Dosimetric Comparison Between Split Ring Applicator and Tandem and Ovoids for HDR Brachytherapy Boost in Cervical Cancer

Abstract

HDR brachytherapy boost for cervical cancer is commonly performed using tandem and ovoids (T&O). The split ring applicator (SRA) is a new delivery device that combines the tandem with two half rings, offering a wide range of implantation and source dwell configurations. To date, no published studies have investigated the dosimetric differences between the SRA and T&O applicators. Between 2010 and 2011, 15 patients (12 T&O, 3 SRA) with FIGO Stage IB1-IVA cervical cancer underwent definitive chemoradiation. Patients received whole pelvis EBRT to 45Gy ± parametrial boost to 50.4 - 54Gy. Following EBRT, patients received afterloaded Ir-192 HDR brachytherapy boost of 28Gy in 4 fractions prescribed to Point A. These were given over the course of 2 consecutive weeks, with 2 consecutive daily fractions given each week. Absolute doses were calculated for ABS reference points: A, B, P, Bladder and Rectum. Conversion to LDR equivalent doses was performed using the linear-quadratic formula with α/β = 10 for points A, B, P and α/β = 3 for Bladder and Rectum. Multivariate Analysis of Variance (MANOVA) was performed using the dose at each reference point as dependent variables and comparing SRA versus T&O as independent variables. MANOVA results using the Wilks' Lambda distribution demonstrate a statistically significant difference between the SRA and T&O groups, F(8,6) = 6.000, p = 0.021. Univariate results do not show a significant difference based on Point A dose: Point A-right F(1,13) = 1.144, p = 0.304; Point A-left F(1,13) = 0.155, p = 0.700. Similarly, no significant differences were seen with Points B, Bladder or Rectum. Point P-left did show statistical significance, F(1,13) = 6.847, p = 0.021, but Point P-right did not, F(1,13) = 0.717, p = 0.412. HDR brachytherapy using SRA versus T&O yields significantly different dose distributions based on the combined analysis of ABS points. However analysis of individual points does not demonstrate significant differences in Points A, B, Bladder or Rectum, suggesting clinical equivalence. Dosimetric analysis using non-ABS criteria may be able to better define the nature of the differences in SRA versus T&O, and long term follow-up of clinical outcomes on a larger cohort of patients is needed to define the clinical significance of any differences.Tabled 1Median HDR and LDR Equivalent Doses for ABS Reference PointsABS PointMedian HDR Dose (Gy)HDR Range (Gy)LDR Equivalent (Gy)LDR Range (Gy)A27.8324.28 - 30.0943.7036.15 - 48.83B6.934.77 - 12.367.544.94 - 15.1P5.633.63 - 9.485.963.67 - 10.87Bladder18.396.21 - 30.7246.889.49 - 109.42Rectum12.807.4 - 19.7426.5312.01 - 52.31 Open table in a new tab