Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Abstract

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via stereotactic radiotherapy where positioning accuracy allows delivering very high doses per fraction in a small volume of the patient. Dosimetric measurements on medical accelerators are conventionally realized using air-filled ionization chambers. However, in small beams these are subject to nonnegligible perturbation effects. This study focuses on liquid ionization chambers, which offer advantages in terms of spatial resolution and low fluence perturbation. Ion recombination effects are investigated for the microLion detector (PTW) used with the Cyberknife system (Accuray). The method consists of performing a series of water tank measurements at different source-surface distances, and applying corrections to the liquid detector readings based on simultaneous gaseous detector measurements. This approach facilitates isolating the recombination effects arising from the high density of the liquid sensitive medium and obtaining correction factors to apply to the detector readings. The main difficulty resides in achieving a sufficient level of accuracy in the setup to be able to detect small changes in the chamber response.

High resolution ion chamber array delivery quality assurance for robotic radiosurgery: Commissioning and validation

“…Furthermore, we investigated and confirmed in a multi-site context the source-detector-distance (SDD) and dose-per-pulse (DPP) dependence of the 1000SRS array when used for the CyberKnife beams. This has previously been reported for the MicroLion chamber and for the 1000SRS array for other delivery systems [12][13][14]. Following the results obtained in this study, we highlighted the necessity of acquiring a center specific SDD response curve prior to the clinical use of the array and the use of a SDD dependent cross-calibration factor, considering that the beam SDD can vary during CyberKnife plan delivery.…”

“…2). The difference between 1000SRS and TM60019 and TM60017 (Monte Carlo corrected according to [18]) for 7.5 mm and 5 mm collimator was in the order of 1.5% and 3.0%, respectively, agreeing well with previous publications on CyberKnife output factors [14,17,18]. As the dose-per-pulse effect with smaller fields is in the order of 0.35% [14] volume averaging effects in the 1000 SRS would probably be responsible for the 3% under-response for the 5 mm collimator output factor and warrant further investigation.…”

“…Recently, a new high-resolution liquid-filled ion-chamber array (Octavius1000SRS, PTW, Germany) with 2.5 mm chamber spacing at the center was developed purposefully for small field radiosurgery DQA. The general dosimetric properties of the 1000SRS are promising [12,13], but specific questions regarding angular and dose-per-pulse (DPP) dependences [14] originating from small non-coplanar beams with variable SDD remain unanswered for the applicability to CyberKnife DQA. Specifically, the following questions need to be addressed before validating CyberKnife treatment plans using the 1000SRS:…”

“…(2) Does the DPP and thus SDD dependence of the 1000SRS influence the DQA measurements for clinical CyberKnife plans with variable SDDs (typically 80-90 cm)? Liquid filled ion-chambers are subject to much larger recombination effects [14]. Specifically, the volumetric recombination, which refers to the recombination of two ions coming from different ionization events, are dependent on dose rate and therefore on the distance of the beam source and the detector [14].…”

“…Liquid filled ion-chambers are subject to much larger recombination effects [14]. Specifically, the volumetric recombination, which refers to the recombination of two ions coming from different ionization events, are dependent on dose rate and therefore on the distance of the beam source and the detector [14]. (3) Are the different incident angles of a CyberKnife plan (typically 0-110°) influencing the DQA measurements performed using the 1000SRS?…”