the phantom measurements, the in vivo measurements were corrected for distance and angle dependencies. Differences between planned and measured dose in patients are shown in Figure 1, sorted by measurements. The majority of dose differences are smaller than 5%. Reproducibility between fractions in similar needles is good for 6 out of 8 measurements (2SD 5 5%). Conclusions: In vivo dosimetry using microMOSFETs in the MUPIT applicator is feasible. Re-imaging should be performed after detection of
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Abstracts / Brachytherapy 15 (2016) S21eS204
Figure 1. Differences between planned and measured dose with microMOSFET dosimeters in patients.
large differences (O10%) between planned and measured dose to verify the applicator configuration. Potentially, patients can be identified who may benefit from re-planning to improve overall treatment quality.
PO134 Evaluation of Dose Response in 106Ru Eye Plaque Brachytherapy Using a Novel Software Tool Gerd Heilemann, MSc1, Lukas Fetty, BEng1, Matthias Blaickner, PhD2, Nicole Nesvacil, PhD1, Iosif Birlescu, MSc1, Roman Dunav€olyi, MD3, Dietmar Georg, Prof4. 1Department of Radiation Oncology, Medical University Vienna, Vienna, Austria; 2Health and Environment Department Biomedical Systems, Austrian Institute of Technology GmbH, Vienna, Austria; 3Department of Ophthalmology and Optometry, Medical University Vienna, Vienna, Austria; 4Department of Radiation Oncology/ Christian Doppler Laboratory for Medical Radiation Research, Medical University Vienna, Vienna, Austria. Purpose: A software tool for 106Ru brachytherapy of uveal melanoma was inhouse developed that allows three dimensional dose calculations for the tumor and various ophthalmic organs at risk. This software is based on a flexible eye model and pre-calculated dosimetric lookup tables for different types of BEBIG eye plaques (CCA, CCB and COB), generated from MC simulations using MCNP6. The aim of the study was to evaluate dose-response relationships using this software package and to determine prognostic values for tumor control and morbidity when treating uveal melanomas with 106Ru eye brachytherapy. Materials and Methods: 39 patients undergoing 106Ru eye plaque brachytherapy were included in this retrospective study. Individual recalculation of dose distributions was performed using a re-sizable 3D eye model created with the software package Sidefx Houdini. The eye model consists of a dome-shaped tumor model, where apex height and basal diameter are adjustable parameters, and various critical structures (e.g. lens, ciliary body, optic nerve, macula, retina and sclera). The position and shape of the tumor was modelled using the above degrees of freedom and reported data on the distance between tumor and macula as well as tumor and optical nerve. Additionally, the fundus images were projected onto the 3D eye model in order to account for the individual tumor shape. For each patient the dose distribution was based on MC generated dose libraries of BEBIG eye plaques (CCA, CCB and COB), that were individually positioned. Finally, dose and dose volume parameters as well as dose volume histograms (DVH) were generated for the tumor and the various organs at risk. Tumor coverage was reported in terms of D98; the dose to the retina was reported as Dmean and correlation with retinopathy examined. Tumor control and morbidity correlations were checked with a two-sample t-test. For quality assurance purposed film measurements were performed to provide an estimation of the dosimetric uncertainties and safety margins inherent to 106Ru brachytherapy of uveal melanoma. Results: The software tool enabled to calculate full 3D dose distributions on an adjustable 3D eye model with a dose calculation grid of 200 mm. For the 39 patients analyzed in this study the local tumor control was 95%. A
statistically significant correlation between tumor coverage in terms of D98 (p 5 0.013) was observed. In our patient cohort seven patients developed retinopathy, which was in turn found to be significantly influenced by the mean dose to the retina (p 5 0.021). Safety margins can be evaluated considering the dosimetric implications. Conclusions: A novel software tool for treatment planning in 106Ru brachytherapy of uveal melanoma was developed, that enables retrospective studies for dose response analysis. Furthermore it is flexible enough to support treatment planning in prospective studies, with treatment planning features common in external beam therapy, such as tumor and organ at risk segmentation, three dimensional dose calculation and dose-volume metrics. Future work will try to extend the retrospective analysis on the basis of a larger patient cohort to provide a broader range of prognostic values.
