The impact of corrected field output factors based on IAEA/AAPM code of practice on small‐field dosimetry to the calculated monitor unit in eclipse™ treatment planning system

Abstract

The objective of this study was to investigate the effect of field output factors (FOFs) according to the current protocol for small‐field dosimetry in conjunction to treatment planning system (TPS) commissioning. The calculated monitor unit (MU) for intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans in Eclipse™ TPS were observed. Micro ion chamber (0.01 CC) (CC01), photon field diode (shielded diode) (PFD), and electron field diode (unshielded diode) (EFD) were used to measure percentage depth doses, beam profiles, and FOFs from 1 × 1 cm2 to 10 × 10 cm2 field sizes of 6 MV photon beams. CC01 illustrated the highest percentage depth doses at 10 cm depth while EFD exhibited the lowest with the difference of 1.6% at 1 × 1 cm2. CC01 also produced slightly broader penumbra, the difference with other detectors was within 1 mm. For uncorrected FOF of three detectors, the maximum percent standard deviation (%SD) was 5.4% at 1 × 1 cm2 field size. When the correction factors were applied, this value dropped to 2.7%. For the calculated MU in symmetric field sizes, beam commissioning group from uncorrected FOF demonstrated maximum %SD of 6.0% at 1 × 1 cm2 field size. This value decreased to 2.2% when the corrected FOF was integrated. For the calculated MU in IMRT‐SRS plans, the impact of corrected FOF reduced the maximum %SD from 6.0% to 2.5% in planning target volume (PTV) less than 0.5 cm3. Beam commissioning using corrected FOF also decreased %SD for VMAT‐SRS plans, although it was less pronounced in comparison to other treatment planning techniques, since the %SD remained less than 2%. The use of FOFs based on IAEA/AAPM TRS 483 has been proven in this research to reduce the discrepancy of calculated MU among three beam commissioning datasets in Eclipse™ TPS. The dose measurement of both symmetric field and clinical cases comparing to the calculation illustrated the dependence of the types of detector commissioning and the algorithm of the treatment planning for small field size.

Evaluation of Acuros® XB accuracy for static small fields dose calculations based on the IAEA/AAPM TRS-483 recommendation

“…The algorithm's accuracy using these jaw openings not only has an impact on static fields but also may impact the dose calculation in modulated fields [10]. Moreover, the dose calculation accuracy of Table 4 Comparison of the average field output factors from this work and those reported by Dufreneix et al [16] and Huq et al [9] for flattened and unflattened beams.…”

“…This experimental procedure was followed for 6 WFF and FFF beams. The measured field output factors were averaged for each field size, and they were compared with those field output factors reported by other authors using the same experimental conditions [8][9][10]. The measured field output factors were used to evaluate Acuros® XB's accuracy to calculate the dose in small static fields (≤ 3 cm length size) at the central beam axis shaped by the MLC.…”

“…For example, a 10 cm × 10 cm nominal field size is about 11 cm × 11 cm at 10 cm depth using an SSD of 100 cm. It is common to consider a factor of 1.1 between the field size at surface and measurement depth [10]. This factor was validated to apply in small fields (see Supplement material).…”

“…(5) might be consulted in the Supplement material. The data were compared to those field output factors reported by Mamesa et al [10] for flattened beams. Up to date, to the best of our knowledge, there are not jaw-field output factors reported for 6 MV unflattened beams measured with an SSD of 100 cm at 10 cm depth.…”

“…The standard deviation is reported at 1σ. Data were compared with values reported by Mamesa et al[10] for 6 MV flattened beams. The differences (Δ(%)) were calculated using Eq.…”