Bag and loop small bowel contouring strategies differentially estimate small bowel dose for post-hysterectomy women receiving pencil beam scanning proton therapy

<b>Background</b> Small bowel (SB) dose-volume relationships established during initial computed tomography (CT) simulations may change throughout therapy due to organ displacement and motion. We investigated the impact of organ motion on SB dose-volume histograms (DVHs) in women with gynecologic malignancies treated with pencil beam scanning (PBS) proton therapy and compared PBS SB DVHs to intensity-modulated radiation therapy (IMRT). <b>Material and methods</b> Post-hysterectomy patients (<i>n</i> = 11) treated for gynecologic cancers were enrolled on an image-guided proton therapy protocol involving CT simulation with full (CT_F) and empty (CT_E) bladders and weekly/biweekly on-treatment scans. IMRT plans were generated for comparative analysis. SB was contoured as bowel loops or bowel bag. Wilcoxon signed-rank tests were used for matched-pair comparisons of SB, bladder, and rectum dose-volumes between CT scans and between PBS and IMRT plans. <b>Results</b> In PBS loops analysis, on-treatment DVH was significantly higher than CT_F for doses &lt;45 Gy (<i>p</i> E. Specifically, V15 for loops was higher on-treatment (median 240 cm³) compared to CT_F (median 169 cm³, <i>p</i> = 0.03). In PBS bag analysis, on-treatment DVH was not significantly different from CT_F across all dose ranges. Bowel bag V45 was not significantly different between on-treatment (median 540 cm³) and CT_F (median 499 cm³, <i>p</i> = 0.53). Decreasing bladder volume was associated with increasing V15 for loops and V45 for bowel bag (<i>p</i> F. <b>Conclusions</b> SB DVHs were well estimated by CT_F bowel bag and underestimated by CT_F loops in the setting of inconsistent bladder filling. Verifying bladder filling prior to treatment or using CT_E for planning may more conservatively estimate SB dose-volume relationships.