Precision dosimetry for narrow photon beams used in radiosurgery—Determination of Gamma Knife^® output factors

Abstract

Treatment units for radiosurgery, like Leksell Gamma Knife and adapted, or dedicated, linear accelerators use small circular beams of ionizing radiation down to 4 mm in diameter at the isocenter. By cross-firing, these beams generate a high dose region at the isocenter together with steep dose gradients of up to 30% per mm. These units are used to treat small complex shaped lesions, often located close to critical structures within the brain, by superimposing several single high dose regions. In order to commission such treatment units for stereotactic irradiations, to carry out quality assurance and to simulate treatment conditions, as well as to collect input data for treatment planning, a precise dosimetric system is necessary. Commercially available radiation dosimeters only partially meet the requirements for narrow photon beams and small field sizes as used in stereotactic treatment modalities. The aim of this study was the experimental determination of the output factors for the field defining collimators used in Gamma Knife radiosurgery, in particular for the smallest, the 4 mm collimator helmet. For output factor measurements a pin point air ionization chamber, a liquid ionization chamber, a diode detector, a diamond detector, TLD microcubes and microrods, alanine pellets, and radiochromic films were used. In total, more than 1000 measurements were performed with these different detection systems, at the sites in Munich and Zurich. Our results show a resultant output factor for the 4 mm collimator helmet of 0.8741 +/- 0.0202, which is in good agreement with recently published results and demonstrates the feasibility of such measurements. The measured output factors for the 8 mm and 14 mm collimator helmets are 0.9578 +/- 0.0057 and 0.9870 +/- 0.0086, respectively.

Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors

“…Mack et al 35 performed a thorough examination and review of output factors of a Gamma Knife ® ͑Elekta Instrument AB, Stockholm, Sweden͒ using several detectors including a PinPoint ® ionization chamber ͑PTW, Freiburg, Germany͒, a liquid ionization chamber, a diode, a diamond detector, a radiochromic film, and thermoluminescent dosimeters, among others. Absent from their detector list is any form of plastic scintillator.…”

Evaluation of a synthetic single‐crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system

“…Many studies have been carried out in the past for the other LGK models. 5,[9][10][11][12][13][14] When performing film dosimetry, however, transition dose must be accounted of. This dose has to be evaluated independently with another device, typically a small field detector.…”

Development of a Stereotactic Device for Gamma Knife Irradiation of Small Animals

“…8) Due to the small ionizing volume of the 4 mm helmet, the 18 mm helmet is usually used to measure absolute dose of a Gamma Knife. 2,13,17) The dose distribution inside the stereotactic device well corresponds to dose distribution of Leksell Gamma Plan in the region of 50% or higher. However, it is a little bit wider in the range, around 30-40% region.…”