Clinical implementation of intensity modulated proton therapy for testicular seminoma.

Background and purpose: We have previously shown that proton therapy results in considerable reduced doses to abdominal organs at risk (OAR) which likely reduces the patient's risk of a second malignant tumor, which is vital for this young population with favourable prognosis. Here, we present dosimetric results after implementing intensity modulated proton therapy (IMPT) as a national standard for seminoma. Patient/material and methods: Thirty patients with stage IIA and IIB (< 3 cm) seminoma were treated with five-field robustly multi-field optimised (MFO) proton therapy to 20-24 Gy (relative biological effectiveness [RBE]) to the dog-leg retroperitoneal volume followed by a boost of 10-16 Gy (RBE) to the nodal metastasis. Control CTs were performed routinely, and target coverage evaluated. A standard two cone-beam CT (CBCT) set-up strategy with four match structures was developed, enabling implementation of a standard adaptive scheme.

Results: The median clinical target volume (CTV-E) length in the craniocaudal direction was 26.9 cm, with a median volume of 551.4 cm3. Target coverage V95% = 100% for the nominal plan and V95% ≥ 98% for worst-case scenarios were fulfilled for all treatment plans and the 46 recalculated plans on control CTs. Kidney V17Gy was 0-6% and mean kidney dose 0-6 Gy across all plans. Bowel bag V15Gy was 194-698 cm3. All other OAR showed low doses. Four patients had replans (1-2 per patient). The median time for our image guidance (IG) strategy was 14:07 min across all patients with two CBCTs.

Interpretation: We have established a robust setup for treatment planning, IG strategy, treatment delivery and adequate response to replanning. Therefore, we suggest considering IMPT for testicular seminoma whenever available.