- Email: [email protected]
45. Salvage re-irradiation for local failure of prostate cancer after curative radiotherapy: Dosimetric and radiobiological modelling of rectal toxicity
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G. Dipasquale et al. / Physica Medica 44 (2017) 1–27
and 3D (c2D and c3D, criteria: 2%/2 mm, 30% threshold). Then, dose differences were reported in terms of 95% target coverage, parotid glands mean dose (Dmean) and PRV spinal cord maximum dose (D2%). Results. On average, c3D (percentage of points with gamma < 1) was 93.4% with patient HU-D relation, 96.6% with phantom HU-D relation, 97.4% with 3 classes threshold and 97.7% with the pseudo-CT. The difference in parotid glands Dmean could reach 5% while differences for spinal cord D2% and PTV coverage were less than 1.5%, whatever the method. The gamma analysis showed location of discrepancy areas: near body contour, regarding shoulders and in presence of heterogeneities (Fig. 1). These differences can be explained by the poor image quality and the HU inconsistency of CBCT images in some areas. The method with pseudo-CT seems to be the best to pass over these issues because densities come from planning CT. Conclusions. The four evaluated methods show that dose differences can reach 5% for parotid glands and are lower than 2% for spinal cord and target volumes. The 3D gamma analysis results (2%/2 mm) are higher than 95% for three among four methods. However, additional uncertainties have to be taken into consideration in case of CBCT image delineation and/or calculation on truncated images. This preliminary study shows CBCT dose calculation feasibility to quantify impact of patient anatomical variations during treatment and trigger a new plan based on CT if necessary. The future work is real delivered dose monitoring, based on CBCT image, in perspective of dose guided adaptive radiotherapy. https://doi.org/10.1016/j.ejmp.2017.10.068
approach can be easily implemented for some anatomical localizations but identifying correspondences on the surface of smooth organs or in large homogeneous tissues is not always feasible. In that case, DIR validation will have to verify that the estimated DVF respect physical considerations such as a reasonable range of local volume variations measured with the Jacobian determinant of the DVF. DIR algorithms can also be tested with images derived from numerical or physical phantoms which provide a ground truth deformation. Phantoms have demonstrated that traditional DIR algorithms could fail to estimate complex deformations such as a sliding motion between organs and justified the need to develop anatomical site-specific models. Finally, a limited number of studies have addressed DIR of longitudinal images presenting inconsistent content (i.e. in case of tumor growth or erosion) and a special attention should be paid in those situations. In parallel of efforts to develop more accurate DIR methods, it is important today to define a commissioning strategy for available DIR solutions to be used in clinical trials involving dose accumulation. In that sense, a Task Group of the American Association of Medical Physicists recently reported its recommendations. https://doi.org/10.1016/j.ejmp.2017.10.069
45. Salvage re-irradiation for local failure of prostate cancer after curative radiotherapy: Dosimetric and radiobiological modelling of rectal toxicity G. Dipasquale a, T. Zilli a, C. Fiorino b, M. Rouzaud a, R. Miralbell a a
Session 10 – Common Session SFRO/SFPM re-irradiations 44. Deformable image registration for dose accumulation: Principle and evaluation G. Cazoulat The University of Texas MD Anderson Cancer Center, United States Most Treatment Planning Systems for adaptive radiotherapy today propose Deformable Image Registration (DIR) tools which enable contour propagation or dose accumulation between images acquired over the course of treatment. While the results of DIR for contour propagation can be easily assessed and eventually manually corrected, the evaluation of DIR for dose accumulation relies on a knowledge of anatomical point correspondences and is therefore much more challenging. This talk will review the methodological principle of DIR for dose accumulation and current evaluation methods. Accumulating dose in a tissue element contained in a voxel of the reference image requires to determine the location of that element in the considered longitudinal images. The role of DIR is to automatically establish those spatial correspondences usually in the form of displacement vector fields (DVF). The first natural way to evaluate DIR results is to deform the longitudinal image onto the reference image based on the DVF and to measure the resulting similarity between the images. However, as demonstrated in previous studies, popular DIR algorithms for contour propagation can lead to high similarity between the deformed and reference images and critically wrong anatomical point correspondences. For this reason, image similarity measures or visual assessment with image fusion tools should be considered in clinical practice only after the local accuracy (in millimeters) has been quantified for a significant number of cases. Among DIR validation methods for dose accumulation, the most reliable one consists of manually identifying pairs of landmarks in the images and to relate the misalignment of those landmark pairs after DIR to the spatial resolution of the dose distributions. This
