# Carbon ion radiotherapy optimization techniques for pancreatic cancer: accounting for the effect of bowel gas variation

## TL;DR

This study explores how to optimize carbon ion radiotherapy for pancreatic cancer by addressing dose uncertainties caused by changes in bowel gas.

## Contribution

The study introduces and evaluates different density-override techniques to reduce dosimetric uncertainties in carbon ion radiotherapy due to bowel gas variation.

## Key findings

- Pattern 2 provided the best tumor coverage for the anterior beam but had poorer dose homogeneity for the lateral beam.
- Pattern 3 showed better homogeneity and lower gastrointestinal doses compared to pattern 2 for certain beam setups.
- Density overrides are recommended to address bowel gas variations, with pattern 3 being preferable for tumors near the GI tract.

## Abstract

Dose variation due to changes in bowel air poses significant challenges for carbon radiotherapy in pancreatic cancer. This retrospective study evaluated a density-override optimization technique to mitigate dosimetric uncertainties caused by bowel air changes.

Planning CT and cone-beam CT data from 8 patients with locally advanced pancreatic cancer undergoing stereotactic ablative radiotherapy were analyzed. Treatment simulations used a dose of 55.2 GyE in 12 fractions with a four-field setup (anterior, lateral, posterior, posterior oblique). Four density-override patterns were compared: pattern 0 (no override), pattern 1 (replacing bowel gas with water), pattern 2 (replacing the entire bowel with mean bowel HU), and pattern 3 (replacing bowel gas with mean bowel HU). Dose evaluations included fraction-wise and accumulated dose analyses, focusing on target coverage, homogeneity index, and organs at risk doses.

Pattern 2 achieved the largest clinical tumor volume coverage and the fewest fractions with > 5% coverage loss for the anterior beam, followed by pattern 3. However, pattern 2 demonstrated poorer homogeneity for the lateral beam compared to patterns 1 and 3 and a higher gastrointestinal (GI) dose for the anterior beam.

This study evaluated the importance of density overrides to address bowel air variations. For patients where a more uniform dose is desirable or whose tumor is adjacent to the GI tract, a pattern 3 density-override should be considered.