PO135 On the Utility of Pre Treatment kV-CBCT for MRI-Based Planning of Cervical Cancer Patients Oana Craciunescu, PhD, Austin Faught, PhD, Zhang Chang, PhD, Jing Cai, PhD, Beverly A. Steffey, MS, Sheridan Meltsner, PhD, Irina Vergalasova, PhD, Junzo Chino, MD. Radiation Oncology, Duke University Medical Center, Durham, NC, USA. Purpose: MRI-based treatment planning has become the gold standard for HDR treatment of cervical cancer. In most clinical implementations, there is a span of 2-4 hours between applicator insertion/imaging and treatment. Moreover, MRI machines are rarely part of the HDR treatment delivery room, meaning that patients are moved several times before the actual treatment. The purpose of this study is to expand an initial pilot study meant to determine if it is feasible to use updated (adapted) doses to organs at risk (OAR) as determined by a pre-treatment kV-CBCT imaging performed with patient in treatment position. Materials and Methods: Cervical cancer patients undergoing external beam radiotherapy followed by an HDR boost were included in this study. MRI (1.5T GE Excite, GE Healthcare) and kV-CBCT (Acuity, Varian Medical Corporation) imaging were performed after applicator insertion. After image registration, applicator delineation (on kVCBCT, 2.5 mm acquisition, 1 mm reconstruction), contouring of HRCTV and IRCTV and normal tissue (on MRI), a treatment plan was generated that met the constraints imposed by the GEC-ESTRO guidelines. Before treatment, another kV-CBCT (2.5 mm slice thickness) was performed with patient in treatment position (preTX). Normal tissues were recontoured on kV-CBCT. The treatment plan was also reproduced on the preTX CBCT and the dose to 2 cm3 for bladder, rectum, sigmoid, bowel, and vagina were compared between planning and preTX using a two-sided paired t test. P values #0.05 were considered significant. In addition, for each OAR, we calculated the percentage of instances with difference between planning and preTX O10%, 20% respectively, and the percent of cases where these differences were positive, meaning the dose calculated on the preTX scan is larger than initially estimated by the treatment plan. This scenario was considered as it depicts the cases where reported dose would be potentially underestimated. Results: Twenty cervical cancer patients were included for a total of 78 fractions. Table 1 shows the summary statistics for all OARs for the treatment plan (Plan) and the CBCT-based preTX plan. The image quality generated with kV-CBCT is known to suffer from reduced imaging contrast, increased cupping, streaking artifacts, and less accurate HUs. While the last shortcoming is not important in dose calculations based on TG43 formalisms, the reduction in image quality leads to uncertainty in bowel/uterus and bladder/uterus interface, superior-inferior delineation between rectum and sigmoid, and bowel/sigmoid identification, leading to large variations in contours when compared to MRI. Statistically significant differences were found for bladder and rectum for which in almost half the cases the differences in the dose to 2 cm3 of organ were greater than 10%, and in approximately 40% of cases the differences in dose were greater than 20%. The main two reasons identified for the large variations are: 1) differences between kV-CBCT and MR in soft tissue
PO132 Retrospective Review of Skin HDR Cases Using the Varian Catheter Flap Ileana Iftimia, PhD, Eileen T. Cirino, MS, Per H. Halvorsen, MS. Radiation Oncology, Lahey Clinic, Burlington, MA, USA. Purpose: This study is a retrospective review of skin HDR cases using the Varian catheter flap technique. Our previously published Standard Operating Procedure for the scalp/face was refined to include back/ extremity cases. Materials and Methods: Two simulation sessions are scheduled for each patient. During the first session an immobilization device and/or mask are created. The physician delineates a PTV over the skin (transcribed on the mask, when used). For the upper back cases the mask covers the shoulders. The required