Hôpitaux universitaires de Genève (HUG), Rue Gabrielle-Perret-Gentil 4 CH-1211 Genève 14, Switzerland b San Raffaele Scientific Institute (HSR), via Olgettina 60, 20132 Milan, Italy Introduction. To assess the impact of radiation dose and clinical parameters on rectal toxicity following salvage external beam radiation therapy (EBRT) with or without a brachytherapy (BT) boost for exclusive local failure after primary-EBRT for prostate cancer. Methods. Fourteen patients with no residual toxicity after primary EBRT (BT) were re-irradiated after a median time interval of 6.1 years (4.7–10.2). The median NTD2Gy was 74 Gy (66–98.4) at primary-RT and 85.1 Gy (70–93.4) at re-irradiation. Rectal dose volume histograms of both treatments (converted to NTD2Gy) and the corresponding normal-tissue-complication-probability (NTCP) values for gastro-intestinal (GI) toxicity were calculated. Results. The 5-year Grade P3 GI toxicity-free survival rate was 57.1 ± 13.1%. Five patients developed Grade 4 GI toxicity. Rectal V70Gy and the maximum dose to 1 cc of rectum at primary EBRT were both predictive for Grade P3 GI toxicity; 12.2% vs. 3.8%, p = .042 and 72.2 Gy vs. 66.8 Gy, p = .0027, respectively. When adding primary-RT and re-irradiation plans, the median maximum dose to 1 cc of rectum was 139.8 Gy (126.7–147.8) vs. 125.9 Gy (99.1– 133.1) (p = .0063) for Grade P3 and Grade 62 GI toxicity groups. Higher NTCP values at primary-RT were predictive for Grade P3 toxicity (p < .05). Conclusions. A higher rectal NTCP value, even in the absence of high-grade late-toxicity after primary-RT, is correlated with an increased risk of severe rectal side-effects after salvage reirradiation. Rectum doses greater than 70 Gy at primary-RT, and NTCP values of more than 10%, might predict for grade P3 rectal toxicity at re-irradiation, with a possible threshold for total rectum dose of around 130 Gy. https://doi.org/10.1016/j.ejmp.2017.10.070
G. Dipasquale et al. / Physica Medica 44 (2017) 1–27
and 3D (c2D and c3D, criteria: 2%/2 mm, 30% threshold). Then, dose differences were reported in terms of 95% target coverage, parotid glands mean dose (Dmean) and PRV spinal cord maximum dose (D2%). Results. On average, c3D (percentage of points with gamma < 1) was 93.4% with patient HU-D relation, 96.6% with phantom HU-D relation, 97.4% with 3 classes threshold and 97.7% with the pseudo-CT. The difference in parotid glands Dmean could reach 5% while differences for spinal cord D2% and PTV coverage were less than 1.5%, whatever the method. The gamma analysis showed location of discrepancy areas: near body contour, regarding shoulders and in presence of heterogeneities (Fig. 1). These differences can be explained by the poor image quality and the HU inconsistency of CBCT images in some areas. The method with pseudo-CT seems to be the best to pass over these issues because densities come from planning CT. Conclusions. The four evaluated methods show that dose differences can reach 5% for parotid glands and are lower than 2% for spinal cord and target volumes. The 3D gamma analysis results (2%/2 mm) are higher than 95% for three among four methods. However, additional uncertainties have to be taken into consideration in case of CBCT image delineation and/or calculation on truncated images. This preliminary study shows CBCT dose calculation feasibility to quantify impact of patient anatomical variations during treatment and trigger a new plan based on CT if necessary. The future work is real delivered dose monitoring, based on CBCT image, in perspective of dose guided adaptive radiotherapy. https://doi.org/10.1016/j.ejmp.2017.10.068
