ESTRO 2023 - Abstract Book

S1719

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ESTRO 2023

(29%) which was up to 16 Gy[RBE] due to positive margins. V95% was equal to or higher than 95% of the target volume in all plans. Median differences in dosimetric parameters for selected OARs were all statistically significant and in favour of protons (P <0.05) except for thyroid V20 (absolute volume of gland receiving a dose ≤ 20 Gy[RBE]) which was comparable in both rival plans. All median ∆ NTCP PH-PT for the studied endpoints were statistically significant and in support of IMPT (P <0.05), with the exception of ∆ NTCP for hypothyroidism (P >0.05) and radiation dermatitis ≥ grade 2, which was significantly higher for IMPT (median value 43,27 % versus 11,25 % for VMAT). Median ∆ NTCP PH-PT are shown in Table 1. Conclusion In clinical practice, VMAT DIBH for LSBC pts is a satisfactory option to spare OARs while ensuring optimal target coverage. In this analysis, IMPT plans provided a statistically relevant reduction of OARs doses leading to significant differences for almost all the NTCP model considered. Nevertheless, clinical validation in a bigger dataset of pts is warranted to confirm these results. E. Visser 1 , S. Petersen 1 , S. Tilbæk 1 , L. Bentzen 2 , H. Rønde 1 , L. Stolarczyk 1 , A. Vestergaard 1 , T. Johansen 1 , R. Klitgaard 1 , S. Pilskog 3,4 , J. Søndergaard 5 , M. Høyer 1 , L. Muren 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Vejle Hospital, University of Southern Denmark, Department of Oncology, Vejle, Denmark; 3 University of Bergen, Department of Physics and Technology, Bergen, Norway; 4 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway; 5 Aalborg University Hospital, Department of Oncology, Aalborg , Denmark Purpose or Objective Proton therapy for prostate cancer patients is challenged by inter-fractional anatomical variations. In preparation for a randomized trial, the aim of this study was to evaluate the robustness towards anatomical changes of a proton treatment planning class solution for high-risk prostate cancer patients and to identify geometric predictors for dose degradations. Materials and Methods Treatment plans and 74 repeat CT scans (rCTs) from eight patients with high-risk prostate cancer were analyzed. The four field treatment planning class solution (two lateral oblique and two posterior oblique beams) created on the planning CTs was applied to the rCTs, matched on prostate fiducial markers and subsequently recalculated. A rectum sub-section was created of the rectum on the axial CT-slices that also contained the prostate. Plan robustness was investigated by determining if V95% ≥ 98% for both the prostate (p-CTV) and the combined regional elective lymph node and seminal vesicles target (ln/sv-CTV). Changes in bladder volume, rectum sub-section and femoral position were associated with p-CTV coverage and the potential violation of normal tissue dose constraints by utilizing a linear mixed model. If a statistically and clinically significant correlation was found, finding a cut-off value useful for clinical application was explored. Results The treatment plan was robust with respect to p-CTV coverage in 69/74 rCTs and in 65/74 rCTs for the ln/sv-CTV. The rectum V75Gy dose constraint ( ≤ 3%) was exceeded on 27/74 rCTs. An increase in rectum sub-section volume, increase in air in the rectum sub-section and posterior femoral translation were correlated with both p-CTV V98% dose degradation and rectum V75Gy dose constraint violation. Increase in rectum sub-section volume was correlated with p-CTV V98% and with rectum V75Gy with regression coefficients of -0.9%/cm^3 and 0.3%/cm^3 respectively (p<0.001) (Fig. 1). Posterior femoral translation was correlated with p-CTV V98% and with rectum V75Gy with regression coefficients of -2.5%/mm and 0.9%/mm respectively (p<0.001) (Fig. 2). A cut-off value for an increase in rectum sub-section volume of 10cm^3 and for a posterior femoral translation of 5mm resulted in a sensitivity for detecting rectal dose violation of 44% and 33% respectively and a specificity of 91% and 94% respectively in this cohort. PO-1957 Anatomical robustness of a proton planning class solution for high-risk prostate cancer