PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Delivery of high dose radiation within a prescribed period of time is associated with local control when treating primary or recurrent gynecologic cancers. A limited number of patients are ineligible for an interstitial brachytherapy boost. We update our experience using stereotactic body radiotherapy (SBRT) boost as an alternative in this context.
Materials/Methods: From 2012 to 2014 eight patients with locally advanced squamous cell carcinoma of the uterine cervix, recurrent endometrial cancer at the vaginal cuff, or primary vaginal cancer received an SBRT boost after pelvic external beam (EBRT) radiotherapy (45-50 Gy). One patient received SBRT after EBRT and two HDR brachytherapy fractions. Patients either refused brachytherapy or were unsuitable from medical comorbidities. Patients were immobilized using a CIVCO body frame (CIVCO medical solutions, Coralville, IA) with abdominal compression. Intravaginal and/or interstitial fiducial markers were used to localize tumor at simulation and treatment. Doses were prescribed to D90 of the PTV (5 mm expansion on CTV excluding rectum when not involved by tumor). Dose was limited by organ-at-risk tolerances (EQD2Gy D2cc planning objectives included bladder < 90 Gy, rectum < 75 Gy, sigmoid < 75 Gy, small bowel < 60 Gy). The treatment planning system was used to generate VMAT plans. Treatment was delivered using a medical linear accelerator. Daily cone beam CTs (CBCTs) were performed using rectum, bladder, visible tumor, and markers for image guidance and stereotactic x-ray was used to ensure precision of delivery. Tumor status and toxicities were recorded at regular follow up intervals; toxicity was graded according to CTCAE v. 4.0.
Results: Dose/fractionation schemes were 7 Gy x 2 for one patient (after two HDR brachytherapy fractions), 6 Gy x 5 for six pts, and 5.8 Gy x 5 for one pt. Cumulative EQD2Gy to D90 of the target volume ranged from 74.6 Gy to 84.3 Gy (mean 81.3 Gy). Mean D2cc rectum and bladder doses were 66.3 Gy (range 59.4 – 75.8 Gy) and 77.3 Gy (range 69.6 – 83.6 Gy) respectively. Mean overlap between rectum as contoured on daily conebeam CTs and the PTV was 0.29 cc (range 0.00 – 1.42 cc). At a median follow up of 55 months for survivors, two local recurrences have been noted. One patient recurred at 16 months and the other at 34 months. One of these patients went on to develop distant metastatic disease and one was salvaged with surgery and has no evidence of disease. One patient developed distant metastases at seven months. One patient developed a rectovaginal fistula requiring surgical intervention after developing distant metastatic disease at 2 years. Two patients developed grade 3 vaginal stricture (inability to visualize vaginal apex/cuff on exam).
Conclusion: Although treatment delivery is often technically feasible, SBRT should not be used as a substitute for brachytherapy boost unless there are absolute contraindications to interstitial brachytherapy delivery.
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