L. Perna

Patient-reported intestinal toxicity from whole pelvis intensity-modulated radiotherapy: First quantification of bowel dose–volume effects

BACKGROUND AND PURPOSE Intestinal toxicity is commonly experienced during whole-pelvis intensity-modulated radiotherapy (WPRT) for prostate cancer. The aim of the current study was to assess bowel dose-volume relationships for acute patient-reported intestinal symptoms of patients treated with WPRT for prostate cancer. MATERIALS AND METHODS Complete data of 206 patients were available; the median dose to pelvic nodes was 51.8Gy (range 50.4-54.4, 1.7-2Gy/fr). Intestinal symptoms were assessed as changes in the Inflammatory Bowel Disease Questionnaire scores relative to the Bowel Domain (IBDQ-B) between baseline and radiotherapy mid-point/end. The 25th percentiles of the most severe worsening from baseline (ΔIBDQ-B) were set as end-points. The impact of bowel loops and sigmoid colon dose-volume/surface parameters as well as selected clinical parameters were investigated using multivariate logistic regression. RESULTS Analyses were focused on the four questions showing a median ΔIBDQ-B>0. No dose volume/surface parameters were predictive, other than ΔIBDQ5≥3 (loose stools): when grouping patients according to bowel DVHs (high risk: V20>470cc, V30>245cc, V42>110cc; low risk: all the remaining patients), a two-variable model including high-risk DVH-shape (OR: 9.3) and age (protective, OR: 0.94) was assessed. The model showed good calibration (slope: 1.003, R2=0.92) and was found to be robust after bootstrap-based internal validation. CONCLUSIONS Constraining the bowel loops may reduce the risk of loose stools. The risk is higher for younger patients.

Validation of a method for “dose of the day” calculation in head-neck tomotherapy by using planning ct-to-MVCT deformable image registration

PURPOSE The aim of this study was to test the feasibility and dosimetric accuracy of a method that employs planning CT-to-MVCT deformable image registration (DIR) for calculation of the daily dose for head and neck (HN) patients treated with Helical Tomotherapy (HT). METHODS For each patient, the planning kVCT (CTplan) was deformably registered to the MVCT acquired at the 15th therapy session (MV15) with a B-Spline Free Form algorithm using Mattes mutual information (open-source software 3D Slicer), resulting in a deformed CT (CTdef). On the same day as MVCT15, a kVCT was acquired with the patient in the same treatment position (CT15). The original HT plans were recalculated both on CTdef and CT15, and the corresponding dose distributions were compared; local dose differences <2% of the prescribed dose (DD2%) and 2D/3D gamma-index values (2%-2mm) were assessed respectively with Mapcheck SNC Patient software (Sun Nuclear) and with 3D-Slicer. RESULTS On average, 87.9%±1.2% of voxels were found for DD2% (on average 27 slices available for each patient) and 94.6%±0.8% of points passed the 2D gamma analysis test while the 3D gamma test was satisfied in 94.8%±0.8% of bodys voxels. CONCLUSIONS This study represents the first demonstration of the dosimetric accuracy of kVCT-to-MVCT DIR for dose of the day computations. The suggested method is sufficiently fast and reliable to be used for daily delivered dose evaluations in clinical strategies for adaptive Tomotherapy of HN cancer.

EP-1815: Towards adaptive Tomotherapy: planning CT to MVCT deformable image registration for dose calculation

S851 ________________________________________________________________________________ Conclusion: A method has been developed to assist the adaptive planning process for lung patients receiving FFF VMAT radiotherapy. This provides a means of assessing the dosimetric effect of tumour changes to determine whether a new treatment plan is necessary. It showed that for 25% of patients who received full treatment replans no replan was necessary, as the dosimetric effect of tumour shrinkage was insignificant in terms of both target coverage and OAR doses. Therefore it allows significant time savings in the treatment replanning process. Use of the technique is limited to patients who display tumour volume changes with no other significant changes to internal/external anatomy.

EP-1725: Predictors of diarrhea after whole-pelvis post-prostatectomy radiotherapy

S807 ________________________________________________________________________________ was maintained as long as the effect metric used for Cox regression had a linear correlation with the true effect metric of at least 0.50. The conclusions held if the trial cohort consisted of an expected high benefit population (22% reduced sample size), but the effect was even stronger if the cohort was a population with modest expected benefit (31% reduced sample size).

PO-0762: Dose-volume predictors of radio-induced effects after SRS for uveal melanoma

S357 ________________________________________________________________________________ and distant metastasis-free rates were 86%, 67%, and 38% at 1 year and 86%, 38%, and 16% at 3 years, respectively. Thirteen patients died; the cause of death was tumor progression in 10 patients, infectious pneumonia in two, and old age in one. The overall and cause-specific survival rates were both 73% at 1 year and 23% and 44% at 3 years, respectively. The median survival time was 16 months. Although all 17 patients developed grade 1–2 radiation dermatitis, there were no therapy-related toxicities of grade ≥3.

Second Tumor Induction Risk in IMRT for Prostate Cancer: An Unbalanced Comparison Between Surgery and Radiotherapy?

AbstractThe aim of this study was to evaluate the risk of second tumor induction for prostate patients treated with volumetric modulated arc therapy in age classes 50–70. Based on both age-dependent models and doses to critical organs, the risk of second tumor induction was evaluated simulating the small field (prostate and seminal vesicles) and large field (whole pelvis) for Helical Tomotherapy and Rapid Arc. The doses to the organs closest to the treatment volume were derived from treatment planning system data. Whereas, due to the lack of calculation algorithms where leakage and internal radiation scattering are unreliable at a large distance from target, the doses to the organs outside the treatment volume were measured in an anthropomorphic phantom. Doses from Image Guided Radiotherapy (IGRT) were also assessed on phantom measurements. The Lifetime Attributable Risk (LAR) for second tumor induction increases from 2.2 to 13.7% as irradiated volume increases and age decreases. IGRT could add a non-negligible factor to the risk when daily set-up verification with high-resolution modality is included. As prostate cancer is detected earlier, the probability of an increase in early stage patients rises, and life expectancy thus increases. Radiotherapy has improved its capability in the tailoring of the dose around the target at the cost of a greater dose to surrounding organs, thus increasing the risk of second tumor induction, especially for those patients expected to survive 15 y or more.