Rectal Gas-Induced Dose Changes in Carbon Ion Radiation Therapy for Prostate Cancer: An In Silico Study.

Purpose: This study aims to determine dosimetric influence of rectal gas in carbon ion radiation therapy (CIRT) for prostate cancer and to establish a procedure for removal rectal gas in clinical scenarios.

Materials and methods: We analyzed 18 prostate cancer cases with bulky rectal gas. The dose distribution was recalculated on computed tomography (CT) with bulky rectal gas (gasCT) after creating the initial plan on a CT without bulky rectal gas, and the doses were transformed using a displacement vector field. This created a dose distribution simulation irradiated with the residual rectal gas. Among 12 fractions (fx) for prostate cancer CIRT, different residual rectal gas fx were used to develop 12 dose distributions, each of which was compared with that in the initial plan. Clinical target volume (D_min, D_99.5%), rectum, and rectal wall (V_95%, V_80%) parameters were assessed. We investigated the indicators associated with these dose changes using digital reconstruction radiograph (DRR) images.

Results: The dosimetric changes in the clinical target volume were not significantly different from that in the initial treatment plan for both D_min and D99.5%. Compared to the initial plan, the dose-volume histogram parameters showed changes exceeding 1 cm³ when residual rectal gas was present in the following number of fractions: 8 fx for V_95% rectum, 5 fx for V_80% rectum, 10 fx for V_95% rectal wall, and 11 fx for V_80% rectal wall. Changes in rectal and rectal wall parameters were highly correlated with the extent of rectal gas assessed on DRR images.

Conclusion: Rectal gas removal may not be necessary up to 4 fx. Moreover, indicators related to dose changes based on DRR images were highly correlated with dose changes, revealing the possibilities of estimating dose changes due to rectal gas from kV-x-ray images and using gas effect evaluation during CIRT irradiation.