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Post Treatment Dosimetry Using Applicator-To-Applicator Rigid Registration Followed by Adaptive Contouring in Intravaginal High Dose Rate Brachytherapy

S Park

S Park*, J Neylon , M Hagio , J Raince , D Demanes , UCLA, Los Angeles, CA


TU-C3-GePD-T-6 (Tuesday, August 1, 2017) 10:30 AM - 11:00 AM Room: Therapy ePoster Lounge

Purpose: To evaluate the feasibility of post treatment dosimetry using rigid registration of multichannel vaginal cylinder (MCVC) applicator in high dose rate (HDR) gynecologic brachytherapy.

Methods: 36 plans for 8 patients treated with intravaginal HDR brachytherapy using MCVC applicators (Elekta/Nucletron) were retrospectively reviewed. For initial treatment plan, patient specific vaginal wall target, applicator, and organs at risk (OAR) were delineated on a CT. 7-9 channels were reconstructed manually. Inverse planning simulated annealing optimization was used to deliver prescription dose in 2-7 fractions to the target while minimizing OAR doses. The initial plan was used for subsequent treatments. Prior to treatment, applicator placement (the depth of insertion and cylinder rotation) and organ deformation were evaluated with pre-treatment verification CT scans. The CT was then fused with the initial CT using applicator-to-applicator rigid registration. Isodose distribution and contours were transferred from the initial CT to the pre-treatment CT. The contours were modified to adapt to the organ motion. We evaluated interfractional normal tissue dose variation with D0.1cc, D1.0cc, and D2.0cc to OAR and compared statistical significance using a paired two-tailed t-test.

Results: Percent change of D0.1cc, D1.0cc and D2.0cc was 1.2±11.8% (-14.4 – 45.5%), -0.1±6.2% (-10.9 – 17.6%), and -0.5±5.1% (-9.8 – 11.7%) to the bladder, -2.7±8.2% (-19.0 – 10.1%), -1.2±6.3% (-10.9 – 9.2%), and -1.5±6.1% (-11.1 – 8.7%) to the rectum, 21.8±29.1% (-30.9 – 78.0%), 14.5±20.6% (-17.5 – 60.0%), and 13.8±16.6% (-10.1 – 51.7%) to the bowel, respectively. Interfractional dose variation to the bladder and rectum was not significant (p = 0.1-0.94), but the dose variation to the bowel was statistically significant (p = 0.001).

Conclusion: This study demonstrated that applicator-to-applicator rigid registration followed by adaptive contouring provides a simple, fast, and accurate post treatment dosimetry. Clinical follow-up is necessary to determine toxicity correlated to the interfractional dose variation to OAR.

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