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Dosimetric Impact of Changing Tandem and Ring Prescription From Point A to Volumetric Prescription of HRCTV for Locally Advanced Cervical Cancer

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A Powers

A Powers*, R Sheu , V Gupta , Mount Sinai Medical Center, New York, NY


SU-I-GPD-T-56 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In order to safely change the institutional practice from prescribing to point A to prescribing to a CTV for locally advanced cervical cancer (LACC) it is imperative to thoroughly understand the dosimetric impact this will have on targets and adjacent normal structures.

Methods: 10 LACC patients received tandem/ring HDR brachytherapy prescribed to point A, accompanying a course of pelvic EBRT. The ‘high risk’ CTV was retroactively contoured on a planning CT using ABS consensus guidelines for one fraction of each of these patients, as well as all three HDR treatments of the most recent patient. The GEC-ESTRO dose volume parameters were generated for the critical OAR’s and HRCTV, and the plans were renormalized such that the HRCTV had a D90 approximately equal to the prescription dose and a V100 greater than 90%. The dwells were manually set for all cases. In all plans, the EBRT and brachytherapy combined EQD2 for 2cc of the rectum, sigmoid, and bowel, were kept below 75Gy and 2cc of the bladder below 90Gy.

Results: In the original plans prescribed to point A, D90 of the HRCTV varied from 59.7% to 141.0% with an average variation of 112.3%±20.6% of the prescription dose. The average HRCTV volume was 32.4cc±14.3cc. For nine of the ten cases the re-normalization resulted in lowering the dose to point A from an average of 100% to 86.8%±9%. Prescribing to HRCTV for the most recent patient would have lowered the EQD2 to 2cc of the bladder, rectum, sigmoid, and bowel, by 8.5Gy, 8.9Gy, 11.6Gy, and 7.4Gy respectively over the course of treatment.

Conclusion: For most of the patients in this study, prescribing to point A results in a higher dose to HRCTV than is considered necessary by volumetric standards, so dose to the OAR’s can be lowered while maintaining sufficient HRCTV coverage.

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