Evaluation of quality indices during multifractionated pelvic interstitial brachytherapy for cervical cancer.

Purpose: To evaluate the impact of needle displacements on quality indices during multifractionated pelvic interstitial brachytherapy (IBT).

Methods And Materials: Patients undergoing boost IBT were included. Postprocedure planning and verification CT scans were obtained. Three-dimensional needle displacements were measured. Clinical target volume and organ at risk were delineated. Coverage index (CI), dose homogeneity index (DHI), dose nonuniformity ratio (DNR), V170, V200, V250, and dose received by 2cc of organs at risk were obtained at baseline. The displacements were simulated by shifting dwell positions, and dose point optimized and graphically optimized plans were generated. Wilcoxon test determined statistical significance of differences in quality indices.

Results: Fifteen patients were included and received five fractions of IBT over 3 days. Maximum displacements were observed in caudal direction (average, 19.1mm). At baseline, CI of 0.94 (range, 0.91-0.99), DHI of 0.90 (range, 0.80-0.94), and DNR of 0.10 (range, 0.05-0.10) were attained. The CI, DHI, and DNR in Day 3 dose point optimized plans were 0.76 (range, 0.4-0.99), 0.76 (range, 0.40-0.94), and 0.23 (range, 0.06-0.64), respectively. The difference in CI, DHI, and DNR between baseline and Day 3 dose point optimized plans was statistically significant (p = 0.002, 0.007, and 0.001, respectively). Day 3 graphically optimized plans were superior to Day 3 dose point optimized plans (CI, 0.82 vs. 0.76; p = 0.01). Graphically optimized could however improve CI without compromise in DHI, DNR, V170, V200, and V250 only in patients wherein caudal displacements ≤15mm.

Conclusions: Caudal needle displacements during multifractionated IBT cause significant deterioration of quality indices. Replanning with graphically optimized and/or needle repositioning maybe required for maintaining the quality of IBT.