Dose distribution for gynecological brachytherapy with dose accumulation between insertions: Feasibility study

Purpose: For gynecological treatments, it is standard to acquire CT images and preferably also MR images before each treatment to calculate the dose of the day. The dose of the complete treatment is calculated by adding the dose metrics of each fraction. It makes the conservative assumption that the same part of the organs at risk always receives the highest dose. The dose calculated this way often limits the prescription dose or the target coverage. We investigated the use of deformable image registration (DIR) as an alternative method to assess the cumulative dose for a treatment course.

Methods and materials: Rigid registration is preformed on CT images, followed by DIR. DIR can be based either solely on the three-dimensional images or combined with organ contours. To improve DIR in the pelvic region with low CT contrast, we propose (1) using contours drawn on CT or (2) modifying artificially the contrast in certain volumes. The dose matrix from fraction_n (n > 1) is deformed using a calculated deformation field.

Results: The use of the contrast-enhanced images or of contour information helps to guide the DIR. However, because of the very high dose gradients involved in brachytherapy, the uncertainty on the accumulated dose remains of the order of 5-10%. Even for good contour matching, a small local error in the deformation can have significant consequences for the dose distribution.

Conclusions: Using DIR, based on image features and contours, allows to accumulate the dose from different brachytherapy fractions. A robust validation procedure should be developed.