Previous studies have reported data on internal rectal motion of rectal cancer patients treated in the prone position. With the introduction of intensity modulated techniques, more patients are treated in the supine position. Data informing specific margins for this treatment position is sparse as is data comparing rectal motion characteristics and factors in male and female patients. The purpose of this retrospective study is to quantify and compare the inter-fractional rectal movement characteristics of male and female rectal cancer patients treated with long course chemo radiotherapy in the supine position. As well as to generate Internal Target Volume (ITV) margins accounting for this organ’s internal physiological movements. Cone Beam Computer Tomography (CBCT) images were acquired from 20 male and 17 female rectal cancer patients on the first 3 days of treatment and weekly thereafter. The rectum, bladder and femoral heads were delineated on the planning CT (PCT) and six CBCT for each patient. Overall, 249 images were analyzed. The site of disease varied along the rectum. All patients were treated with a full bladder. The rectum was divided into three 5cm segments (inf, med, and sup) The motion of the rectum was quantified by documenting the anteroposterior and lateral distances as measured using fixed anatomical landmarks namely from the anterior aspect of the sacrum and mid-left femoral head, respectively. These measurements were taken at 1-cm intervals from the inferior border of L5. Sigmoid was excluded from these measurements. Estimate of systemic and random physiological movement error was determined, and margins were calculated using the Van Herk formula. 249 CBCT data sets (120 male, 102 female) on patients who had undergone long course chemotherapy were analyzed. The mean of the per-patient standard deviation (σ) was largest for the mid rectum in all directions (Anterior, Left and Right) except for the posterior motion of the lower rectal wall. There were statistically significant differences in σ between males and females in the lateral directions in all three rectal segments. Suggested ITV margin guidelines are therefore non-isotropic and vary per segment of rectum and sex. Table 1 shows the ITV margins calculated based on this study’s patient population. In our present study, rectal motion is shown to be significantly different between males and females. Our data suggests using asymmetrical sex specific margins in all patients should be considered.Abstract 3377; TableFEMALE ITV MARGINSUP [cm]MID [cm]INF [cm]POST1.940.931.39ANT1.751.671.56LEFT1.691.471.03RIGHT1.941.61.22MALE ITV MARGINSUP [cm]MID [cm]INF [cm]POST1.260.990.89ANT1.411.451.19LEFT0.961.060.85RIGHT1.291.161.06 Open table in a new tab