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

Abstract

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. 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 1×1cm2 -5×5cm2 , shifting each segment 1cm 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. CFs exhibited an exponential decrease with increase in elementary segment size. CFs were also invariant with beam energy, changing by ≤1% from 6-15MV. 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. 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 3×3cm2 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.