Author: Kazi Towmim Afrin, Imad M. Ali, Jordan West 👨🔬
Affiliation: University of Oklahoma Health Sciences Center, University of Oklahoma 🌍
Purpose: To evaluate quantitatively the dosimetric uncertainties associated with dose calculation and measurement of small photon fields used in stereotactic photon therapy.
Methods: The output factors (Sc,p) for small and intermediate photon field ranging from 0.5x0.5cm2 to 10x1cm2 were measured using four detectors including Edge, PTW-diode, SRS-green and micro-diamond. The 6MV beam from an Edge machine was used in the dose measurement and calculation for the small fields which is used mostly ins stereotactic radiation therapy. The corresponding Sc,p factors were calculated using three dose calculation algorithms that include: Iplan from Brainlab, AAA and Acuros in the Eclipse treatment planning system.
Results: The dose discrepancies between the measured and calculated photon small fields increased as the field size decreased. While the dose deviations where within 2% for fields larger than 2.0x2.0cm2, it increased upto 13% for the smallest field (0.5x0.5cm2). The output factors calculated by the Acuros-dose calculation algorithm agreed more with the measured output factors in comparison with the AAA dose calculation algorithm. The output factors measured by the different detectors were consistent within 2%, however, the green diode data were more than 2%. Particularly for the small fields, the discrepancies between the measured and calculated doses resulted from dose extrapolation using large field data that were used in the commissioning and beam configuration and failure of the dose calculation algorithm to model the variations in beam quality and lack of charge particle equilibrium conditions.
Conclusion: The output factors calculated by the AAA and Acuros-dose calculation algorithms agreed well the measured factors for large and intermediate fields, while there were large deviations in the small fields. The dose calculation algorithms should include small field data in the commissioning data and beam configuration that can account for variations in beam quality and other dosimetric artifacts associated with small field dosimetry.