Multimodality treatment for patients with Wilms tumor has improved patient survival, but is associated with acute and long-term toxicity, partially due to irradiation. Proton therapy using pencil beam scanning (PBS) is a promising technique to reduce dose to organs at risk (OAR). In this study, we evaluate PBS plans for postoperative irradiation in patients with Wilms tumor.

Patients were treated with anterior-posterior-posterior-anterior (AP-PA) photon fields encompassing the preoperative tumor volume. Patients requiring whole lung irradiation were treated with AP-PA photon fields covering the bilateral lungs. Prescription doses were generally 1,080 and 1,200 cGy, respectively. Flank PBS plans encompassing the ipsilateral retroperitoneum and para-arotic nodes were generated for dosimetric evaluation.

Treatment records and comparison plans of 11 patients were reviewed. Mean dose and median dose to 50% or more of the contralateral kidney (D50) were 135 cGy and 139 cGy with photons and 52 cGy relative biological effectiveness (RBE) (P = 0.009) and 5 cGy RBE (P = 0.000001) with PBS. Mean dose and median D50 to bowel was 639 cGy and 979 cGy with photons and 379 cGy RBE (P = 0.001) and 47 cGy RBE (P = 0.004) with PBS. Mean dose and median D50 to the liver were 755 cGy and 1,013 cGy with photons and 411 cGy RBE (P = 0.02) and 132 cGy RBE (P = 0.02) with PBS. For patients with right-sided tumors, mean liver dose following sequential whole lung irradiation was 1,252 cGy with photons and 845 cGy RBE (P = 0.04) with PBS.

PBS proton therapy is a feasible method for irradiating the retroperitoneum and provides significant sparing of dose to critical OAR. This may translate to improved long-term health outcomes for patients and warrants further clinical investigation.