A Randomized Crossover Trial to Compare Two Rectum Separation Techniques during High-Dose-Rate Intracavitary Brachytherapy with Tandem-Ring Applicators for Cancer Cervix

Continuous interactions among the radiation oncologists and physicists engaged in intracavitary brachytherapy (ICBT) operative procedures and radiotherapy treatment planning over the years was useful, especially in the understanding of treatment planning related confounding variables. The broad aim of prescribing the dose to point A and keeping the doses to organs at risk (OARs) within the range of their tolerance limits while recapitulating the classical pear-shaped dose distribution can be very well achieved for a particular ICBT fraction in 2-D planning workflow, whether one adheres to traditional Manchester or to Alternate loading pattern. But due to different geometric distribution of active dwell positions in two loading patterns, the percentage of prescribed (point A) dose that reaches the OARs (ICRU rectal and bladder points) may differ. Therefore, when two consecutive ICBT fractions performed on one patient which differ only with regard to organ separation techniques are to be compared on the OARs doses, the loading pattern should also be similar alongside other physical parameters of the applicators like ring size, tandem angle, tandem length and point A definition. Either Manchester or Alternate loading pattern was used to prescribe the dose to point A in all the ICBT fractions in this study. It is therefore important to elaborate on the definition of these two loading patterns we utilized in this study. Manchester loading pattern is derived from the Classical Manchester system which used intra-uterine rubber tubes of three different lengths, capable of taking in line either one, two or three radium tubes of 2cm overall length, depending upon the anatomy of a particular patient, to place radium sources in the uterus. Special containers made of rubber or plastic called ovoids were used in pairs inserted across the vagina, one in each lateral fornix at the level of the cervix. Ovoids of three different sizes were used depending upon the capacity of a particular patient, to place radium sources in the vagina. They were locked in position by a spacer which fixes them 1cm apart. The Manchester system devised loadings of each size of applicator so that, whichever combination of central uterine tube and pair of vaginal ovoids is used, the exposure rate at point A is the same, within a few percent of 57.5+/- 0.8 Roentgen per hour for standard arrangements. The tip side of intra-uterine tubes were densely loaded compared to the flange side, so that when either 6cm or 4cm intra-uterine tube was used in combination with either small, medium or large ovoids the dose rate of 57.5 R/hr at point A was achieved with around 65 % contribution from intrauterine sources and 35% contribution from the vaginal sources [1]. All ICBT implants included in this study were performed utilizing tandem-ring (TR) applicators. Treatment planning was done on treatment planning system (TPS) Oncentra Brachy (version 4.6.0). High-dose-rate (HDR) fractions were delivered with Microselectron HDRV3 (Nucletron BV, 18 channels) utilizing single moving Ir-192 source welded to the end of a flexible cable, through the selected channels. The source could be precisely positioned at any point in the implanted applicators by programming its dwell position and dwell time and desired isodose distribution could be obtained. Within this workflow, our Manchester loading pattern almost conforms to the above described basic principle and is defined as the loading pattern where in the utero-cervical canal, the dense loading towards the tip end is achieved by keeping 5mm interval (gap of one step when step size is 2.5mm) and sparse loading towards the flange end of central tandem by keeping 7.5mm interval (gap of two steps) between two adjacent dwell positions. An interval of 2.5mm or 5mm (no gap or a gap of 1 step) is kept between two adjacent dwell positions in the pair of four or six lateral loadings within the ring placed in the vaginal fornices surrounding the external os, when dwell times are constant for all dwell positions. For reference, please go through the loading patterns of patient no. 1, 3, 4, 6, 10, 13, 14, 17, 18, 19, 20, 21, 22, 23, 26, 27, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42, 43, 44, 45, 46 in the uploaded datasheet of this study. The alternate loading can be defined as the loading pattern where in the utero-cervical canal an interval of 5mm (gap of one step when step size is 2.5mm) is uniformly kept between the two adjacent dwell positions starting from the tip end to the flange end of the central tandem and an interval of 2.5mm or 5mm (no gap or a gap of one step) is kept between two adjacent dwell positions in the pair of four or six lateral loadings within the ring placed in the vaginal fornices surrounding the external os. For reference, please go through the loading patterns of patient no. 2, 5, 7, 8, 9, 12, 15, 16, 24, 25, 30, 41, 47 in the uploaded datasheet of this study. Both methods achieve the ratio of...