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OC-0360: Dose warping uncertainties for the cumulative rectal wall dose from brachytherapy in cervical cancer
S192 ESTRO 36 _______________________________________________________________________________________________ Conclusion Overall, good agreement is found between ACE and MC dose calculations in front of the eye plaques in water. The consistent difference of ~3-4% observed for all comparisons with MC simulations is potentially due to differences in the MC simulation codes used to generate the data, and scaling of the ACE dose distribution in water to match TG-43 data in OcB. Updated seed models will be used to investigate this discrepancy. The good level of agreement indicates that further investigation of ACE in applications involving a virtual, voxelized eye phantom, and patient CT datasets, is warranted. OC-0359 Microdosimetric evaluation of intermediateenergy brachytherapy sources using Geant4-DNA G. Famulari1, P. Pater1, S.A. Enger1,2 1 McGill University, Medical Physics Unit, Montreal, Canada 2 McGill University Health Centre, Department of Radiation Oncology, Montreal, Canada Purpose or Objective Recent interest in alternative radionuclides for use in high dose rate brachytherapy (Se-75, Yb-169, Gd-153) with average energies lower than Ir-192 has triggered the investigation of the microdosimetric properties of these radionuclides. A combination of Monte Carlo Track Structure (MCTS) simulations and track sampling algorithms was used to predict the clinical relative biological effectiveness (RBE) for fractionated radiotherapy at relevant doses and dose rates. Previous studies have concluded that the dose mean lineal energy in nanometre-sized volumes is approximately proportional to the α-ratio derived from the linear-quadratic (LQ) relation in fractionated radiotherapy in both low-LET and high-LET radiation. Material and Methods Photon sources were modelled as point sources located in the centre of a spherical water phantom with a radius of 40 cm using the Geant4 toolkit. The kinetic energy of all primary, scattered and fluorescence photons interacting in a scoring volume were tallied at various depths from the point source. Electron tracks were generated by sampling the photon interaction spectrum, and tracking all the interactions following the initial Compton or photoelectric interaction using the event-by-event capabilities of Geant4-DNA. The lineal energy spectra were obtained through random sampling of interaction points and overlaying scoring volumes within the associated volume of the tracks. Results For low-LET radiation, the dose mean lineal energy ratio was approximately equal to the α-ratio in the LQ relation for a volume of about 30 nm (Fig 1). The weighting factors (often denoted clinical RBE) predicted were 1.05, 1.10, 1.14, 1.19 and 1.18 for Ir-192, Se-75, Yb-169, Gd-153, and I-125, respectively (Fig 2). The radionuclides Se-75, Yb169, and Gd-153 are 5-14 % more biologically effective than current Ir-192 sources. There is little variation in the radiation quality with depth from the source.
Fig 1: Dose mean lineal energy ratios between Co-60 and 100 kVp Fig 2: Dose mean lineal energy ratios as a function of scoring diameter X-rays as a function of scoring diameter. The dotted line corresponds for various brachytherapy sources. to α-ratio of 1.20. Conclusion Currently, the International Commission on Radiation Protection (ICRP) assigns a radiation weighting factor of unity for all photon emitting sources, equating the RBE of high and low energy photon sources. However, the clinical RBE for lower energy brachytherapy sources are considerably above unity and should be taken into account during the treatment planning process, to ensure that the equivalent dose delivered to the tumour is similar for different sources. OC-0360 Dose warping uncertainties for the cumulative rectal wall dose from brachytherapy in cervical cancer L.E. Van Heerden1, N. Van Wieringen1, C. Koedooder1, C.R.N. Rasch1, B.R. Pieters1, A. Bel1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Brachytherapy (BT) is part of radiotherapy for women with locally advanced cervical cancer; nowadays, BT is commonly given in multiple applications to the tumour area. In clinical practice, the 2 cm3 receiving the highest dose (D2cm3) in the rectum is calculated by assuming that the high dose volumes overlap for each treatment. To account for rectal deformation due to differences in filling and/or the presence of air, many authors state it is preferable to sum the 3D dose distributions using dose warping after deformable image registration (DIR). However, little is known about the reliability of DIR for dose warping. The purpose of this study is to quantify the dose warping uncertainty in the rectum using a physically realistic model, which describes rectal deformation.
