SU‐FF‐J‐34: Influence of Volumes of Prostate, Rectum and Bladder On Treatment Planning CT‐Day On Inter‐Fraction Motion of Prostate During BAT Image‐Guided IMRT

Abstract

Purpose: To study the relationship between prostate volume/location, bladder and rectum volumes on treatment‐planning CT‐day and prostate shift in XYZ directions on treatment‐days. Method and Materials: Prostate, SV, bladder and rectum (rectosigmoid‐flexure to anorectal‐verge), were contoured on CT‐images. Isocenter was 6 cm posterior to the tip of pubic‐arch and 1 cm inferior to the pubic‐brim. IMRT plans were prepared. Contours were exported to BAT‐system. Patients were positioned on couch using skin marks. US‐probe was used to obtain US‐images of prostate, bladder and rectum and aligned with CT‐images. Shifts in XYZ directions as recommended by BAT‐system were made and recorded. 4698 couch‐shifts for 42 patients were analyzed to study a correlation between prostate shifts vs. bladder and rectum volumes and prostate volume/location on CT‐day. Spatial location of prostate was defined as distance of prostate base to isocenter. Dose to 50% of bladder vs. volume was also studied. Pearson's correlation coefficient r, and P values were used for statistical analysis. Results: Mean and range of volumes (cc): bladder: 179, 42–582, rectum: 108, 28–223 and prostate: 55, 21–154. Mean prostate shifts (cm, ±SD): R/L (X): −0.047±0.16, AP/PA (Y): 0.14±0.3 and S/I (Z): 0.19±0.26. Lateral, AP/PA and S/I shifts were not correlated with volumes of bladder, rectum and prostate; bladder and prostate; and bladder and rectum, (P>0.2), respectively. Smaller the rectal volume (P<0.001) or diameter (P<0.05) of rectum, larger was the anterior shift and vice‐versa. Smaller the prostate base distance to isocenter or volume, larger was superior shift and vice‐versa (P<0.05). Dose to bladder decreased with increase in volume up to 300cc, reaching a plateau with further increase in volume (P<0.001). Conclusions: Prostate location/volume and rectal‐volume, but not bladder‐volume on CT‐day influence prostate position. Bladder with 200–300cc volume, but not full bladder, would be optimum for patient comfort, minimizing bladder dose and US‐image quality.