Integral quality monitor (IQM^®) signal correction factors for small fields to predict larger irregular segment output signals

Abstract

Purpose To develop a method of correcting for the inaccuracies of small adjoined field segments in their contribution to larger fields in order to get a better match between their combined signals and the measured integral quality monitor (IQM) open field signals. This would enable the pre‐calculation of known irregular segment output signals per monitor unit (MU), which would be later useful for patient‐based dose calculations for treatment verification during pre‐treatment treatment validation using the IQM output signal per MU. Methods Small fields exhibit source obscurity and loss of scatter, resulting in smaller signals being measured by the IQM and the subsequent underestimation of IQM output signals of larger segments obtained by combining small segment signals. Larger field segments were broken down into a set of smaller, regular, abutted segments, whose individual signals were added together to get the predicted output signal of the larger field. The signal/MU for each smaller constituent segment was extracted at its exact location from measured IQM response maps, generated by irradiating the IQM with small elementary segments ranging from 1 × 1 cm2 –5 × 5 cm2, shifting each segment 1 cm at a time and measuring its corresponding output signal/MU throughout the entire IQM sensitive area. The predicted signal was weighed against the IQM‐measured signal of the open field to calculate a signal correction factor (CF) of each elementary segment size. The CFs were applied to known signals of each set of elementary fields before summation in order to pre‐calculate signals of larger irregular fields more accurately. The dependence of CFs on elementary segment size, location of the open field, and beam energy was investigated. Results CFs exhibited an exponential decrease with increase in elementary segment size. CFs were also invariant with beam energy, changing by ≤1% from 6–15 MV. Uncorrected signals for regular fields had relative errors of above 5% whilst signal correction reduced these errors down to ~0.4% (i.e., 99.6% accuracy). For irregular fields, signal correction reduced calculation errors from ~10% to well below 1.5%. Larger signal prediction errors were found when smaller segments were used to reconstruct the field. Open field size and location had a great impact on measured signals but virtually no significance on CFs. Conclusions Results indicate that summation of small segment signals cannot sufficiently reproduce the same output given by an open field if individual elementary segment signals are not weighted with their respective CFs. This effect is particularly predominant for elementary segments 3 × 3 cm2 and for irregular fields. The method outlined enabled the calculation of signal CFs in order to match predicted signals with measured signals to 98.5% accuracy, thus enabling the pre‐calculation of irregular segment output signals/MU for future patient dose calculations.

Impact of ovary-sparing treatment planning on plan quality, treatment time and gamma passing rates in intensity-modulated radiotherapy for stage I/II cervical cancer

“…[ 46 , 47 ] Shiba et al [ 48 ] revealed that low number of segments will increase the gamma passing rates. Some authors [ 49 – 51 ] reported that a high percentage of small-area segments can result in a decrease in the gamma passing rates, because small-area segments will lead to the insufficient scattering and insufficient resolution of the detector. Some studies [ 52 , 53 ] had found that the high dose rates, the MU controller communication delay and the low two-dimensional matrix sampling frequency will lead to the dose deviation of small-MU segments.…”

Resistive electrode array (REA) for radiotherapy beam monitoring and quality assurance

“…The REA collects integral signals from multiple points at the edges of the electrode to provide spatial information about the shape of the beam. REA measures multiple signals that are collected from a single large electrode, which is similar to the way segmented monitor chambers and integral quality monitor (IQM) operate (Valdes 1954, Sjögren et al 1999, Mahuvava and Du Plessis 2019, Oderinde and Plessis 2019, McCaw et al 2020. Segmented monitor chambers are used in accelerator beamlines to control dose rate, flatness, and symmetry thanks to their specific segmentation suitable for derivation of these clinical parameters (Valdes 1954, Sjögren et al 1999, McCaw et al 2020.…”

“…Segmented monitor chambers are used in accelerator beamlines to control dose rate, flatness, and symmetry thanks to their specific segmentation suitable for derivation of these clinical parameters (Valdes 1954, Sjögren et al 1999, McCaw et al 2020. Similarly, IQM has positional sensitivity for linac QA and IMRT fields due to its angled electrodes along the axis of the MLC motion (Mahuvava andDu Plessis 2019, Oderinde and. However, in REA the discrimination of radiation fields is achieved not via segmentation or angling (differential sensitivity to x-rays) but rather via the finite resistance of the collecting electrode.…”