Session Item

The LDR prostate brachytherapy procedure has recently been modified at our institution, due to changes in the seed model and type of strand used. The aim of this work is to describe the adopted program performed ahead of its clinical implementation, including source data validation, TPS commissioning and end-to-end procedure test.

The established program involved four main steps:

1) The template definition in the brachytherapy software of the US device and its calibration against the physical template, used to insert the needles, and the TPS grid.

2) The validation of the source specification data (Bebig Model I25.S17 plus I-125) available in Variseed TPS v.9.0 (Varian), through comparison to the consensus datasets provided in AAPM TG 43;

3) The completion of a set of tests to assess the TPS dose calculation accuracy and plan evaluation tools.  These tests included, first, the verification of the dose calculated by TPS for a single source and multiple sources, against reference values provided in AAPM TG 43, and manual calculations using the 2D formalism. Then, the assessment of the dose display by creating five isodose lines and checking if they appeared in the correct location. Finally, the evaluation of the accuracy of the volume and DVH calculation by considering objects with known dimensions;

4) The performance of an end-to end test, using the prostate phantom model 053S (CIRS), to simulate the entire process of a typical LDR prostate treatment, from US image acquisition, to treatment planning and seeds implant. This step aimed at reducing the time spent in the operating room by defining and training the workflow to be adopted.

The source data provided by the vendor, including the dose rate constant, the radial dose function, the anisotropy function, active length and half-time, was matched to the consensus datasets.

The dose values calculated by TPS were in close agreement (<2%) with the reference values provided in TG 43 and manual calculations, for various positions (up to 4 cm) along and away of the source. The correct sum of the dose from two sources was also verified (difference <2%), for points at multiple locations. The isodose lines test confirmed the correct dose display. A good consistency between DVH data and dose display was also checked. The difference between the structure volume calculation and the known value was less than 1%, for the considered geometrical shapes.

The end-to-end test went through smoothly, being the whole process successfully simulated. The correct calibration of the templates was confirmed, the continuous image acquisition was performed through the US transversal probe, the contours and the source placement tools were tested and the seeds implant was checked in live mode.

A comprehensive quality assurance program for implementation of LDR prostate brachytherapy procedures has been established, and will be used for QA or training purposes.