001007 Holloway et al Dose to the sigmoid in cervi

Slide1: 

SIGMOID DOSE DELIVERED BY HIGH-DOSE-RATE BRACHYTHERAPY VERSUS LOW-DOSE-RATE BRACHYTHERAPY FOR CERVICAL CANCER Caroline L Holloway, MD, Desmond A O’Farrell, CMD, Robert A Cormack, PhD, and Akila N Viswanathan, MD, MPH, Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA Background 2D planning for low dose rate tandem and ovoid (LDRT/O) brachytherapy in cervical cancer is based on reference points for both the prescription point (Point A) and the dose to the rectum and bladder (ICRU rectal and bladder points). Dose to the sigmoid with LDR or HDR tandem and ovoid is unknown. 3D CT planning allows visualization and dosimetry of the sigmoid. High dose rate tandem and ovoid/ring (HDRT/O) treatment planning permits optimization of dose to the sigmoid by varying dwell times. Purpose To compare the dose to the sigmoid between HDR and LDR cervical brachytherapy with a central tandem in place. Materials and Methods 26 patients were treated for cervical cancer between 04/2004 – 04/2006 at the Brigham and Women’s Hospital with 3D planning techniques. 79 HDR fractions were delivered to 16 patients with tandem and ovoid (10 patients) tandem and ring (4 patients) or both (2 patients). 16 LDR fractions were delivered to 10 patients with LDRT/O (4 patients), LDR interstitial (LDRT/INT) (5patients) or both (1patient). The dose range for HDR was: 24 – 27.5 Gy in 4 – 5 fractions, LDR: 39.7 – 50 Gy (1 – 2 fractions at 0.4 –0.8 Gy/hr). Post implant contours of the sigmoid were made and dose volume histograms (DVH) generated for the D0.1 cc, D1 cc, and D2 cc. 3D manual optimization of the HDR plans kept the D2cc dose to the sigmoid, bladder and rectum to < 80% of the prescribed dose. Equivalent doses were calculated using a BED formula normalized to 2 Gy/fraction with an /3 (EQ2/3) (Formula 1). A two tailed t-test compared the dose to the sigmoid: HDRT/O vs LDRT/O and LDRT/INT Results Median tandem length for HDR was 6 cm (range [rg.] 5 -6 cm), LDR 6cm (rg. 5 – 7 cm). 17% (14/79) of the HDR fractions did not utilize the top dwell positions. The median sigmoid D0.1 cc EQ2/3 was significantly higher in the HDR fractions compared with the LDR (Table 1). There was no significant difference between the HDR or LDR D1 cc or D2 cc EQ2/3 (Table 2). The dose to the D2cc in both the HDR and LDR was approximately 80% of prescription dose. Conclusions The point dose to the sigmoid as estimated by the D0.1cc is significantly higher in HDR treatments. There is no significant difference in the sigmoid D2 cc between standard HDR and LDR treatments for cervical cancer when the HDR plans are optimized to keep the sigmoid < 80% of the prescribed dose. Long-term follow-up is necessary to determine whether the dose to a 0.1 cc or 2cc volume is predictive of complications to the sigmoid. Table 1: Sigmoid D0.1 cc EQ2/3 Table 2: Sigmoid D1 cc and D2 cc EQ2/3 HDR = D 1+ d/α/β3 1+ 2Gy/α/β3 LDR = D 1+ 2* R μ*α/β3 μ= 0.46 R= dose rate Formula 1: BED normalized to 2 Gy/fraction with an /3 (EQ2/3) Figure 1: LDRT/O isodose distribution in relation to the sigmoid colon Figure 2: HDRT/O isodose distribution in relation to the sigmoid colon