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Volume 6, Number 1, 2001

Physical and dosimetric aspects of quality assurance in stereotactic radiotherapy

Rostkowska J, Kania M, Bulski W, Kawczyńska M, Fijuth J, Wysocka A.

Summary:

A quality assurance system in stereotactic radiosurgery and stereotactic fractionated radiotherapy, concerning the physical and dosimetric aspects, may be divided into three elements: (1) the preparation of reliable basic data for the computerized treatment planning system; (2) a control of the accelerator parameters prior to patient treatment; (3) preparation of the optimal treatment plan with the treatment planning system.

Due to the small size of the beams formed by circular collimators (7.5-35mm diameter, BrainLab System) the smallest available detectors should be used for measurements - a diamond diode (0.3 mm thickness) and a 0.015 cm3 ionization chamber (PTW Freiburg) are adequate to measure precisely TMR curves, beam profiles and output factors required for the treatment planning system BrainScan.

The full control of accelerator parameters (Clinac 2300 C/D) necessary to safely carry out the treatment requires a comprehensive list of tests (an extended list of weekly checks including Winston-Lutz test). Testing procedure carried out with a set of specialized devices (Med-Tec, Radak, BrainLab) takes about two hours. Proper accelerator check and regulations allow for very precise patient positioning.

Treatment planning (with the treatment planning system BrainScan) is based on a series of CT and MR scans with target volume and organs at risk marked on each slice by the radiotherapist. The planner has to select the positions of isocentres (up to 3), collimator diameters, number and range of the arcs. Additional parameters for optimization procedure are the total dose proportions delivered by each arc. The treatment plan evaluation is based on the analysis of DVHs for target volume and also for organs at risk (orbits, optical nerves, brain stem) in order to minimize the dose and volume irradiated. It was accepted that the dose uniformity factor, defined as a ratio Dmin/Dmax within the target volume, should be not less than 0.8, and should approach 0.9 as much as possible.

The above-presented system of quality control, specifying tolerance limits of controlled parameters, assures safe and precise dose delivery in stereotactic radiotherapy.

Signature: Rep Pract Oncol Radiother, 2001; 6(1) : 53-53


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http://www.sciencedirect.com/science/journal/15071367/19/2