Variation in High-Dose-Rate Brachytherapy Dose Contribution Among Pelvic Lymph Node Groups in Locally Advanced Cervical Cancer

Purpose/Objective(s): The standard treatment for locally advanced cervical cancer (LACC) is concurrent chemoradiation therapy followed by brachytherapy. However, there is institutional variation of the optimal radiation prescription dose to metastatic pelvic lymph nodes (LNs) in LACC. Previous work from our group showed high dose rate (HDR) brachytherapy contributed about 7% of the total Equivalent Dose in 2-Gray Fractions (EQD2) of external beam radiation therapy (EBRT) and HDR brachytherapy. Due to the location in the pelvis and subsequent distance from the implant, LN groups receive differing amounts of brachytherapy contribution. In this study, we investigate a hypothesis that there is variation in HDR brachytherapy dose contribution among anatomical pelvic LN groups in LACC. Materials/Methods: Twenty-one patients with 45 positive pelvic LNs (9 common iliac (CI), 15 external iliac (EI), 12 internal iliac (II) and 9 obturator (Ob) LNs) treated in two institutions from Oct 2007 to Aug 2011 were included in this retrospective analysis. All patients received EBRT to the pelvis with a supplemental boost to the involved pelvic node, plus HDR brachytherapy. Pathologically involved LNs were contoured on the planning EBRT image as well as the 4 to 5 brachytherapy planning images. The mean received dose of each LN from the EBRT and brachytherapy plans was recorded and EQD2 was calculated. A one-way Analysis of Variance (ANOVA) test was performed to determine if the mean brachytherapy EQD2 was significantly different among 4 pelvic LN groups. Results: The average prescribed doses from the EBRT, including the initial pelvic fields and boost contribution to CI, EI, II and Ob LNs, were 54.60Gy, 54.53Gy, 53.15Gy and 54.42Gy respectively. The average prescribed HDR doses to International Commission on Radiation Units and Measurements (ICRU) point A were 26.83Gy, 27.84Gy, 29.79Gy and 28.49Gy accordingly. The average dose delivered to CI, EI, II and Ob LNs were 53.19Gy, 55.14Gy, 53.26Gy and 55.10Gy (EBRT), and 2.65Gy, 4.31Gy, 5.46Gy and 5.77Gy (HDR) respectively, with the corresponding EQD2 of 52.26Gy, 54.36Gy, 52.42Gy and 54.42Gy (EBRT), and 2.36Gy, 4.00Gy, 5.09Gy and 5.47Gy (HDR). The HDR contribution to CI, EI, II and Ob LNs was 4.10%, 6.93%, 8.83% and 9.48% of the total EQD2 (EBRT+HDR, 57.69Gy) of all LN groups respectively. There was a statistically significant difference in brachytherapy EQD2 among the 4 pelvic LN groups (p < 0.05), with the Ob LN receiving the most dose. Conclusions: Our study highlights the 4.1% to 9.5% variation in brachytherapy dose contribution of the total EQD2 among pelvic LN groups. This difference in HDR contribution needs to be considered when prescribing EBRT boost dose to each pelvic LN group for the optimal therapeutic total dose. Author Disclosure: Y. Lee: None. D.L. Rash: None. R.L. Stern: None. M. Mathai: None. D. Deer: None. F. Wang: None. J.S. Mayadev: None.