Assessment of Imprecise Small Photon Beam Modeling by Two Treatment Planning System Algorithms

Hadi Keivan, Daryoush Shahbazi-Gahrouei, Ahmad shanei, Alireza Amouheidari


Background: Dosimetric accuracy in intensity‑modulated radiation therapy (IMRT) is the
main part of quality assurance program. Improper beam modeling of small felds by treatment
planning system (TPS) can lead to inaccuracy in treatment delivery. This study aimed to evaluate
of the dose delivery accuracy at small segments of IMRT technique using two‑dimensional (2D)
array as well as evaluate the capability of two TPSs algorithm in modeling of small felds.
Materials and Methods: Irradiation were performed using 6 MV photon beam of Siemens Artiste
linear accelerator. Dosimetric behaviors of two dose calculation algorithms, namely, collapsed cone
convolution/superposition (CCCS) and full scatter convolution (FSC) in small segments of IMRT
plans were analyzed using a 2D diode array and gamma evaluation. Results: Comparisons of
measurements against TPSs calculations showed that percentage difference of output factors of small
felds were 2% and 15% for CCCS and FSC algorithm, respectively. Gamma analysis of calculated
dose distributions by TPSs against those measured by 2D array showed that in passing criteria of
3 mm/3%, the mean pass rate for all segment sizes is higher than 95% except for segment sizes
below 3 cm × 3 cm optimized by TiGRT TPS. Conclusions: High pass rate of gamma index (95%)
achieved in planned small segments by Prowess relative to results obtained with TiGRT. This study
showed that the accuracy of small feld modeling differs between two dose calculation algorithms.

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