Interfraction Variation and Dosimetric Changes in Cervical Cancer Patients Treated with Intracavitary Brachytherapy

Abstract

Introduction
Intracavitary brachytherapy (ICB) is one of the cornerstones in the definitive management of locally advanced cervical carcinoma. Due to possible interfraction variation due to patient and tumor anatomy, placement and stability of brachytherapy applicator, the current standard of care is replanning every fraction. Some proponents suggested eliminating replanning to certain subset of patients to save waiting, sedation and manpower time especially in a resource limited setting.

Purpose:
We examined if there would be a significant change in the relative dosimetry if the source positions and calculated dwell times generated during the plan of the first fraction were applied to subsequent brachytherapy insertion using the orthogonal films taken for the subsequent fractions.

Materials and methods:
Retrospective review of charts and orthogonal films were done of patients treated with four fractions of ICB from 2007 to 2012. Institutional review board approval was obtained prior to commencement of the study. Eligible cases were patients with histologically confirmed non-metastatic, cervical carcinoma treated with definitive concurrent chemoradiation with at least four fractions of brachytherapy using Henschke applicators with same dose prescription to point A for all fractions. Uterine dilatation length should not differ by more than 1 cm among brachytherapy fractions. The same tandem curvature should have been employed for all four fractions. Dose on point A, point B, bladder and rectum point were collected from actual plans using ICRU defined points. Replanning to create a hypothetical plan on the first set of orthogonal plates were done and the source position and dwell time were subsequently applied to the remaining fractions using the orthogonal films of that fraction. Analysis of difference between actual plan vs hypothetical plan, and first fraction to the remaining fractions were done using Chi square test for independence, Wilcoxon-signed rank test and Friedmann test. P value <0.05 were considered significant.

Results:
Twenty nine patients were included in this study with a total of 116 plans via orthogonal films. Comparison of the actual plan and hypothetical plan was done on each fraction. The result showed that the cervical, rectal and bladder dose between actual plan and hypothetical plan were not significantly different (p values > 0.05).

In the hypothetical plan, the source activity and dwell time of the first plan were applied to the orthogonal films of second, third and fourth brachytherapy application. The mean doses to point A were 648cGy, 629cGy and 642 cGy for the second, third and fourth fractions respectively. In each case, the dose was not significantly different from that obtained in the first fraction (p>0.05). The mean doses to the cervix were 1443cGy, 1405cGy and 1484 cGy for the second, third and fourth fractions respectively. In each case, the dose was not significantly different from that obtained in the first fraction (p>0.05). The mean doses to bladder were 330cGy, 346cGy and 293 cGy for the second, third and fourth fractions respectively. Comparing each fractions to plan of first fraction, again there is no significant difference dose received to the bladder point (p>0.05). The mean dose to rectum was 297cGy, 295cGy and 285 cGy for the second, third and fourth fraction respectively. Comparing each fractions to plan of first fraction, there is no significant difference dose received to the rectum point (p>0.05).

Conclusions:
The result of this study showed the safety of using source position and dwell time of the first plan to subsequent fractions. The result of this study is investigational. Until further studies using volumetric planning are done, it is still recommended to perform replanning with every fraction whenever feasible.