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Objective
Goal of our investigation is to evaluate, whether an observed central area of unusual low dose and shape can affect clinical use and common absolute dosimetry procedures for the considered surface applicator set.
Materials and Methods
Surface dose measurements of all applicator inserts were performed with Gafchromic EBT3 films (Ashland Inc. Covington, KY). Generation of the calibration curve and dosimetric evaluation was performed with the software PTW Verisoft 4.2.2.0 (PTW Freiburg, Germany). The term “therapeutic dose” is defined as the 85% level of prescribed dose in this work.
Results
An underdosed area unusual for vertical type surface applicators is observed for all applicators. Mean size is 2.0 mm × 3.6 mm, situated 2.2 mm from applicator central axis, dosimetrically lower than 85% of nominal dose.
Conclusion
Since the under-dosage is situated in the central region and therefore in the gross tumor area, tumor control is possibly affected. In addition, when using small chambers for commissioning a considerable error can arise when the chamber is situated in the cold spot area. We therefore recommend film dosimetry to be always part of the commissioning process for vertical type HDR surface applicator sets. We recommend changing the applicator orientation between different fractions and splitting hypo-fractions into multiple sub-fractions with varied application orientation to avoid permanently under-dosed surface areas.