Genitourinary Cancer

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SU_28_2288 - Effect of Margin Size and Image Guidance Method on Biochemical Failure and Toxicity in Prostate Cancer Treated with IMRT

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

Effect of Margin Size and Image Guidance Method on Biochemical Failure and Toxicity in Prostate Cancer Treated with IMRT
S. Kost, C. A. Reddy, K. Bittinger, K. L. Stephans, R. D. Tendulkar, and P. Xia; Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): Intensity modulated radiation therapy (IMRT) has improved the ability to safely deliver high-dose conformal radiation. Dose escalated IMRT has led to an improvement in biochemical relapse free survival (BRFS) of prostate cancer (CaP). Furthermore, the use of image guidance for setup verification has improved target positioning resulting in reduced PTV margins. The aim of this study was to evaluate the effects of image guidance method and margin size on BRFS and risk of normal tissue toxicity in CaP patients treated with IMRT.

Materials/Methods: We identified 204 consecutive patients with CaP treated between 2006 and 2013 with definitive intent radiation to 78 Gy in 39 fractions from an IRB approved registry. Treatments were delivered with three types of image guided radiotherapy (IGRT): ultrasound (US), cone beam CT (CBCT), and electromagnetic tracking. Dosimetric data was extracted from the medical record, and an “average margin” was identified by considering a spherical prostate volume and relating CTV:PTV ratios using the radius cubed. We then used Cox proportional hazards regression to examine the correlation of patient, tumor, and treatment factors including margin size and IGRT type on BRFS. Toxicity was assessed using Fine’s test and Fine and Gray regression. BRFS was defined using the Phoenix nadir+2 definition, and toxicity was scored using CTCAE v.4.0 criteria.

Results: The median follow up was 76 months. NCCN risk group distribution was skewed to higher risk patients: 3.9% low, 45.1% intermediate, and 51% high. ADT was administered to 64.7% of patients for a median duration of 6 months. The mean PTV margin for all patients was 8.4 mm (range: 4.8–10.1 mm). The mean PTV margin for was 8.8 mm, 8.4 mm, and 7.4 mm for US, CBCT and electromagnetic tracking respectively (p<0.0001). The 5-yr BRFS rates by IGRT type were 83%, 75%, and 77% for US, CBCT and electromagnetic tracking respectively (p=0.43). On univariate analysis, no factor was correlated with BRFS: iPSA (p=0.50), biopsy Gleason Score (p=0.37 7 vs 6, p=0.26 8-10 vs 6, and p=0.65 8-10 vs 7), clinical stage (p=0.91), ADT (p=0.85), IGRT type (p=0.43) and margin size (p=0.09). The 5-yr cumulative incidence rates of late Grade >3 genitourinary (GU) toxicity by IGRT type were 6%, 8%, and 3% for US, CBCT and electromagnetic tracking respectively (p=0.64). The 5-yr cumulative incidence rates of late Grade >3 gastrointestinal (GI) toxicity by IGRT type were 2%, 10%, and 12% for US, CBCT and electromagnetic tracking respectively (p=0.06). Similarly IGRT type was not predictive of late Grade ≥2 GI (p=0.28) or GU toxicity (p=0.13). Margin size was not correlated with toxicity either (p=0.13 for GI grade ≥ 2, p=0.17 for GI grade > 3, p=0.38 for GU grade ≥ 2, and p=0.27 for GU grade > 3).

Conclusion: The treatment margins in this study were adequate for target coverage and did not correlate with rates of BRFS or toxicity. Similarly, the choice of image guidance modality did not effect BRFS or rates of GU/GI toxicity.

Author Disclosure: S. Kost: None. C.A. Reddy: Consultant; IJROBP. K. Bittinger: None. K.L. Stephans: None. R.D. Tendulkar: None. P. Xia: Employee; Cleveland Clinic. Research Grant; Philips Healthcare.

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