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

TLDR

Radiosurgery units such as the Leksell Gamma Knife use small circular photon beams down to 4 mm, creating a high‑dose isocenter with steep gradients up to 30 % per mm, enabling treatment of small, complex brain lesions near critical structures. The study aimed to experimentally determine the output factors for Gamma Knife collimators, especially the 4 mm helmet, to support commissioning, quality assurance, and treatment‑planning. Output factors were measured with a suite of detectors—including ionization chambers, diodes, diamond detectors, TLDs, alanine pellets, and radiochromic films—across over 1,000 readings at sites in Munich and Zurich. The measured output factors were 0.8741 ± 0.0202 for the 4 mm helmet, 0.9578 ± 0.0057 for the 8 mm helmet, and 0.9870 ± 0.0086 for the 14 mm helmet, confirming feasibility and agreement with published data.

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.