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

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, normalPion, 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, normalPion 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, normalPion was 1.002 and 1.015 for the 10 MV flattened and FFF beams, respectively. All normalPion 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 ≤ normalPion < 1.02). The normalPion 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

Characteristics of flattening filter free beams at low monitor unit settings

“…Although FFF beams used a higher dose per pulse than the 6X beam, the variation in P ion was small, as supported by previous investigations of the FFF beams of TrueBeam TM . [30][31][32] Typical linacs generate 1-3 patterns of photon beam energy with the same number of flattening filters. In contrast, some of newer commercial linacs can generate flattened beams and several patterns of FFF beam energy because FFF beams do not require flattening filters.…”

Development of a silicon diode detector for skin dosimetry in radiotherapy

“…The ratio between the charge measured by the silicon diode to the charge measured by the Attix ionization chamber represents the DPP dependence of the detector and is normalized to the dose per pulse of 2.78 10 −4 Gy/pulse corresponding to the reference SSD of 100 cm. This method avoids variation of the spectrum of the radiation and can be used to characterize only the dose rate dependence assuming that the ion chamber's dose per pulse dependence is negligible …”

“…This method avoids variation of the spectrum of the radiation and can be used to characterize only the dose rate dependence assuming that the ion chamber's dose per pulse dependence is negligible. 42…”

Monte Carlo validation of the TrueBeam 10XFFF phase–space files for applications in lung SABR

“…Commissioning measurements in FFF beams must be carried out with special consideration of the high dose per pulse, particularly in the 10XFFF beam. [14][15][16] Ionization chambers may suffer from recombination effects and other dosimeters react to high dose rates in different ways depending on the design and underlying principles of physics at play in the detector. Output factors (the ratio of dose to a specified point in a clinical field of interest to dose to a specified point in a reference field) are difficult to measure accurately for small fields.…”

“…A growing number of papers have been published on ion recombination effects in ion chambers [14][15][16] and diligence is required to account for these effects to ensure accuracy and independence of the measured result from the detector type and model. When reporting on measurements of FFF beam data, it is important for investigators to report on whether correction has been made for changes in detector sensitivity at high dose per pulse.…”