shape/ size of the flap are evaluated, and needles are subsequently placed inside the flap channels and labeled. The flap is then sutured over the mask with no air gap over the PTV region. For extremities a mask may not always be suitable. For such cases the required shape/size for the flap are evaluated based on the PTV contoured on the skin. For the second simulation session a wire is placed over the PTV contour on the inside part of the mask (when used) or directly on the skin for the other cases. The CT images are acquired with the mask/flap in place and dummy wires inserted in each channel. A careful evaluation of patient position and immobilization devices is performed prior to scanning. Thinner slices (!3 mm) are used for strongly curved treatment regions. The CT images are exported to the Brachytherapy TPS, where needles are identified and wire contoured. The PTV is defined with a thickness of 3 mm, and limited in the other two dimensions by the wire. Critical structures are contoured as needed. The prescription is 51 Gy in 17 fractions (2-4 times/week) for SCC/BCC patients, and 30 Gy in 15 fractions (5 times/week) for lymphomas. A plan is performed using a volume optimization approach with the following criteria: PTV D90% O 100%; goal PTV V100% O 95%; skin D 0.04cc ! 145%Rx; goal skin D 1cc ! 125%Rx. A 2nd check is performed using independent software. The checklist for verifying the plan includes prescription, needles (number/direction), total/maximum/minimum dwell time, accurate transfer to the HDR unit, and DVH criteria review. A dry test is performed prior to the treatment start to ensure that the needles have no obstruction and that their curvature is not problematic. Standardized treatment checklists are filled out before each treatment. Critical structures are shielded as deemed necessary. OSL dosimeters are used to measure the dose to organs at risk (e.g. eye lens). Results: To date we have treated 12 skin HDR patients following the procedure described above. Two patients were treated for lymphomas. Three cases were extremities, one upper back, and the rest scalp/face. The number of channels used for these patients was in the range 9-20 (average 15). For one case the physician decided to prescribe the dose to 4 mm depth, for all other cases the dose was prescribed to 3 mm depth. For all patients studied here the PTV D90% was O 100%Rx (range 101.5106.8%Rx; mean value 102.9%Rx) and the PTV V100% was in the range
S159
94.7%-99.9%Rx (mean value 96.9%Rx). For a single case PTV V100 was ! 95%Rx (our goal). The skin D 0.04cc was in the range 124.7-144.0% Rx (mean value 135.0%Rx) and the skin D 1cc was in the range 117.7131.7%Rx (mean value 124.6%Rx). For 5 cases the skin D 1cc was slightly higher than 125%Rx (our goal). The agreement for the calculation point dose was within 1%. The nominal total dwell time range was 124.5 - 693.9 sec (average 448.1 sec). The maximum dwell time per position was kept below 25 sec for the first 3 cases before we decided to lower this threshold to 10 sec to reduce hotspots. Eye shields were used and OSL measurements performed for two patients for whom the PTV was close to the eye. OSL measurements were also performed for the upper back patient to assess neck dose. Conclusions: Our scalp/face standardized procedure was updated for other anatomical locations of skin cancer and planning consistency was improved. The proposed DVH criteria were in general met, except for the skin D 1cc for which the values for 5 patients were slightly higher than our goal of 125%Rx. For future cases if the current DVH criteria for the skin are not met we will evaluate if skin D 0.1cc is ! 135%Rx. When possible, the skin dose should be further reduced while assessing if the PTV coverage is still acceptable. Care must be taken to evaluate the immobilization at the time of simulation and the fit for every fraction. Dose to anatomy in close proximity to the surface application should be evaluated. Distance can be optimized at the time of simulation with creative positioning and immobilization.
