S399
Clinical - Gynaecological
ESTRO 2026
Digital Poster 37 Potential reduction in normal tissue toxicity with robustly optimized IMPT for vulvar cancer: a dosimetric and NTCP comparison with VMAT Anna C Prins 1,2 , Raymond de Boer 2 , András G Zolnay 2 , Remi A Nout 2 , Mischa S Hoogeman 2,1 , Kelvin Ng Wei Siang 1,2 1 Department of Medical Physics & Informatics, Holland Proton Therapy Center, Delft, Netherlands. 2 Department of Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands Purpose/Objective: Photon radiotherapy, typically delivered with volumetric modulated arc therapy (VMAT), is the current standard for adjuvant or definitive treatment of vulvar cancer. However, intensity-modulated proton therapy (IMPT) may further reduce normal tissue toxicity due to its dose deposition properties. Currently, no clinical trials or dosimetric studies exist that investigate the potential of proton therapy for vulvar cancer. To our knowledge, this is the first study to investigate IMPT for vulvar cancer by comparing robustly optimized IMPT with VMAT plans. Material/Methods: Thirty patients treated with VMAT (59.4-64.5 Gy(RBE) to boost CTV, 45-49.5 Gy(RBE) to elective CTV, in 27-33 fractions) were retrospectively planned with IMPT using composite minimax robust optimization (CMRO). Four- and six- beam arrangements were used, following the same fractionation as VMAT plans with either simultaneous integrated boost (SIB) or sequential (SEQ) fractionation techniques. IMPT plans were evaluated for target coverage based on the dose criteria of D98 ≥ 95% and D2 < 107%, using the voxelwise minimum and maximum dose distributions. All treatment plans were evaluated on organs at risk (OARs) dose constraints for bladder, bowel bag, anorectum, femoral heads, and pelvic bone.
Normal tissue complication probabilities (NTCPs) were calculated for eight toxicity endpoints across relevant OARs, including skin. Skin dose distributions were also analyzed using dose–surface maps, and associations between NTCP and target volumes were assessed. Results: All IMPT plans achieved robust target coverage, meeting the dose coverage criteria similar to those of the PTV-based VMAT plans. IMPT significantly reduced doses to all OARs compared with VMAT (p < 0.05), translating into lower NTCPs for nearly all endpoints (p < 0.05), see Figure 1.
Although IMPT showed similar or reduced grade 3 dermatitis risk, dose–surface maps revealed more high-dose regions localized in the lower abdominal/inguinal skin areas, potentially increasing skin toxicity, see Figure 2.
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