SU-E-T-13: An Evaluation of RadCal Software in Its Monitor Unit Calculation Accuracy for Small Dynamic Fields Using HDMLC

Abstract

Purpose: To investigate the accuracy of an independent monitor unit (MU) verification program for small fields used in dynamically modulated highdefinition multileaf-collimator (HDMLC)-based stereotatic radiosurgery (SRS). Methods: HDMLC-SRS patient-treatment plans were calculated using the BrainLab-pencil beam dose-calculation algorithm implemented in the BrainLab-iPlan treatment planning system (TPS) (v.4.1.2) for the Novalis-TX 6X-SRS beam. Percent depth doses, beam profiles and output tables from this clinical beam were utilized to commission the RadCalc software (v.6.2) for independent MU verification. The MU difference between the MU calculation of the treatment planning system and the independent MU verification program for 187 dynamic arcs and 44 intensity modulated beams corresponding to 41 intracranial lesions from 35 patients was calculated. Field sizes of the arcs and beams studied ranged from 15 to 34 mm in their longest extension. Results: The average difference in the MUs with one standard deviation was −0.1% +/− 1.0% for the dynamic arcs and −0.0%+/− 2.7% for the intensity-modulated beams. The intensity modulated beams verifications showed a much higher spread in the difference compared to the non-modulated dynamic arcs; however, the average results can be considered clinically acceptable. The MU difference correlated inversely with the lesion size, indicating the limitation of the TPS algorithm with smaller field sizes. Conclusion: Use of the RadCalc software as an independent monitor unit verification system for both modulated and non-modulated small fields based on high-definition multileaf collimation is adequate to independently validate the point dose calculation accuracy. The system can be utilized as a part of the SRS quality assurance program.