Abstract 1
Background and purpose: Photon radiotherapy, typically delivered with volumetric modulated arc therapy (VMAT), is the current standard for adjuvant or definitive treatment of vulvar squamous cell carcinoma (SCC), but intensity-modulated proton therapy (IMPT) may reduce normal tissue toxicity. To our knowledge, this is the first study to investigate IMPT for vulvar SCC by comparing robustly optimized IMPT with VMAT plans.
Materials and 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. Plans were evaluated based on target coverage, organs at risk (OARs) dose constraints for bladder, bowel bag, rectum, femoral heads, iliac crests, and bone marrow. 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 trends between NTCP and target volumes were assessed.
Results: Robust CTV coverage was achieved for all IMPT plans, similar to 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). 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. A trend toward higher NTCP with larger target volumes was observed, with IMPT showing similar or smaller increases than VMAT. Six-beam and SIB plans demonstrated improved robustness and OARs sparing compared with four-beam and SEQ plans.
Conclusions: IMPT can achieve robust target coverage and substantial reductions in OARs doses and NTCPs compared with VMAT for vulvar SCC. Careful consideration of high-dose skin areas, hence skin toxicity, is required to guide the clinical implementation of IMPT.

Abstract 2
Background and purpose: Proton minibeam radiation therapy (pMBRT) is a novel technique that may reduce normal tissue toxicity through spatial fractionation of proton beams. RayStation has recently introduced a Monte Carlo (MC) dose calculation method incorporating multislit collimators (MSCs). This study investigates the influence of MSC slit width and center-to-center (CTC) distance on robust target coverage and peak-to-valley dose ratio (PVDR) in single-beam pMBRT.
Materials and methods: Simulations were performed in RayStation 24B-IonPG using a validated MC dose engine including MSCs. A homogeneous cubic water phantom with a spherical clinical target volume (CTV) was modelled. Nineteen pMBRT plans were generated with slit widths 0.04–0.20 cm and CTC distances 0.2–0.6 cm, plus a broad beam reference. Four single-beam geometries were simulated, representing different entrance-to-target depths (6–24 cm) and angles. Robust composite minmax optimization (CMRO) with 28 scenarios (5 mm setup, 3% range uncertainty) was applied. Robust target coverage and PVDR at beam entrance and CTV center were analyzed. Tumor control probability (TCP) was calculated for illustrative purposes.
Results: A trade-off was observed between entrance PVDR and robust target coverage. High PVDR values at entrance reduced coverage, while sufficient coverage reduced PVDR. CTV depth and beam geometry were key factors: larger depths and oblique angles reduced entrance PVDR (<2.7) and coverage. Intermediate depths (6–15 cm) provided the most favorable balance. TCP was comparable between pMBRT and broad beams when PVDR in the target was <1.2, but decreased with heterogeneous target doses.
Conclusions: This study demonstrates that robust single-beam pMBRT planning in RayStation is feasible but can be limited by a trade-off between PVDR and robust target coverage, and heavily dependent on beam depth and geometry. These findings provide insights into the effects of beam geometry and collimator parameters on PVDR and robustness, guiding future investigations on
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