Intensity-modulated proton therapy for hippocampal-sparing prophylactic cranial irradiation: a planning comparison with photon therapy.

Background: The purpose of the study was to evaluate the dosimetric characteristics of volumetric modulated arc therapy (VMAT), helical tomotherapy (HT), and intensity-modulated proton therapy (IMPT) and to compare the dosimetric differences between the two IMPT plans with coplanar and non-coplanar beams in prophylactic cranial irradiation (PCI) with hippocampal-sparing for small cell lung cancer (SCLC).

Methods: Twenty-five patients diagnosed with limited-stage SCLC and received PCI were enrolled in the study. Four treatment plans were designed: VMAT, HT, and two IMPT plans with coplanar and non-coplanar beams (referred to as IMPT-cop and IMPT-noncop, respectively). The prescription dose was 25 Gy in 2.5 Gy(RBE) fractions. The PTV was optimized in both the VMAT and HT plans. In IMPT plans, multifield optimization and CTV robust optimization with a 3-mm setup uncertainty and 3.5% range uncertainty were used. According to the RTOG 0933 protocol, the dose limits for the hippocampus were the dose received by 100% volume (D₁₀₀) ≤ 9 Gy and the maximum dose (D_max) ≤ 16 Gy.

Results: For the target, the two IMPT plans significantly improved the V₁₀₀, D₉₈, the homogeneity index (HI) and gradient index (GI) compared with VMAT and HT plans. The HT plans showed the highest conformity index (CI) compared to the other three plans. The two IMPT plans significantly reduced the D₁₀₀, D_max and D_mean of the hippocampus, the mean dose of bilateral eyeballs and parotids, the maximum dose of bilateral lenses and lenses PRV compared to the VMAT and HT plans. For D₁₀₀ in hippocampus, the IMPT-cop and IMPT-noncop plans reduced by 43.23%, 42.55%, 41.14%, and 40.43%, respectively, relative to VMAT and HT plans. For D_max in hippocampus, the IMPT-cop and IMPT-noncop plans decreased by 8.22%, 8.29%, 7.86%, and 7.93%, respectively, relative to VMAT and HT plans. For hippocampal D_mean, IMPT-cop and IMPT-noncop plans decreased by 23.1%, 22.48%, 20.55%, and 19.91% compared with VMAT and HT plans, respectively. VMAT plans showed the lowest values for the maximum dose to the bilateral eyeballs among the four plans. When comparing the two IMPT plans, IMPT-cop plans significantly reduced the mean dose to the hippocampus, and increased the D_mean and D_max of bilateral eyeballs, and the D_max of bilateral lenses and lenses PRV compared to IMPT-noncop plans.

Conclusions: Compared with photon plans, proton plans significantly reduce the dose to the hippocampus, lenses, eyeballs and parotids in hippocampal-sparing PCI. Compared to IMPT plans with coplanar beams, IMPT plans with non-coplanar beams have shown dosimetric advantages in eyeballs and lenses, with no benefit for dose sparing in the hippocampus.