Abstract
Flattening filter‐free radiation beams have higher dose rates that significantly increase the ion recombination rate in an ion chamber's volume and lower the signal read by the chamber‐electrometer pair. The ion collection efficiency correction (Pion) accounts for the loss of signal and subsequently changes dosimetric quantities when applied. We seek to characterize the changes to the percent depth dose, tissue maximum ratio, relative dose factor, absolute dose calibration, off‐axis ratio, and the field width. We measured Pion with the two‐voltage technique and represented Pion as a linear function of the signal strength. This linear fit allows us to correct measurement sets when we have only gathered the high voltage signal and to correct derived quantities. The changes to dosimetric quantities can be up to 1.5%. Charge recombination significantly affects percent depth dose, tissue maximum ratio, and off‐axis ratio, but has minimal impact on the relative dose factor, absolute dose calibration, and field width.PACS number: 87.55.N‐
Highlights
Flattening filter-free (FFF) beams offer superior dose rates for radiotherapy treatments than flattened beams
Field sizes less than 6 × 6 cm2 are as flat as the flattened beams, making the FFF beam ideal for 3D conformal planning techniques that utilize small fields, such as stereotactic ablative radiotherapy (SABR)
We confirmed Pion is independent of the monitor unit rate by measuring it with a 10 × 10 cm2 field and the ion chamber placed in phantom at depth-of-dose maximum in a source-to-axis distance (SAD) setup
Summary
Flattening filter-free (FFF) beams offer superior dose rates for radiotherapy treatments than flattened beams. Field sizes less than 6 × 6 cm are as flat as the flattened beams, making the FFF beam ideal for 3D conformal planning techniques that utilize small fields, such as stereotactic ablative radiotherapy (SABR). Larger fields are not flat, but the nonuniformity can be overcome with IMRT or VMAT techniques. Special attention needs to be paid to the FFF beams as recommended by AAPM Therapy Emerging Technology Assessment Work Group.[1]. The report discusses considerations unique to FFF beams in facility planning, commissioning, QA programs, treatment planning systems, treatment delivery, patient specific QA, practical clinical limitations, and safety. Increasing the dose per pulse increases the ion recombination rate in an ion chamber’s volume
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