Estimation of absorbed dose in clinical radiotherapy linear accelerator beams: Effect of ion chamber calibration and long-term stability

Abstract

The measured dose in water at reference point in phantom is a primary parameter for planning the treatment monitor units (MU); both in conventional and intensity modulated/image guided treatments. Traceability of dose accuracy therefore still depends mainly on the calibration factor of the ion chamber/dosimeter provided by the accredited Secondary Standard Dosimetry Laboratories (SSDLs), under International Atomic Energy Agency (IAEA) network of laboratories. The data related to Nd,water calibrations, thermoluminescent dosimetry (TLD) postal dose validation, inter-comparison of different dosimeter/electrometers, and validity of Nd,water calibrations obtained from different calibration laboratories were analyzed to find out the extent of accuracy achievable. Nd,w factors in Gray/Coulomb calibrated at IBA, GmBH, Germany showed a mean variation of about 0.2% increase per year in three Farmer chambers, in three subsequent calibrations. Another ion chamber calibrated in different accredited laboratory (PTW, Germany) showed consistent Nd,w for 9 years period. The Strontium-90 beta check source response indicated long-term stability of the ion chambers within 1% for three chambers. Results of IAEA postal TL “dose intercomparison” for three photon beams, 6 MV (two) and 15 MV (one), agreed well within our reported doses, with mean deviation of 0.03% (SD 0.87%) (n = 9). All the chamber/electrometer calibrated by a single SSDL realized absorbed doses in water within 0.13% standard deviations. However, about 1-2% differences in absorbed dose estimates observed when dosimeters calibrated from different calibration laboratories are compared in solid phantoms. Our data therefore imply that the dosimetry level maintained for clinical use of linear accelerator photon beams are within recommended levels of accuracy, and uncertainties are within reported values.

Experimental and theoretical dosimetry of the RIC-100 phosphorus-32 brachytherapy source for implant geometries encountered in the intraoperative setting

“…Reference doses, in addition to those used to generate a dose to film density calibration file in the Film QA Pro 3.0 software, were obtained by exposing EBT3 film placed at a depth of maximum dose, d max , in solid water (Model 457, Gammex, Middleton, WI) using a 6 MV beam from a linear accelerator (Varian Inc., Palo Alto, CA). Expected uncertainty in the linear accelerator's absolute calibration dose is less than 2%, with the largest component arising from the ADCL Co-60 N D,W uncertainty for the ion chamber used to calibrate the linear accelerator (11,12). Measured dose uncertainty, due to intrafilm heterogeneity and fog, is the greater of 1% SD or 2 cGy.…”

Design and Development of a Low-cost Integrated Dosimeter for External Beam Dosimetry in Radiation Oncology

“…The input charge to be integrated is q in , N D,w,Q is the diode calibration factor for absorbed dose to water in a beam of quality Q , D w is the absorbed dose to water as measured by the diode, v out is the front-end output voltage signal digitized by the microcontroller unit (MCU) ADC. For a v out of 1V, the dose measured by the low-cost detector with a 22 nF capacitor will be approximately 0.063 Gy (equation [ 3 ]).…”

“…[ 2 ] Clinical radiotherapy medical physicists play a crucial role in overseeing the meticulous administration of prescribed dose distributions to patients. The use of ionization chambers, calibrated against primary standards,[ 3 , 4 ] ensures the precise measurement of absolute dose delivered to the tumor volume. Complementing this approach, thermoluminescent dosimeters,[ 5 ] metal–oxide–semiconductor field-effect transistors,[ 6 ] semiconductor detectors such as diodes,[ 7 ] and diamond detectors[ 8 ] have been used.…”