Purpose: To assess the relationship between daily variations in rectal dose and rectal filling during a hypofractionated course of prostate radiotherapy. Methods: Rectum volumes were delineated on daily CBCT images of 20 prostate cancer patients who received 60 Gy in 20 fractions. Daily doses were calculated by registering planned beams to the CBCTs and exported for analysis along with CT and ROI structure files. Gas content was quantified using HU thresholding. Dose to the anterior, posterior, and full rectal wall was quantified by way of dose-surface maps of the rectal wall. The relationship between changes in daily rectal doses and rectal volume/gas content from planning baseline was evaluated using Spearman correlation and multivariate linear regression. Results: A weak correlation existed between changes in rectal volume and changes in rectal dose to the full and anterior rectal wall areas (0.22<⍴<0.39) with a slightly better correlation for the anterior wall. A weaker correlation was observed between changes in rectal gas content and full and anterior rectal wall dose (0.14<⍴<0.30). No correlations were observed for the posterior wall. Multivariate regression using rectal volume and gas content did not improve correlations. Conclusions: Rectal volume and gas filling data are not sufficient to adequately explain daily changes in rectal wall dose during the course of prostate radiotherapy. Additional data such as bladder filling or prostate motion may be necessary to create a reliable model.