S193 ESTRO 36 _______________________________________________________________________________________________ Material and Methods Seven patients were studied, treated with MRI-guided PDR BT (two times 24 x 0.75 Gy, given in two applications BT1 and BT2). DIR was performed using the Feature-Based Deformable Registration (FBDR) tool, connected to a research version of Oncentra®Brachy (Elekta Brachytherapy, Veenendaal, the Netherlands). The delineated rectums were converted to 3D surface meshes, and a mapping was established to propagate elements on the surface of rectumBT1 to the surface of rectumBT2. The transformation vectors were used to deform the BT1 dose distribution. Next, the BT1 and BT2 doses were summed voxel-by-voxel. To investigate the dose warping uncertainty a physically realistic model (PRM) describing rectal deformation was used. In this model the central axes of rectumBT1 and rectumBT2 were constructed. The axes were assumed to be fixed in length. For both rectumBT1 and rectumBT2, orthogonal planes were reconstructed at 5 evenly spaced positions on the axis (Fig. A). 100 points were evenly distributed over the intersection curve of each plane with the rectal wall. It is assumed that the most dorsal point of the rectum is fixed and also that the rectal wall only stretches perpendicularly to the central axis. For point pairs on rectumBT1 and rectumBT2 that were at the same location according to the PRM, the dose for BT1 and BT2 was added (DPRM) and compared as a 'ground truth” to the DIR accumulated dose (DDIR) in the BT2 point. For BT, the high dose regions in the OAR are most relevant and points within the 2 cm3 volume receiving the highest dose should be correctly identified. We therefore evaluated the percentage of points where DPRM and DDIR were both >D2cm3.
Results Over all patients, DDIR varied between 1.1-44.4GyEQD2 and DPRM varied between 1.1-40.1GyEQD2 (α/β=3Gy for late OAR toxicity, T1/2=1.5 hours). For point pairs, the absolute difference between DDIR and DPRM was 0-8.3GyEQD2 (Fig. B). The 2 cm3 volumes receiving the highest dose according to
the two models have an overlap of 66% (Fig. C).
Conclusion With the rectal model it is feasible to quantify dose warping uncertainties, which could be as high as 8.3 GyEQD2. Most points (>66%) in high dose regions were correctly identified as part of D2cm3. OC-0361 Commissioning of applicator-guided SBRT with HDR Brachytherapy for Advanced Cervical Cancer S. Aldelaijan1, S. Wadi-Ramahi1, A. Nobah1, N. Jastaniyah2 1 King Faisal Specialist Hospital and Research Center, Biomedical Physics, RIyadh, Saudi Arabia 2 King Faisal Specialist Hospital and Research Center, Radiation Oncology, RIyadh, Saudi Arabia Purpose or Objective There is emerging evidence that dose escalation to the “GEC ESTRO defined” high-risk clinical target volume leads to improved clinical outcome in patients with cervical cancer. For those with large residual disease or with unfavorable topography of parametrial spread, achieving such high doses is limited by the dose to organs at risk. Options include a parametrial boost by EBRT which lack precision and lead to prolongation of overall treatment time or the addition of interstitial needles which require a specialized brachytherapy (BT) program. The option of combining brachytherapy with SBRT, using the applicator as a guide, is being explored at our institution. The purpose of this work is to show how this idea can be successfully implemented using an EBT3 Gafchromic film-based dosimetry system. The effect of positional inaccuracies on overall dosimetric outcome is studied as well. Material and Methods A cube phantom was constructed to snuggly accommodate an intrauterine tandem (IU), Fig1a. Pieces of EBT3 film were taped on both sides of the IU to capture the dose distribution. The phantom was CT-scanned and the physician contoured a CTV mimicking large residual parametrial disease, Fig1b. The plan was such that the 7Gy isodose adequately covers the near-distance CTV. The BT plan was used as input for the SBRT plan and the 7Gy to 2.0Gy dose gradient were used to create dose shells, each having its own dose objective and constraint. Three VMAT arcs were used to achieve the goal of D98% > 95% to the entire CTV. Later, HDR BT treatment was delivered using microSelectron v2 and the SBRT was delivered using TrueBeam®. Positioning accuracy of the phantom was done using CBCT imaging with the applicator for image registration. Films were scanned with 10000XL EPSON scanner at 127 dpi and dosimetry was done using the green channel and an in-house MATLAB routine. Intentional
Fig 1: Dose mean lineal energy ratios between Co-60 and 100 kVp Fig 2: Dose mean lineal energy ratios as a function of scoring diameter X-rays as a function of scoring diameter. The dotted line corresponds for various brachytherapy sources. to α-ratio of 1.20. Conclusion Currently, the International Commission on Radiation Protection (ICRP) assigns a radiation weighting factor of unity for all photon emitting sources, equating the RBE of high and low energy photon sources. However, the clinical RBE for lower energy brachytherapy sources are considerably above unity and should be taken into account during the treatment planning process, to ensure that the equivalent dose delivered to the tumour is similar for different sources. OC-0360 Dose warping uncertainties for the cumulative rectal wall dose from brachytherapy in cervical cancer L.E. Van Heerden1, N. Van Wieringen1, C. Koedooder1, C.R.N. Rasch1, B.R. Pieters1, A. Bel1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Brachytherapy (BT) is part of radiotherapy for women with locally advanced cervical cancer; nowadays, BT is commonly given in multiple applications to the tumour area. In clinical practice, the 2 cm3 receiving the highest dose (D2cm3) in the rectum is calculated by assuming that the high dose volumes overlap for each treatment. To account for rectal deformation due to differences in filling and/or the presence of air, many authors state it is preferable to sum the 3D dose distributions using dose warping after deformable image registration (DIR). However, little is known about the reliability of DIR for dose warping. The purpose of this study is to quantify the dose warping uncertainty in the rectum using a physically realistic model, which describes rectal deformation.
