Purpose: To quantitatively measure rectal gas content over the course of prostate cancer radiotherapy and to examine how rectal filling type (gas or feces) impacts rectal dose.
Methods: Rectums were manually contoured on daily cone-beam CT images of 9 prostate cancer patients previously treated with hypofractionated radiotherapy. Rectal filling contents were labelled as gas or non-gas by sorting voxels based on Hounsfield unit values. Daily rectum doses were calculated twice: once with the true material densities of the rectum and once with the rectum treated as water. The impact of filling type on rectal dose was evaluated by examining the differences between DVHs calculated using the two methods.
Results: Rectal gas content was observed to vary more in some patients than others. While the majority had minimal (3) gas levels, three patients regularly had much larger gas volumes during the treatment course. When investigating the effect filling type had on rectal dose, it was observed that large gas volumes (>20 cm3) posterior to the PTV led to increased V30Gy and V35Gy values compared to the same volume of fecal filling. DVHs were most impacted by filling type above the prescription dose of 60 Gy due differences in beam attenuation between gas and feces.
Conclusions: The presence of gas in the rectum during prostate radiotherapy results in increased portions of the rectum receiving moderate doses and also influences the proportion of the rectum receiving doses above prescription.