Ion recombination corrections of ionization chambers in flattening filter-free photon radiation.

Yuenan Wang,Joseph Y Ting,Stephen B Easterling

doi:10.1120/jacmp.v13i5.3758

Abstract

The flattening filter free (FFF) X‐rays can provide much higher dose rate at the treatment target compared to the conventional flattened X‐rays. However, the substantial increase of dose rate for FFF beams may affect the ion recombination correction factor, which is required for accurate measurements using ionization chambers in clinical dosimetry. The purpose of this work is to investigate the ion recombination of three types of commonly used ion chambers (Farmer, PinPoint and plane‐parallel) in the FFF photon radiation. Both 6 MV and 10 MV flattened and FFF beams were fully commissioned on a Varian TrueBeam linear accelerator. The ion recombination correction factor, Pion, was determined using the two‐voltage technique for a 0.6 cc Farmer chamber, a 0.015 cc PinPoint chamber, and a 0.02 cc parallel‐plate chamber at different source‐to‐axis distances (SAD) in a solid water phantom or water tank phantom at a depth of 10 cm in a 10×10 cm2 field. Good repeatability of measurements was demonstrated. Less than 1% difference in Pion between the flattened and FFF photons for all three ion chambers was observed. At a SAD of 100 cm and a depth of 10 cm for a 10×10 cm2 field, Pion for the Farmer chamber was 1.004 and 1.008 for the 6 MV flattened and FFF beams, respectively. At the same setup using the Farmer chamber, Pion was 1.002 and 1.015 for the 10 MV flattened and FFF beams, respectively. All Pion results for the Farmer, PinPoint, or parallel plate chamber in the 6 MV and 10 MV flattened and FFF beams were within 2% from the unity (1 ≤ Pion < 1.02). The Pion ratio of the FFF to flattened beams was 0.99~1.01 for both 6 MV and 10 MV photons. The ion recombination effect of the Farmer, PinPoint, and plane‐parallel chamber in the FFF beams is not substantially different from that in the conventional flattened beams.PACS number: 87.56.bd

Highlights

263 Wang et al.: Ion recombination of chambers in unflattened photons fifteen years and the recent development of volumetric-modulated arc therapy (VMAT),(8-9) the need to have a flattened photon beam from a linear accelerator has vanished
The ionization chamber is the most widely used dosimeter for accurate dose calibrations in radiotherapy quality assurance.[10,11] For example, a standard 0.6 cc Farmer chamber is commonly used in the radiation beam calibration protocols.[10]. A microsized PinPoint chamber can be used to determine relative and absolute dosimetry of small photon fields in the stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) procedures.[12]. A plane-parallel chamber, on the other hand, has flat surfaces with thin foils or membranes, which can cause minimal perturbation for incident photons and electrons, and can be used in radiation beam calibration.[10]. Since the Farmer, PinPoint, and parallel-plate ion chambers are gas-filled detectors, as charged particles pass through a gas, free electron and positive ion are generated through the ionization process along the track of radiation
The purpose of this study is to evaluate the ion recombination for typical thimble and plane-parallel chambers in the flattening filter free (FFF) photon radiation to facilitate the quality assurance procedure and accurate dose calibrations for the FFF X-rays

Summary

Introduction

263 Wang et al.: Ion recombination of chambers in unflattened photons fifteen years and the recent development of volumetric-modulated arc therapy (VMAT),(8-9) the need to have a flattened photon beam from a linear accelerator has vanished. The ionization chamber is the most widely used dosimeter for accurate dose calibrations in radiotherapy quality assurance.[10,11] For example, a standard 0.6 cc Farmer chamber is commonly used in the radiation beam calibration protocols.[10] A microsized PinPoint chamber can be used to determine relative and absolute dosimetry of small photon fields in the SRS and SBRT procedures.[12] A plane-parallel chamber, on the other hand, has flat surfaces with thin foils or membranes, which can cause minimal perturbation for incident photons and electrons, and can be used in radiation beam calibration.[10] Since the Farmer, PinPoint, and parallel-plate ion chambers are gas-filled detectors, as charged particles pass through a gas, free electron and positive ion are generated through the ionization process along the track of radiation. According to Attix,(13) there are two types of recombination: initial and general recombination The former process occurs most likely in densely ionized track such as α-particles and is independent of dose rate. The latter process occurs when positive and negative ions from different tracks recombine on their way to the collecting electrode and is dose-rate dependent

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