S193 ESTRO 36 _______________________________________________________________________________________________ Material and Methods Seven patients were studied, treated with MRI-guided PDR BT (two times 24 x 0.75 Gy, given in two applications BT1 and BT2). DIR was performed using the Feature-Based Deformable Registration (FBDR) tool, connected to a research version of Oncentra®Brachy (Elekta Brachytherapy, Veenendaal, the Netherlands). The delineated rectums were converted to 3D surface meshes, and a mapping was established to propagate elements on the surface of rectumBT1 to the surface of rectumBT2. The transformation vectors were used to deform the BT1 dose distribution. Next, the BT1 and BT2 doses were summed voxel-by-voxel. To investigate the dose warping uncertainty a physically realistic model (PRM) describing rectal deformation was used. In this model the central axes of rectumBT1 and rectumBT2 were constructed. The axes were assumed to be fixed in length. For both rectumBT1 and rectumBT2, orthogonal planes were reconstructed at 5 evenly spaced positions on the axis (Fig. A). 100 points were evenly distributed over the intersection curve of each plane with the rectal wall. It is assumed that the most dorsal point of the rectum is fixed and also that the rectal wall only stretches perpendicularly to the central axis. For point pairs on rectumBT1 and rectumBT2 that were at the same location according to the PRM, the dose for BT1 and BT2 was added (DPRM) and compared as a 'ground truth” to the DIR accumulated dose (DDIR) in the BT2 point. For BT, the high dose regions in the OAR are most relevant and points within the 2 cm3 volume receiving the highest dose should be correctly identified. We therefore evaluated the percentage of points where DPRM and DDIR were both >D2cm3.
Results Over all patients, DDIR varied between 1.1-44.4GyEQD2 and DPRM varied between 1.1-40.1GyEQD2 (α/β=3Gy for late OAR toxicity, T1/2=1.5 hours). For point pairs, the absolute difference between DDIR and DPRM was 0-8.3GyEQD2 (Fig. B). The 2 cm3 volumes receiving the highest dose according to
the two models have an overlap of 66% (Fig. C).
Conclusion With the rectal model it is feasible to quantify dose warping uncertainties, which could be as high as 8.3 GyEQD2. Most points (>66%) in high dose regions were correctly identified as part of D2cm3. OC-0361 Commissioning of applicator-guided SBRT with HDR Brachytherapy for Advanced Cervical Cancer S. Aldelaijan1, S. Wadi-Ramahi1, A. Nobah1, N. Jastaniyah2 1 King Faisal Specialist Hospital and Research Center, Biomedical Physics, RIyadh, Saudi Arabia 2 King Faisal Specialist Hospital and Research Center, Radiation Oncology, RIyadh, Saudi Arabia Purpose or Objective There is emerging evidence that dose escalation to the “GEC ESTRO defined” high-risk clinical target volume leads to improved clinical outcome in patients with cervical cancer. For those with large residual disease or with unfavorable topography of parametrial spread, achieving such high doses is limited by the dose to organs at risk. Options include a parametrial boost by EBRT which lack precision and lead to prolongation of overall treatment time or the addition of interstitial needles which require a specialized brachytherapy (BT) program. The option of combining brachytherapy with SBRT, using the applicator as a guide, is being explored at our institution. The purpose of this work is to show how this idea can be successfully implemented using an EBT3 Gafchromic film-based dosimetry system. The effect of positional inaccuracies on overall dosimetric outcome is studied as well. Material and Methods A cube phantom was constructed to snuggly accommodate an intrauterine tandem (IU), Fig1a. Pieces of EBT3 film were taped on both sides of the IU to capture the dose distribution. The phantom was CT-scanned and the physician contoured a CTV mimicking large residual parametrial disease, Fig1b. The plan was such that the 7Gy isodose adequately covers the near-distance CTV. The BT plan was used as input for the SBRT plan and the 7Gy to 2.0Gy dose gradient were used to create dose shells, each having its own dose objective and constraint. Three VMAT arcs were used to achieve the goal of D98% > 95% to the entire CTV. Later, HDR BT treatment was delivered using microSelectron v2 and the SBRT was delivered using TrueBeam®. Positioning accuracy of the phantom was done using CBCT imaging with the applicator for image registration. Films were scanned with 10000XL EPSON scanner at 127 dpi and dosimetry was done using the green channel and an in-house MATLAB routine. Intentional