# Geometric target margin strategy of proton craniospinal irradiation for pediatric medulloblastoma

## TL;DR

This paper proposes a new geometric method for proton irradiation planning in pediatric medulloblastoma patients to ensure uniform dose delivery.

## Contribution

A novel geometric target margin strategy is introduced to improve consistency in proton craniospinal irradiation planning.

## Key findings

- The proposed geometric method produced simulated target volumes with median Hausdorff distances of ~9 mm across spinal regions.
- Mean distance-to-agreement was under 3.3 mm in all spinal sections, indicating good alignment.
- Dice and Jaccard coefficients showed moderate to high overlap between simulated and conventional target volumes.

## Abstract

In proton craniospinal irradiation (CSI) for skeletally immature pediatric patients, a treatment plan should be developed to ensure that the dose is uniformly delivered to all vertebrae, considering the effects on bone growth balance. The technical (t) clinical target volume (CTV) is conventionally set by manually expanding the CTV from the entire intracranial space and thecal sac, based on the physician’s experience. However, there are differences in contouring methods among physicians. Therefore, we aimed to propose a new geometric target margin strategy. Nine pediatric patients with medulloblastoma who underwent proton CSI were enrolled. We measured the following water equivalent lengths for each vertebra in each patient: body surface to the dorsal spinal canal, vertebral limbus, ventral spinal canal and spinous processes. A simulated tCTV (stCTV) was created by assigning geometric margins to the spinal canal using the measurement results such that the vertebral limb and dose distribution coincided with a margin assigned to account for the uncertainty of the proton beam range. The stCTV with a growth factor (correlation between body surface area and age) and tCTV were compared and evaluated. The median values of each index for cervical, thoracic and lumber spine were: the Hausdorff distance, 9.14, 9.84 and 9.77 mm; mean distance-to-agreement, 3.26, 2.65 and 2.64 mm; Dice coefficient, 0.84, 0.81 and 0.82 and Jaccard coefficient, 0.50, 0.60 and 0.62, respectively. The geometric target margin setting method used in this study was useful for creating an stCTV to ensure consistent and uniform planning.