Dosimetric Comparison of Tandem and Ovoids Versus Tandem Alone Intracavitary Applicators for Treatment of Locally-Advanced Cervical Cancer

Abstract

Standard therapy for locally advanced cervical cancer includes intracavitary brachytherapy. Tandem and ovoids (TO) are traditionally used in the US, but when the vaginal fornices are obliterated by tumor, ovoid placement is difficult. Practice patterns surveys in Canada reveal 9% of practitioners there use tandem only. This study compares the dosimetry of TO versus tandem (T) alone insertions with respect to the dose to ICRU points A, B, bladder and rectum, volumetric dose to bladder and rectum and volume of tissue receiving various dose gradients. One hundred eight brachytherapy cases from 27 women with stage II-IV cervical cancer were treated consecutively for cervical cancer with HDR brachytherapy with 4 applicator insertions. The prescribed dose of each insertion was 7 Gy to ICRU point A with additional goals of minimizing bladder and rectal doses. Patients were treated with either TO or T alone at physician discretion based on the clinical evaluation of the vaginal fornices at the time of the procedure. Parameters compared were ICRU point A and B doses, volume receiving 95%, 85%, 50%, and 20% of the prescription dose, ICRU bladder and rectal point doses, and maximal dose to 2 cc of bladder and rectum (D2 cc). The Mann-Whitney U test was used to compare these parameters. The mean dose to ICRU point A was not significantly different between the TO and T applicators, 681 cGy vs 670 cGy (p = 0.09). The mean dose to ICRU point B was significantly higher with the TO applicator than T alone, 200 Gy vs 177 cGy (p = 10-7). Dose distribution was different between the two applicators, with significantly increased volume receiving 95%, 85%, 50%, and 20% of the prescription dose using the TO applicator. The mean V95 was 109.5 cc for TO vs 96.4 cc for T (p = 0.001) and the mean V20 was 1041.7 cc for TO vs 923.9 cc for T (p = 0.009). There was no significant difference in ICRU rectal point doses between the applicators, 361.8 cGy TO vs 375.1 cGy T (p = 0.7). However, there were significantly increased doses to the ICRU bladder point, and D2 cc of the rectum and bladder with the T alone. The bladder point dose was 294.9 cGy TO vs 360.6 cGy T (p = 0.02), bladder D2 cc 403.7 cGy TO vs 632.7 cGy T (p = 0.001) and rectal D2 cc 289.7 cGy TO vs 224.8 cGy T (p = 0.01). TO and T applicator insertions for cervix brachytherapy provide equivalent doses to the ICRU point A, however, the volumetric distribution is significantly different between the 3 channel applicator and single channel tandem. The TO applicator provides increased volumetric dose within the patient, but allows better sparing of the rectum and bladder. In trained providers, T may be considered when the vaginal fornices prohibit appropriate placement of ovoid applicators to provide equivalent Point A doses.