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| Home > Publications database > Effects of air on the dosimetric robustness of treatment plans for prostate cancer in the presence of intrafractional, anatomical changes during online adaptive radiotherapy. |
| Home > Publications database > Effects of air on the dosimetric robustness of treatment plans for prostate cancer in the presence of intrafractional, anatomical changes during online adaptive radiotherapy. |
| Journal Article | DKFZ-2026-01729 |
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2026
ACMP
Reston, Va.
Abstract: In the online adaptive platform, a linear Boltzmann-transport-equation-solver (LBTE) as calculation algorithm is implemented for dose optimization. This class-C-optimizer is known to counteract the dose decrease at boundaries of small fields due to electronic disequilibrium by increasing the fluence to achieve the goals of dose homogeneity within the PTV.To analyze the dosimetric sensitivity of adaptive treatment plans for prostate cancer to intrafractional anatomic changes in the presence of rectal air using a CBCT-based adaptive platform.In the first part of the study planning CT scans (CTplan) from eight patients treated with an air-filled endorectal balloon and two patients presenting larger rectal air pockets were included in a simulation study. The dataset was used to assess the sensitivity of LBTE-optimized treatment plans to intrafractional density variations and to changes in the overlap between the planning target volume (PTV) and an air cavity (PTV∩Air structure). Density changes resulting from anatomical shifts were simulated using water overrides (WOR) in defined shift regions within the PTV∩Air structure. These shift regions were created by dorsally expanding the clinical target volume (CTV) by margins of 2 mm or 5 mm. Additionally, a WOR of the entire rectum was applied. For each WOR scenario, Dmax, D1 and D1cc values were recorded within the fixed PTV∩Air structure and within the rectal wall, which was shifted according to the applied margin. A mixed linear model was built to describe dosimetric characteristics in dependence of the WOR scenario. Additionally, intrafractional variations of Dmax within the PTV were assessed in 101 dose fractions from 15 patients who were treated with a rectal balloon and underwent online adaptive radiotherapy. All doses were normalized by the prescribed dose.WOR within PTV∩Air-structure overlap resulted in increases of the analyzed dosimetric characteristics. D1cc increased with WOR within shift margins of 2 and 5 mm and within the entire rectal cavity by 2.2% ± 0.7%, 5.5% ± 0.7%, and 8.7% ± 1.0%, respectively (p < 0.0001, F-test). The corresponding dose increases within the rectal wall were slightly smaller, with D1cc increases of 1.4% ± 0.6%, 1.7% ± 0.9%, and 5.0% ± 0.5%. Sensitivity to WOR in the PTV∩Air using a 5 mm shift margin was markedly lower when the same treatment plans were recalculated with AAA compared with LBTE (p = 0.0020, signed-rank test). On average, AAA detected only 43% ± 12% of the WOR effect predicted by LBTE. For prostate cancer patients treated with a rectal balloon, Dmax, D1 and D1cc changes in LBTE-optimized plans showed 90% confidence intervals of [-0.05-0.05] due to intrafractional motion, suggesting that posterior tissue shifts of 5 mm or less do not have a clinically relevant impact.PTV∩Air larger than 5 mm led to LBTE-optimized treatment plans, that are sensitive to anatomic soft tissue shifts larger than 5 mm in posterior direction with respect to dose homogeneity. PTV∩Air-overlaps up to 5 mm led to LBTE-optimized treatment plans, that showed only minor sensitivities to anatomic soft tissue shifts of 2-5 mm in posterior direction with respect to dose homogeneity. During online adaptive radiotherapy, the presence of rectal air pockets or an air-filled rectal balloon did not result in clinically relevant dose increases, provided that intrafractional motion-induced soft tissue shifts remained within 5 mm in the posterior direction.
Keyword(s): Humans (MeSH) ; Prostatic Neoplasms: radiotherapy (MeSH) ; Prostatic Neoplasms: diagnostic imaging (MeSH) ; Prostatic Neoplasms: pathology (MeSH) ; Radiotherapy Planning, Computer-Assisted: methods (MeSH) ; Male (MeSH) ; Radiotherapy Dosage (MeSH) ; Radiotherapy, Intensity-Modulated: methods (MeSH) ; Organs at Risk: radiation effects (MeSH) ; Air (MeSH) ; Algorithms (MeSH) ; Cone-Beam Computed Tomography: methods (MeSH) ; Rectum: radiation effects (MeSH) ; Radiometry: methods (MeSH) ; Dose Fractionation, Radiation (MeSH) ; adaptive radiation therapy ; dose inhomogeneity ; plan robustness ; prostate cancer
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