K.J. Mehta

Do changes in interfraction organ at risk volume and cylinder insertion geometry impact delivered dose in high-dose-rate vaginal cuff brachytherapy?

PURPOSEnWithin a multifraction high-dose-rate vaginal cuff brachytherapy course, we determined if individual variations in organ at risk (OAR) volume and cylinder insertion geometry (CIG) impacted dose and whether planned minus fractional (P - F) differences led to a discrepancy between planned dose and delivered dose.nnnMETHODS AND MATERIALSnWe analyzed vaginal cuff brachytherapy applications from consecutive patients treated with three fractions of 5 Gy after each undergoing a planning CT and three repeat fractional CTs (fCTs). Rectal and bladder D2ccs and volumes were recorded in addition to the x (in relationship to midplane) and y (in relationship to the table) angles of CIG. Paired t-tests and multiple regression analyses were performed.nnnRESULTSnTwenty-seven patients were identified. In comparing the planning CT vs. mean fCT rectal volumes, bladder volumes, x angles, and y angles, only bladder volume was significantly different (planned volume higher, t = 2.433, p = 0.017). The cumulative mean planned OAR D2cc vs. delivered D2cc was only significantly different for the bladder (planned dose lower, t = -2.025, p = 0.053). Regression analysis revealed planned rectal D2cc (p < 0.0003) and a positive (posterior) y insertion angle (p = 0.015) to significantly impact delivered rectal D2cc. Additionally, P - F rectal volume (p = 0.037) was significant in determining rectal delivered dose.nnnCONCLUSIONSnA more posterior y angle of insertion was found to increase rectal D2cc leading us to believe that angling the vaginal cylinder anteriorly may reduce rectal dose without significantly increasing bladder dose. Although attention should be paid to OAR volume and CIG to minimize OAR dose, the clinical significance of P - F changes remains yet to be shown.

Contouring and Constraining Bowel on a Full-Bladder CT Scan as per RTOG Protocol Guidelines May Not Reflect Bowel Position and Dose Certainty in Gynecological EBRT

uf076 FB treatment plans were applied on EB CTs for determining bowel dosevolume changes due to empty bladder. From the resulting dose distributions, EB bowel dose-volume histogram (DVH) data were obtained and compared with respective FB treatment plan bowel DVHs. From these sets of DVHs, biological indices (generalized Equivalent Uniform Dose (gEUD) and Normal Tissue Complication Probability (NTCP) were calculated and compared between FB and EB. NTCP calculations were based on LKB Model (TCD 50 = 55 Gy; m=0.16, n=0.15).