Evaluating Dose Summation in Gynecological Brachytherapy

Purpose Gynecological malignancies present challenges in determining an appropri- ate volumetric dose due to the highly variable physiologic activity of the surrounding tissue. Because of the high doses used in brachytherapy, surrounding structures have the potential to move around in the dose region and receive unknown amounts of radiation. Deformable image registration could overcome challenges in determining the true delivered dose through a dose accumulation process. This study uses two dose summation techniques to determine the efficacy of a deformation registration for the Syed applicators and cylinder applicators.

Methods and Materials Data for patients treated with a vaginal cylinder or Syed template were imported into the MIM software (Cleveland, OH). The bladder, rectum and applicator were contoured on each CT. The deformable registration was applied to structures only by masking other image data to a single intensity with the purpose of focusing the registration on the high dose area, as well as to minimize any uncertainty from the CT data. The deformable registration flow consisted of the following steps: 1) Using a different Hounsfield Unit, the CTs were masked so that each of the structures-of-interest (SOIs) had one unique intensity value; 2) Perform a rigid registration between two image sets with alignment based on the applicator position; 3) Perform a deformable registration; 4) Refine registration by using local manual alignment in area with large contour changes; 5) Repeat steps 1 to 3 to register the desired structure from all the subsequent fractions to the first fraction structure; 5) Transfer each deformed contoured to the first CT. The deformed structure accuracy was measured by a comparison to the first fraction using the dice similarity coefficient. Two dose summation techniques were investigated: a) deform the desired structure to one fraction and determine the fractional dose to these new structures, and b) deform the fractional dose to one CT and accumulate to find the total dose. Point doses, D2cc, were used as a comparison value for each method.

Results The cylinder set of patients had a DSC ranging from 0.82 to 0.96 for bladder and 0.82 to 0.94 for rectum. The contour deformation addition method has variations up to 23% from the initial clinical point dose for the cylinder applicators. The dose deformation accumulation method gave up to a 7.1% difference from the clinical point dose. The Syed applicator patient set has DSC ranging from 0.53 to 0.97 for the bladder and 0.75 and 0.95 for the rectum. These registrations' dose additions varied up to 34.78% and the dose deformation accumulation varied up to 34.97%.

Conclusions With the changing anatomy in brachytherapy, deforming the dose with the end point of dose summation leads to different volumetric doses then when dose is recalculated on deformed structures, raising concerns about the accuracy of the deformed dose. Dice Similarity Coefficients alone cannot be used to establish the accuracy of a deformation for brachy dose summation purpose.