engleski

Dosimetry studies in Gamma Knife Radiosurgery - contribution of RBI and UHC collaboration

Gamma Knife (GK) radiosurgery is highly precise method for treatment of small intracranial lesions with high single radiation dose. Leksell Gamma Knife unit Model C (Elekta AB, Stockholm, Sweden) contains 201 fixed 60Co gamma sources arranged in a concave half-spherical surface. Narrow photon beams are collimated and focused at a single focus (isocenter) by fixed primary and exchangeable secondary collimator system. Secondary collimator system consists of four exchangeable collimator helmets that produce almost spherical dose distribution of diameter 4, 8, 14 or 18 mm around isocenter. In order to conform radiological dose distribution to the size and shape of a tumour volume and to spare neighboring critical healthy tissues, clinical treatment plans are usually built with more than one collimator helmet, and for each helmet more than one isocenter. Dosimetry group from Rudjer Boskovic Institute (RBI) has a long term collaboration with GK group from University Hospital Centre (UHC) Zagreb where the only one GK unit in Croatia operates. In our focus were out-of-field doses due to their correlation with second cancer risks but also dosimetry of narrow GK photon beams and improvements of GK treatment. Within first task concerning out-of-field doses radiophotoluminescent and thermoluminescent dosimeters were used to evaluate organ and surface doses which are implemented for cancer risk estimate. In addition, comparison of different GK collimators and evaluation of the automatic system for repositioning of the patients head during treatment in correlation with tumour dose were investigated. Within second task concerning dosimetry of narrow GK beams, spatial and angular response of several types of detectors (PinPoint, Semiflex, ionization chambers, p-type semiconductor detectors and radiochromic film) were investigated and corrections of measured GK output factors were evaluated. Summary of our dosimetry studies in GK radiosurgery will be presented.

Gamma Knife, dosimetry studies, out-of-field organ doses

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