PO133 In Vivo Dosimetry in Mupit Applications - A Feasibility Study F.J.W.M. Dankers, MSc1, A. Loopstra, BSc2, L.H.M. Mestrom, BSc2, M.A.D. Haverkort, MD2, E.M. van der Steen - Banasik, MD2, M.P.R. Van Gellekom, PhD2. 1Radboudumc, Nijmegen, Netherlands; 2 Radiotherapiegroep, Arnhem, Netherlands. Purpose: To investigate the feasibility of in vivo dosimetry using microMOSFET dosimeters in patients treated with brachytherapy using the MUPIT applicator. Materials and Methods: In Radiotherapiegroep Arnhem, the MUPIT applicator (Martinez Universal Perineal Interstitial Template) is mainly used in patients with vaginal carcinoma or recurrence of endometrial carcinoma. Patients receive external beam radiotherapy, 23 fractions of 2 Gy to the pelvis with a possible boost of 10 Gy to the tumor or the lymph nodes, followed by brachytherapy, 8 to 9 fractions of 2,5 Gy (Ir-192 afterloader, source strength range 15-37 GBq). Treatment planning is performed on MRI, directly after the insertion of the applicator and flexible needles. Frequently, one or more needles are not loaded during treatment planning because adequate tumor coverage has already been achieved. In this study, we placed a microMOSFET dosimeter (TN502RDM, Best Medical) in an empty needle for independent verification of the treatment delivery. Measurements were performed in 4 patients, with 1 to 3 empty needles, and repeated for 2 to 4 fractions, resulting in 24 in vivo measurements. Phantom measurements were used to determine characteristics of the microMOSFETs and improve measurement accuracy. Results: Phantom measurements showed a linear relationship between dose and microMOSFET threshold voltage increase, and were used to determine the calibration coefficient (mV/cGy). The threshold voltage increase was distance (i.e. energy) dependent and microMOSFET sensitivity to radiation dose decreased by 4% over its lifetime (0-20.000 mV). Also, angle dependencies were measured; e.g. a 10% difference was measured between anterior and posterior irradiation of the microMOSFET. Reproducibility of repeated 50 cGy irradiations was 2% (1 SD). Based on the phantom measurements, the in vivo measurements were corrected for distance and angle dependencies. Differences between planned and measured dose in patients are shown in Figure 1, sorted by measurements. The majority of dose differences are smaller than 5%. Reproducibility between fractions in similar needles is good for 6 out of 8 measurements (2SD 5 5%). Conclusions: In vivo dosimetry using microMOSFETs in the MUPIT applicator is feasible. Re-imaging should be performed after detection of
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Abstracts / Brachytherapy 15 (2016) S21eS204
Figure 1. Differences between planned and measured dose with microMOSFET dosimeters in patients.
large differences (O10%) between planned and measured dose to verify the applicator configuration. Potentially, patients can be identified who may benefit from re-planning to improve overall treatment quality.
PO134 Evaluation of Dose Response in 106Ru Eye Plaque Brachytherapy Using a Novel Software Tool Gerd Heilemann, MSc1, Lukas Fetty, BEng1, Matthias Blaickner, PhD2, Nicole Nesvacil, PhD1, Iosif Birlescu, MSc1, Roman Dunav€olyi, MD3, Dietmar Georg, Prof4. 1Department of Radiation Oncology, Medical University Vienna, Vienna, Austria; 2Health and Environment Department Biomedical Systems, Austrian Institute of Technology GmbH, Vienna, Austria; 3Department of Ophthalmology and Optometry, Medical University Vienna, Vienna, Austria; 4Department of Radiation Oncology/ Christian Doppler Laboratory for Medical Radiation Research, Medical University Vienna, Vienna, Austria. Purpose: A software tool for 106Ru brachytherapy of uveal melanoma was inhouse developed that allows three dimensional dose calculations for the tumor and various ophthalmic organs at risk. This software is based on a flexible eye model and pre-calculated dosimetric lookup tables for different types of BEBIG eye plaques (CCA, CCB and COB), generated from MC simulations using MCNP6. The aim of the study was to evaluate dose-response relationships using this software package and to determine prognostic values for tumor control and morbidity when treating uveal melanomas with 106Ru eye brachytherapy. Materials and Methods: 39 patients undergoing 106Ru eye plaque brachytherapy were included in this retrospective study. Individual recalculation of dose distributions was performed using a re-sizable 3D eye model created with the software package Sidefx Houdini. The eye model consists of a dome-shaped tumor model, where apex height and basal diameter are adjustable parameters, and various critical structures (e.g. lens, ciliary body, optic nerve, macula, retina and sclera). The position and shape of the tumor was modelled using the above degrees of freedom and reported data on the distance between tumor and macula as well as tumor and optical nerve. Additionally, the fundus images were projected onto the 3D eye model in order to account for the individual tumor shape. For each patient the dose distribution was based on MC generated dose libraries of BEBIG eye plaques (CCA, CCB and COB), that were individually positioned. Finally, dose and dose volume parameters as well as dose volume histograms (DVH) were generated for the tumor and the various organs at risk. Tumor coverage was reported in terms of D98; the dose to the retina was reported as Dmean and correlation with retinopathy examined. Tumor control and morbidity correlations were checked with a two-sample t-test. For quality assurance purposed film measurements were performed to provide an estimation of the dosimetric uncertainties and safety margins inherent to 106Ru brachytherapy of uveal melanoma. Results: The software tool enabled to calculate full 3D dose distributions on an adjustable 3D eye model with a dose calculation grid of 200 mm. For the 39 patients analyzed in this study the local tumor control was 95%. A
statistically significant correlation between tumor coverage in terms of D98 (p 5 0.013) was observed. In our patient cohort seven patients developed retinopathy, which was in turn found to be significantly influenced by the mean dose to the retina (p 5 0.021). Safety margins can be evaluated considering the dosimetric implications. Conclusions: A novel software tool for treatment planning in 106Ru brachytherapy of uveal melanoma was developed, that enables retrospective studies for dose response analysis. Furthermore it is flexible enough to support treatment planning in prospective studies, with treatment planning features common in external beam therapy, such as tumor and organ at risk segmentation, three dimensional dose calculation and dose-volume metrics. Future work will try to extend the retrospective analysis on the basis of a larger patient cohort to provide a broader range of prognostic values.
