Small field commissioning and verification for the uRT-TPOIS radiotherapy treatment planning system

Stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) plans involve high doses and small fields. Standard commissioning procedures typically do not prioritize small fields, which is why the commission parameters of small field models require a comprehensive and precise verification process to ensure accurate dose calculation for SRS/SBRT plans.

The study employed various methods, including dose validation tests in a water tank with both regular and irregular small fields. The StereoPHAN and SRS MapCHECK were utilized to assess the modelling accuracy of collapse cone convolution (CC) and Monte Carlo (MC) algorithms for both point and planar doses of the small field plans. Additionally, pre-treatment approval of SRS/SBRT plans based on Electronic Portal Imaging Device (EPID) equipped on the machine was conducted. To enhance realism, 19 clinical SRS/SBRT plans were measured and analyzed for their point and planar doses. In addition, a special 27-target plan was measured using 'gold standard' film and compared with the calculations based on parameters from the small field commissioning procedures.

The validation tests conducted in the water tank proved to be reliable and accurate for both regular and irregular small fields. The results indicated that the average error of point dose decreased from 2.29 % with parameters based on standard commissioning procedures to 0.64 % with parameters based on small field commissioning procedures. Furthermore, planar doses acquisition included high-dose and non-high-dose planes, resulting in an increase in the average gamma passing rate from 93.15 % to 99.90 %. The gamma passing rates of EPID pre-treatment verifications exceeded 99 % at a criteria of 2mm2 %. These findings demonstrate the effectiveness of small field commissioning procedures in improving the accuracy of dose calculations for SRS/SBRT plans.

This study marks the first time that small field commissions and verifications of CC and MC algorithms in uRT-TPOIS, along with uRT-linac 506c, were conducted. The reliability and accuracy of the calculation algorithms for small field commissions were verified through the comparisons between measurements and calculations. The results showed that both CC and MC algorithms' small field parameters met the clinical requirements with minimal deviations (lower than 2 %) between measurements and calculations. Additionally, EPID was found to be an effective quality assurance tool for SRS/SBRT plans.