Comparison of proton therapy techniques for treatment of the whole brain as a component of craniospinal radiation

BACKGROUND: For treatment of the entire cranium using passive scattering proton therapy (PSPT) compensators are often employed in order to reduce lens and cochlear exposure. We sought to assess the advantages and consequences of utilizing compensators for the treatment of the whole brain as a component of craniospinal radiation (CSI) with PSPT. Moreover, we evaluated the potential benefits of spot scanning beam delivery in comparison to PSPT. METHODS: Planning computed tomography scans for 50 consecutive CSI patients were utilized to generate passive scattering proton therapy treatment plans with and without Lucite compensators (PSW and PSWO respectively). A subset of 10 patients was randomly chosen to generate scanning beam treatment plans for comparison. All plans were generated using an Eclipse treatment planning system and were prescribed to a dose of 36 Gy(RBE), delivered in 20 fractions, to the whole brain PTV. Plans were normalized to ensure equal whole brain target coverage. Dosimetric data was compiled and statistical analyses performed using a two-tailed Student’s t-test with Bonferroni corrections to account for multiple comparisons. RESULTS: Whole brain target coverage was comparable between all methods. However, cribriform plate coverage was superior in PSWO plans in comparison to PSW (V95%; 92.9 ± 14 vs. 97.4 ± 5, p < 0.05). As predicted, PSWO plans had significantly higher lens exposure in comparison to PSW plans (max lens dose Gy(RBE): left; 24.8 ± 0.8 vs. 22.2 ± 0.7, p < 0.05, right; 25.2 ± 0.8 vs. 22.8 ± 0.7, p < 0.05). However, PSW plans demonstrated no significant cochlear sparing vs. PSWO (mean cochlea dose Gy(RBE): 36.4 ± 0.2 vs. 36.7 ± 0.1, p = NS). Moreover, dose homogeneity was inferior in PSW plans in comparison to PSWO plans as reflected by significant alterations in both whole brain and brainstem homogeneity index (HI) and inhomogeneity coefficient (IC). In comparison to both PSPT techniques, multi-field optimized intensity modulated (MFO-IMPT) spot scanning treatment plans displayed superior sparing of both lens and cochlea (max lens: 12.5 ± 0.6 and 12.9 ± 0.7 right and left respectively; mean cochlea 28.6 ± 0.5 and 27.4 ± 0.2), although heterogeneity within target volumes was comparable to PSW plans. CONCLUSIONS: For PSPT treatments, the addition of a compensator imparts little clinical advantage. In contrast, the incorporation of spot scanning technology as a component of CSI treatments, offers additional normal tissue sparing which is likely of clinical significance.