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

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 side effects.

MATERIALS AND

METHODS

Thirty patients treated with VMAT (59.4-64.5 Gy(RBE, relative biological effectiveness)) 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 of interest (OOI) 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 OOI doses (p < 0.001; e.g., lower pelvic bone marrow V5Gy (RBE), VMAT: 100 [100-100]%, IMPT: 62.9 [31.7-97.8]%), and the NTCPs (e.g., hematologic side effects 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 OOI sparing over four-beam and SEQ.

CONCLUSIONS

Besides target coverage, robustly optimized IMPT achieved substantial OOI dose and NTCP reductions versus VMAT in vulvar cancer. Clinical implementation requires attention to high-dose skin regions.