PO135 On the Utility of Pre Treatment kV-CBCT for MRI-Based Planning of Cervical Cancer Patients Oana Craciunescu, PhD, Austin Faught, PhD, Zhang Chang, PhD, Jing Cai, PhD, Beverly A. Steffey, MS, Sheridan Meltsner, PhD, Irina Vergalasova, PhD, Junzo Chino, MD. Radiation Oncology, Duke University Medical Center, Durham, NC, USA. Purpose: MRI-based treatment planning has become the gold standard for HDR treatment of cervical cancer. In most clinical implementations, there is a span of 2-4 hours between applicator insertion/imaging and treatment. Moreover, MRI machines are rarely part of the HDR treatment delivery room, meaning that patients are moved several times before the actual treatment. The purpose of this study is to expand an initial pilot study meant to determine if it is feasible to use updated (adapted) doses to organs at risk (OAR) as determined by a pre-treatment kV-CBCT imaging performed with patient in treatment position. Materials and Methods: Cervical cancer patients undergoing external beam radiotherapy followed by an HDR boost were included in this study. MRI (1.5T GE Excite, GE Healthcare) and kV-CBCT (Acuity, Varian Medical Corporation) imaging were performed after applicator insertion. After image registration, applicator delineation (on kVCBCT, 2.5 mm acquisition, 1 mm reconstruction), contouring of HRCTV and IRCTV and normal tissue (on MRI), a treatment plan was generated that met the constraints imposed by the GEC-ESTRO guidelines. Before treatment, another kV-CBCT (2.5 mm slice thickness) was performed with patient in treatment position (preTX). Normal tissues were recontoured on kV-CBCT. The treatment plan was also reproduced on the preTX CBCT and the dose to 2 cm3 for bladder, rectum, sigmoid, bowel, and vagina were compared between planning and preTX using a two-sided paired t test. P values #0.05 were considered significant. In addition, for each OAR, we calculated the percentage of instances with difference between planning and preTX O10%, 20% respectively, and the percent of cases where these differences were positive, meaning the dose calculated on the preTX scan is larger than initially estimated by the treatment plan. This scenario was considered as it depicts the cases where reported dose would be potentially underestimated. Results: Twenty cervical cancer patients were included for a total of 78 fractions. Table 1 shows the summary statistics for all OARs for the treatment plan (Plan) and the CBCT-based preTX plan. The image quality generated with kV-CBCT is known to suffer from reduced imaging contrast, increased cupping, streaking artifacts, and less accurate HUs. While the last shortcoming is not important in dose calculations based on TG43 formalisms, the reduction in image quality leads to uncertainty in bowel/uterus and bladder/uterus interface, superior-inferior delineation between rectum and sigmoid, and bowel/sigmoid identification, leading to large variations in contours when compared to MRI. Statistically significant differences were found for bladder and rectum for which in almost half the cases the differences in the dose to 2 cm3 of organ were greater than 10%, and in approximately 40% of cases the differences in dose were greater than 20%. The main two reasons identified for the large variations are: 1) differences between kV-CBCT and MR in soft tissue