Tuning effective spot size parameter in Acuros XB algorithm for EdgeTM radiosurgery machine

Abstract

The small field of stereotactic radiosurgery has become a common technique in radiation therapy. The treatment planning system (TPS) is an important process to deliver the accurate dose to patients. Many parameters in the treatment planning system can affect the dose calculation. One important matter is the effective spot size parameter (ESS). The ESS can affect the calculated beam profile shaping, especially penumbra region and output factor in a small beam. Inappropriate ESS can lead to improper isodose distribution in treatment planning. So, the aim of this research was to tune the ESS for the Acuros XB algorithm and to perform the small field output factor to evaluate the parameter performance. The ESS was varied separately in X and Y direction from 0 to 1.5 mm with 0.25 mm increment. In this research, the MLC field size of 0.5, 1, 2 cm square field with 3×3 cm 2 Jaw fixing, and 3×3 cm 2 Jaw square field with all retracted MLC were employed. The comparisons of beam profile shaping and gamma index evaluation both with TPS calculation and Gafchromic EBT3 film measurement were investigated. Then, the appropriate matching ESS was selected. The field output factors following the TRS-483 guideline were evaluated for dosimetric performance of the selected ESS. The microdiamond and PTW SRS diode were used for field output factor measurement of the small MLC field 0.5, 1, 2 cm square field with 3×3 cm 2 Jaw fixing, and Jaw fixing square field range from 0.6×0.6 cm 2 to 5×5 cm 2 cm with a reference field of 10×10 cm 2 . The appropriate ESSs for this research in (X, Y (mm)) were (0.75, 0), (1, 0), and (1, 0.25). The appropriated ESSs showed an excellent agreement of beam matching profile in crossline and inline direction. The penumbra difference of all appropriated ESSs demonstrated the value less than 0.5 mm while the gamma evaluation revealed the highest value with 5%/0.3 mm gamma criteria. The appropriated ESS demonstrated the good agreement of field output factor in testing performance for all MLC field, while in the Jaw field less than 2×2 cm 2 was limited. The ESS has a significant impact to the small beam in the Acuros XB calculation algorithm in the treatment planning system. The beam profile matching with the high-resolution detector such as film should be used for tuning the ESS. In addition, clinical situation should be performed for the confidence selecting the ESS parameter.