Evaluating the Impact of Rectal Gas on Treatment Dose Distribution In Prostate Tumor Therapy: A Retrospective Analysis
Abstract
Purpose
Prostate cancer is the most common form of cancer, excluding skin cancer, and results in significant genitourinary toxicity. Rectal gas buildup has exacerbated this by changing simulated patient anatomy and introducing tissue inhomogeneity. We are investigating the practical effects of tissue inhomogeneity by retrospectively evaluating the dosimetric impact of rectal gas pockets on target coverage and rectal wall dose in prostate VMAT radiation therapy.
Methods
Patients treated for Volumetric Modulated Arc Therapy (VMAT) prostate cancer were retrospectively reviewed. Rectal gas buildup was visually evaluated for volume and proximity to the Planning Target Volume (PTV). Clinically delivered plans were recalculated using rectal gas density overrides to simulate gas-free anatomy. Density overrides were defined at <-150 HU and overridden to soft tissue equivalent densities (0 HU). Recalculated plans compared hot spots for both the rectal wall and prostate target. We then investigated how dose changes varied with the size of the rectal gas pocket.
Results
Evaluation of the dosimetric differences showcased an association between the localized rectal gas pocket and an increased dose to the rectal wall and the PTV. A linear relationship was observed between the volume of gas proximal to the PTV, and the measured delta between each CT variant. Point dose (dose at 0.03 cc) increases were seen to increase by 7.7% (537.9 cGy) for small volumes (12.6 cc). Larger volume metrics (2 cc) showed similar trend increases, increasing by 1.6% (113.7 cGy) despite small buildup volumes (6.31 cc)
Conclusion
This work demonstrates clinical dose increases of up to 15% in the PTV and rectum wall for prostate VMAT radiation therapy cases even at rectal gas volume as low as 6 cm3. This increase may justify the use of rectal gas volume overrides to mitigate the effects, allowing clinics to plan conservatively and reduce the risk of overdose.