approach can be easily implemented for some anatomical localizations but identifying correspondences on the surface of smooth organs or in large homogeneous tissues is not always feasible. In that case, DIR validation will have to verify that the estimated DVF respect physical considerations such as a reasonable range of local volume variations measured with the Jacobian determinant of the DVF. DIR algorithms can also be tested with images derived from numerical or physical phantoms which provide a ground truth deformation. Phantoms have demonstrated that traditional DIR algorithms could fail to estimate complex deformations such as a sliding motion between organs and justified the need to develop anatomical site-specific models. Finally, a limited number of studies have addressed DIR of longitudinal images presenting inconsistent content (i.e. in case of tumor growth or erosion) and a special attention should be paid in those situations. In parallel of efforts to develop more accurate DIR methods, it is important today to define a commissioning strategy for available DIR solutions to be used in clinical trials involving dose accumulation. In that sense, a Task Group of the American Association of Medical Physicists recently reported its recommendations. https://doi.org/10.1016/j.ejmp.2017.10.069
45. Salvage re-irradiation for local failure of prostate cancer after curative radiotherapy: Dosimetric and radiobiological modelling of rectal toxicity G. Dipasquale a, T. Zilli a, C. Fiorino b, M. Rouzaud a, R. Miralbell a a
Session 10 – Common Session SFRO/SFPM re-irradiations 44. Deformable image registration for dose accumulation: Principle and evaluation G. Cazoulat The University of Texas MD Anderson Cancer Center, United States Most Treatment Planning Systems for adaptive radiotherapy today propose Deformable Image Registration (DIR) tools which enable contour propagation or dose accumulation between images acquired over the course of treatment. While the results of DIR for contour propagation can be easily assessed and eventually manually corrected, the evaluation of DIR for dose accumulation relies on a knowledge of anatomical point correspondences and is therefore much more challenging. This talk will review the methodological principle of DIR for dose accumulation and current evaluation methods. Accumulating dose in a tissue element contained in a voxel of the reference image requires to determine the location of that element in the considered longitudinal images. The role of DIR is to automatically establish those spatial correspondences usually in the form of displacement vector fields (DVF). The first natural way to evaluate DIR results is to deform the longitudinal image onto the reference image based on the DVF and to measure the resulting similarity between the images. However, as demonstrated in previous studies, popular DIR algorithms for contour propagation can lead to high similarity between the deformed and reference images and critically wrong anatomical point correspondences. For this reason, image similarity measures or visual assessment with image fusion tools should be considered in clinical practice only after the local accuracy (in millimeters) has been quantified for a significant number of cases. Among DIR validation methods for dose accumulation, the most reliable one consists of manually identifying pairs of landmarks in the images and to relate the misalignment of those landmark pairs after DIR to the spatial resolution of the dose distributions. This
Hôpitaux universitaires de Genève (HUG), Rue Gabrielle-Perret-Gentil 4 CH-1211 Genève 14, Switzerland b San Raffaele Scientific Institute (HSR), via Olgettina 60, 20132 Milan, Italy Introduction. To assess the impact of radiation dose and clinical parameters on rectal toxicity following salvage external beam radiation therapy (EBRT) with or without a brachytherapy (BT) boost for exclusive local failure after primary-EBRT for prostate cancer. Methods. Fourteen patients with no residual toxicity after primary EBRT (BT) were re-irradiated after a median time interval of 6.1 years (4.7–10.2). The median NTD2Gy was 74 Gy (66–98.4) at primary-RT and 85.1 Gy (70–93.4) at re-irradiation. Rectal dose volume histograms of both treatments (converted to NTD2Gy) and the corresponding normal-tissue-complication-probability (NTCP) values for gastro-intestinal (GI) toxicity were calculated. Results. The 5-year Grade P3 GI toxicity-free survival rate was 57.1 ± 13.1%. Five patients developed Grade 4 GI toxicity. Rectal V70Gy and the maximum dose to 1 cc of rectum at primary EBRT were both predictive for Grade P3 GI toxicity; 12.2% vs. 3.8%, p = .042 and 72.2 Gy vs. 66.8 Gy, p = .0027, respectively. When adding primary-RT and re-irradiation plans, the median maximum dose to 1 cc of rectum was 139.8 Gy (126.7–147.8) vs. 125.9 Gy (99.1– 133.1) (p = .0063) for Grade P3 and Grade 62 GI toxicity groups. Higher NTCP values at primary-RT were predictive for Grade P3 toxicity (p < .05). Conclusions. A higher rectal NTCP value, even in the absence of high-grade late-toxicity after primary-RT, is correlated with an increased risk of severe rectal side-effects after salvage reirradiation. Rectum doses greater than 70 Gy at primary-RT, and NTCP values of more than 10%, might predict for grade P3 rectal toxicity at re-irradiation, with a possible threshold for total rectum dose of around 130 Gy. https://doi.org/10.1016/j.ejmp.2017.10.070