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Craniospinal (CS) radiotherapy (RT) is a part of the standard treatment for pediatric medulloblastoma patients. Protons reduce the dose to many organs-at-risk compared to photons, however, the whole brain still receives full prescription dose which results in a high risk of neurocognitive impairment. To reduce these side effects hippocampal-sparing (HS) CS proton pencil beam scanning (PBS) RT has been proposed, which uses complex treatment plans with sharp dose gradients. The aim of this study was to dosimetrically verify HS PBS proton plans.

Thirteen pediatric medulloblastoma patients previously treated with CS RT were included. In this study, the CTV included the cranial target. Prescribed dose was 23.4 Gy(RBE) in 13 fractions. Proton PBS plans with two lateral and one posterior field (fig 1a) were created in Eclipse (Varian Medical Systems). For each patient, one HS plan with a mean dose of 9 Gy(RBE) to the hippocampi and one conventional plan with homogeneous dose distribution were created. The plans were recalculated in a water phantom and measured field-by-field using the MatriXX PT detector (IBA Dosimetry) and solid water blocks. The measurements were performed at water equivalent depths (WED) corresponding to the central part of the hippocampi (fig 1a). Both the HS and conventional plans were measured for all patients with the standard resolution of the detector (7.6 mm), in total 130 measurements. The measured and planned dose distributions were compared using 2D global gamma evaluation (3%/2mm, threshold 5%) and the resulting pass rates were compared using a paired Wilcoxon test. Ten fields for three HS plans were measured with higher resolution (3.8 mm) by shifting the detector. The measured and planned dose distributions were compared using 2D global gamma evaluation (3%/1mm) within ROIs corresponding to the HS area (ROI_HS) and a surrounding ring (ROI_ring) (fig 2).

Figure 1 a) Absorbed dose in color-wash 95-107% for a HS plan together with gantry angles and measurement depths. b) Median (range) gamma pass rate for HS (green) and conventional (blue) plans.

It was possible to reduce the mean hippocampal dose from 23 to 9 Gy(RBE), while keeping the CTV V95% above 95% for all HS plans. For the standard resolution measurements, the median pass rates were 99.5% (90.4-100) and 99.4% (79.7-100) for the HS and conventional plans, respectively (p < 0.001, fig 1b). For the high-resolution measurements, the median pass rates were 100% (95.9-100) and 97.9% (78.0-100) for ROI_HS and ROI_ring, respectively.

Figure 2 High resolution measured dose distributions for a posterior (a) and lateral (b) field with ROIs corresponding to 1) ROI_HS and 2) ROI_ring.

Using HS proton therapy, it is possible to reduce the hippocampal dose from 23 to 9 Gy(RBE) with plans including very steep dose gradients. Yet, these plans showed great agreement between the planned and measured dose in the hippocampal area. This shows that HS proton therapy is dosimetrically feasible.