Abstract
BackgroundFor 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.MethodsPlanning 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.ResultsWhole 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.ConclusionsFor 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.
Highlights
For treatment of the entire craniospinal axis, many practitioners consider proton therapy the radiation modality of choice [1,2]
The V95% for the cribriform plate was significantly higher for Passive scattering proton therapy without compensator (PSWO) plans, an anatomical area which, if inadequately covered, may be associated with an increased risk of disease recurrence [19]
Without the capacity for distal blocking offered by the addition of a compensator, PSWO plans had significantly higher maximum lens doses (Figure 1A)
Summary
Fifty consecutive brain tumor patients treated with craniospinal radiation were included. Organs at risk (OARs) including the lens and cochlea along with target volumes (whole brain and cribriform plate) were contoured on the simulation computed tomography scan and each reviewed by a staff radiation oncologist. An Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) was used for dose calculations and all plans generated using 2.5 mm slice spacing. For this retrospective study, for each patient PSPT had been previously planned and delivered using a compensator which was manually edited in order to spare both cochlea and lens OARs as much as possible, while maintaining target coverage. The optimization included the cribriform plate as an additional target volume to facilitate prescription dose coverage. Statistical significance was determined by a two-tailed ttest with Bonferroni corrections employed to account for multiple comparisons
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