Problems and uncertainties with multiple point A's during multiple high-dose-rate intracavitary brachytherapy in carcinoma of the cervix.

Abstract

This study evaluates the consequences of point A as a dose prescription point during multiple high-dose-rate (HDR) intracavitary brachytherapy (ICBT) in cancer cervix. Fifty patients who had received teletherapy were randomised into two groups of 25 to receive three HDR ICBT fractions of 6 Gy each at point A with either a flexible Ralstron (Shimadzu Corporation, Japan) or rigid Rotterdam (Nucletron, Netherlands) applicator. The orthogonal radiographs of the 150 applications were evaluated for applicator geometry and point A co-ordinates. Irrespective of the nature and rigidity of the applicators, its various components exhibited a highly significant variation during multiple fractionated HDR ICBT. The Cartesian co-ordinates of point A (left and right) for the applicator geometry also showed significant variation during multiple HDR ICBT procedures. This resulted in an average shift of 9.5 mm (SD= +/-4.4) and 11.1 mm (SD= +/-6.4) in right point A, 10.2 mm (SD= +/-4.5) and 10.8 mm (SD= +/-6.6) in left point A for Ralstron and Rotterdam applicator, respectively, during the three HDR ICBT. Consequently, doses to both right and left point A's showed significant variation during multiple ICBT application and were independent of the applicator type. Applicator variation in the components and spatial position in the pelvis during multiple HDR ICBT results in multiple point A's irrespective of the nature of applicator, leading to uncertainty in the dose prescription. These uncertainties, which have a bearing on clinical end points, could be minimised by shifting from point-based dose prescription to image-based target localisation and treatment planning in ICBT.