S1219
Clinical - Urology
ESTRO 2026
Digital Poster 2200
Lower organ-of-interest doses through reduced PTV margins for high-risk prostate cancer patients treated with CBCT-based online adaptive radiotherapy Bettina Skjold Jasper, Laura Patricia Kaplan, Dorthe Yakymenko, Mette Marie Bruun Nielsen, Eva Samsøe Department of Clinical Oncology and Palliative Care, Radiotherapy, Zealand University Hospital, Næstved, Denmark Purpose/Objective: High-risk prostate cancer (HRPC) treated with radiotherapy often leads to adverse effects due to irradiation of pelvic organs-of-interest. In online adaptive radiotherapy (oART), inter-fraction motion is circumvented and the required PTV margins are decreased. We aimed to establish necessary PTV margins to prostate, seminal vesicles, and pelvic lymph nodes for HRPC patients treated with CBCT-based oART and to evaluate the impact of PTV margin reduction on target coverage, doses to organs-of- HRPC patients treated with 39 fraction oART to the prostate (2 Gy/fx), seminal vesicles (2 Gy/fx), and pelvic lymph nodes (1.44 Gy/fx) on the Ethos systems (Varian, Siemens Healthineers) were included. In part 1, intra-fractional motion was evaluated retrospectively in 22 patients (171 fractions). Post adaptation, directly prior to treatment, a verification CBCT (vCBCT) was captured to verify patient position. The smallest PTV margins covering CTVs on the vCBCT were determined visually. In part 2, using an Ethos Emulator, dose plans for ten patients were re- interest, and NTCP. Material/Methods: optimized with reduced PTV margins determined in part 1, and eight adaptive fractions per patient were simulated. Target coverage (PTV and CTV V95%) and doses to organs-of-interest were compared between simulated fractions and treated fractions with standard PTV margins. NTCP for grade ≥ 2 late rectal bleeding was calculated using a published model and compared [1]. Results: In part 1, a 4 mm margin covered the pelvic lymph nodes in 92% of retrospectively analyzed fractions and the seminal vesicles in 99% of fractions. The prostate was covered by a 3 mm margin in all analyzed fractions. In part 2, target coverage was comparable between simulated and treated fractions. The median [min,max] reduction in bladder V50Gy was 8.3 [ − 1.4,22.7] percentage points (pp) (Figure 1). Median reductions in bladder V70Gy, bowel bag V35Gy, and rectum V70Gy were 5.1 [0.9,16.6], 5.8 [ − 0.6,15.3], and 3.9 [ − 0.8,14.4] pp. Reduction in PTV margins led to a median reduction in NTCP for rectal bleeding of 1.2%
Conclusion: To our knowledge, this is the first investigation on a large prospective cohort demonstrating clear association between lymphopenia and late intestinal symptoms after WPRT. Patients with acute G3+ lymphopenia showed a 2.5 times higher risk of experiencing late symptoms related to frequency of bowel movement, urgency and incontinence. This study was supported by an AIRC grant (IG14603). References: [1] Sini C et al. Radiother Oncol 2016;118:79-84. PMID: 26702990[2] Pavarini M et al. Radiother Oncol 2024;195:110230. PMID 38503355[3] Sanguineti G. et al. Radiother Oncol 2024;192,110088 Keywords: prostate cancer, lymphopenia, intestinal toxicity
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