(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To present a comprehensive method for improved dosimetry in interstitial HDR brachytherapy (IS-BT) for cervix treatments. To quantify the benefit of the proposed method.
Methods: IS-BT plans for treatments of cervix are created post-delivery of 45 Gy in the initial course of external beam (EB) photon therapy. The prescription dose for the HDR treatment is usually 27.5Gy in 5 fractions. IS-BT plan construction required that the tumor D90 should receive the highest possible total dose while the maximum D2cm3 dose constraints to the rectum, bladder and sigmoid (65, 80 and 65 Gy) are respected. The EQD2 calculation is performed prior to delivery of brachytherapy so that the total dose from both courses could be evaluated. EB plans do not include brachytherapy relevant structures and the dose to D2 cm3. Therefore, the IS-BT plans are limited by the quality of the EB plans. In this study, we re-planed and optimized five random EB plans including brachytherapy relevant structures and volumes in order to evaluate any possible increase of the dose to the high-risk-CTV (HR-CTV) and decrease the dose to the organs-at-risk (OAR).
Results: Re-planning of the EB plans resulted in an average dose decrease of 2.1%, 6.4%, 5.9%, and 3.2% to D2cm3 of rectum, bladder, sigmoid and small bowel, respectively, while the same level of dose to the PTV (100 % of the prescription dose to at least 95% of the target volume) was maintained. These dosimetric improvements resulted in a possibility to deliver additional 2-4Gy while the OAR dose was maintained within constraints.
Conclusion: The proposed methodology clearly shows benefits for certain patients. In such cases, the dose to the OAR is not a limited factor to the total dose to HR-CTV. The increased dose to the HR-CTV could potentially improve patient treatment outcomes, which will be studied further.
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