Potential normal tissue sparing in vulvar cancer using intensity modulated proton therapy versus volumetric modulated arc therapy

Abstract Background and purpose Photon radiotherapy, typically volumetric modulated arc therapy (VMAT), is the standard for adjuvant or definitive treatment of vulvar cancer. Here, we explore the potential of intensity modulated proton therapy (IMPT) to further reduce radiation toxicity. Materials and methods Thirty patients treated with VMAT (59.4–64.5 Gy(RBE, relative biological effectiveness¹)) to boost clinical target volume (CTV), 45–49.5 Gy(RBE) to the elective CTV, in 27–33 fractions) were retrospectively planned with IMPT, using four and six beams with simultaneous integrated boost (SIB) or sequential (SEQ) techniques. Target coverage and organs at risk (OARs) dose constraints were evaluated for bladder, bowel bag, anorectum, femoral heads, pelvic bone and skin. Normal tissue complication probabilities (NTCPs) were calculated for eight endpoints. Skin dose–surface distributions and NTCP–target volume trends were analyzed. Results The IMPT plans achieved robust target coverage similar to VMAT while significantly reducing OAR doses (p < 0.001; e.g., lower pelvic bone marrow V5Gy, VMAT: 100 [100–100]%, IMPT: 62.9 [31.7–97.8]%), and the NTCPs (e.g., hematologic toxicity grade ≥ 3, VMAT: 19.4 [4.6–50.2]%, IMPT: 2.3 [0.4–9.5]%). Despite similar grade 3 dermatitis risk (VMAT:79.6 [14.0–99.2]%, IMPT: 76.3 [17.9–96.6]%, p < 0.001), localized high-doses were observed at the lower abdominal/inguinal skin. The NTCPs increased with increasing target volume, less so for IMPT. Six-beam and SIB plans improved robustness and OAR sparing over four-beam and SEQ. Conclusions Besides target coverage, robustly optimized IMPT achieved substantial OAR dose and NTCP reductions versus VMAT in vulvar cancer. Clinical implementation requires attention to high-dose skin regions.