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Purpose: For clinical use of the Varian Ir-192 HDR surface applicator set with vertical Leipzig-style cone the manufacturer-provided table of treatment times for a given dose at 5mm depth is verified and extended to surface doses. Goal of our work is to evaluate the optimal source position to obtain a maximum therapeutical area and a depth dose characteristic suitable for clinical use. Additionally it is determined whether certain well known shortcomings from comparable brachytherapy surface applicator sets apply to the Varian surface applicator set as well and how the influence of dose inhomogenity can be limited in clinical use.
Methods: The manufacturer provided time-dose table is verified with a calibrated PTW 0.3cc semiflexible ionisation chamber for two representative applicator inserts (20mm and 40mm diameter). For the extension of the 5mm reference depth dose in this table to surface dose a 0.02cc PTW soft X-ray chamber is used. High resolution 2D dosimetry is performed with Gafchromic film. Calibration of the films is performed in the reference geometry of the manufacturer with Ir-192 HDR irradiation with the 40mm diameter surface applicator. Dose is normalised in a region of interest similar to the projection of the 0.3cc ionisation chamber in the plane of measurement. For film scanning an Epson V700pro flat bed scanner in transparency mode is used.
Results: The manufacturer provided treatment time to dose table is well in agreement with our measurements, with a deviation smaller than 1%. Therapeutic area is defined as the area of the 90% isodose and found to be maximal with source position 10mm from the applicator tip for the applicator inserts 10mm to 25mm diameter, with a source position of 15mm from tip for all larger applicator inserts of the applicator set. A maximal therapeutic area on the surface is clinically given priority over a reduced depth dose resulting from using the 10mm position. In addition to known flatness problems of vertical type brachytherapy surface applicators a cold spot of size of approximately 3mm × 3mm with 70% of the nominal dose is found asymmetrically nearby the central axis of all applicators. Asymmetricity of the dose distribution presumably arises from a slightly tilted source in treatment position. When alternation of the direction of the afterloader cable is performed from fraction to fraction to force the cable to a different angle in treatment position and when a position deviation of the central axis of ±1mm per fraction is assumed, the central axis region dose is actually 90%, what we consider as clinically acceptable.
Conclusions: Therapeutic area can be optimized with diameter dependent source positions. To limit the influence of the cold spot in the target area in clinical use systematic alternation of the direction of the cable is performed from fraction to fraction. Design of flattening filters similar as has been done for comparable applicator sets has to be considered in future use to improve the dose distribution homogeneity.