Emma Dyce
Prince of Wales Hospital, University of Wollongong
ROMP registrar

Simon Downes
Chief Physicist
Prince of Wales Hospital

Background and Purpose: The Vaginal CT/MR Multi Channel Applicator Set (VMC) (Elekta Brachytherapy, Venendaal, Netherlands) consists of a cylindrical applicator with a central channel and multiple surrounding channels close to the applicator surface. This applicator is designed to give additional options for dose optimisation to the target and reduce dose to nearby organs at risk. A common practice with vaginal brachytherapy is to use standard plans of desired treatment lengths and diameters so a patient can undergo insertion and treatment delivery on the treatment bed without the need for imaging.

This work presents the commissioning and non-image based planning method developed at the Nelune Comprehensive Cancer Centre with the Oncentra planning system and Flexitron HDR unit (Elekta).

Methods:

Physical measurements and CT scans were used to test integrity and design specifications of each cylinder, as well as assess matching to applicator library files. Autoradiographs were performed on cuffed catheters to test the final source position. Dose verification measurements were performed with Gafchromic film.

Without a PTV contoured from CT or MR imaging, the PTV must be defined in terms of dose points that describe the boundaries of the PTV relative to the applicator, both in length and distance away from the applicator surface.

Using applicator library files in Oncentra, source positions defined in various channels can be used to calculate projected dose points needed to define a desired PTV surface required by the Radiation Oncologist. A simple spreadsheet was developed to perform this function which extrapolates the required dose points. Normal optimization to dose points can then be used to optimize coverage.

Results:

Physical dimensions and final dwell positions for the cylinders were found to match manufacturer specifications, and accurately match applicator library files. Integrity checks revealed no cracks or damage to any part of the applicators. All measured dose points were found to be within 10 % of calculated values.

A spreadsheet was created to efficiently identify dose points required to create a plan based on treatment length, depth and target location.

This method reduces the procedure time significantly since no imaging is required. Patients can be treated quickly, accurately and safely when OAR dose calculation is not required, as is usually the case.

Conclusions:

The VMC applicator system has been successfully commissioned for clinical treatment. Future work is planned to evaluate dosimetric differences between the VMC and other vaginal applicators.


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