Characterizing the impact of adaptive planning on image-guided perineal interstitial brachytherapy for gynecologic malignancies.

ABSTRACT

PURPOSE: To determine the dosimetric impact of organ and implant motion/deformation in the context of adaptive planning in image-guided gynecologic brachytherapy using a 3-fraction transperineal approach. METHODS AND MATERIALS Twenty-six patients were analyzed. Each patient was treated with three fractions given over a 24-h period using a single insertion. A planning CT scan (±MRI) was acquired before the first fraction. A verification scan was taken within 1 h following the second fraction. A single plan was delivered for Fractions 1 and 2 with an adaptive plan delivered for Fraction 3. Two evaluation frameworks were established. Framework 1 investigated the effects of motion/deformation from both implant and organs. Framework 2 investigated the impact of implant motion/deformation alone. Differences in high-risk clinical target volume (HRCTV) D90%, V100%, and bladder/rectum D2cc were assessed. RESULTS: From implant to verification, the HRCTV D90% and V100% decreased significantly (5.0%, p < 0.01; 3.1%, p < 0.01) and rectal D2cc was significantly higher (12.2%, p = 0.02). Adaptive planning recouped these changes. Implant changes resulted in a reduction in HRCTV dose and coverage, but no significant effect was seen in the bladder or rectum. CONCLUSIONS: Adaptive planning represents an important aspect of perineal-based interstitial image-guided brachytherapy given in three fractions; its absence would result in plan degradation.