Radiotherapy pre-treatment dose validation: A second verification of monitor units (MU) with a commercial software

Sensitivity Analysis of the Integral Quality Monitoring System® Using Monte Carlo Simulation

The Integral Quality Monitoring (IQM) System is a real-time beam output verifying system that validates the integrity and accuracy of patient treatment plan (TP) data during radiation treatment. The purpose of this study was to evaluate the sensitivity of the IQM to errors in segment using EGSnrc/BEAMnrc Monte Carlo (MC) codes. Sensitivity analysis (SA) techniques were applied to study the significance of small alterations of field sizes (segments) on the IQM signal response. One hundred and eighty multileaf segments were analyzed with methods that include scatter plots (SP), brute force, variance-based (VAR), and standard regression coefficient SA. The segments were altered randomly within ±1, ±2, and ±3 mm leaf steps for 10 MV photon beams. SP analysis gradient and VAR maximum index are 1.045 and 0.556 for the smallest segment while the largest segment has the value of 0.018 and 0.504, respectively. The brute force and standard regression displayed maximum sensitivity indices around the unaltered segments. These tests conclusively indicated that the IQM was more sensitive to alterations of small segments compared to larger segments. This is important since small segment variation will cause a higher dose output variation that should be picked up during online beam monitoring.

Validation and clinical implementation of commercial secondary check software with heterogeneity corrections

AIM

To validate and implement PTW diamond secondary check software (SCS) in a routine clinical use.

BACKGROUND

The secondary independent monitor unit or dose calculation verifications have led to a significant increase in the workflow associated with QA treatments. Modelling, validation and commissioning are necessary steps thereby making it a useful tool for QA.

MATERIALS AND METHODS

PTW Diamond SCS is capable of calculating VMAT fields, based on modified Clarkson integration, accounting for multi-leaf collimators (MLC) transmission and measured collimator scatter factors. Validation for heterogeneity corrections is made using circular phantom with inserts of various density materials. 150 VMAT plans were compared using (i) plans calculated in homogeneous cylindrical phantom and (ii) VMAT plans calculated with heterogeneity corrections using electron density values for each organ.

RESULTS

Diamond SCS calculated dose for homogeneous cylindrical phantom resulted in average deviation of (0.1 ± 2.14%) with Eclipse TPS calculated dose and (-2.0 ± 1.66%) with absolute measured dose. PTW's OCTAVIUS-4D phantom with 729 ion chamber detector array measurements agreed well with Eclipse TPS calculated dose showing an average deviation of (-1.69 ± 1.56%). Diamond SCS dose calculations were performed with heterogeneity corrections for 124 VMAT plans with isocentre at a region above -350 HU. The overall MU variations between Diamond SCS and TPS Acuros-XB algorithms were within ±5%.

CONCLUSION

Hence, the Diamond SCS can be used as an additional tool along with phantom measurements for patient specific quality assurance of VMAT plans with heterogeneity corrections having isocentre at a region above -350 HU.

Results of 1 year of clinical experience with independent dose calculation software for VMAT fields

It is widely accepted that a redundant independent dose calculation (RIDC) must be included in any treatment planning verification procedure. Specifically, volumetric modulated arc therapy (VMAT) technique implies a comprehensive quality assurance (QA) program in which RIDC should be included. In this paper, the results obtained in 1 year of clinical experience are presented. Eclipse from Varian is the treatment planning system (TPS), here in use. RIDC were performed with the commercial software; Diamond^® (PTW) which is capable of calculating VMAT fields. Once the plan is clinically accepted, it is exported via Digital Imaging and Communications in Medicine (DICOM) to RIDC, together with the body contour, and then a point dose calculation is performed, usually at the isocenter. A total of 459 plans were evaluated. The total average deviation was -0.3 ± 1.8% (one standard deviation (1SD)). For higher clearance the plans were grouped by location in: Prostate, pelvis, abdomen, chest, head and neck, brain, stereotactic radiosurgery, lung stereotactic body radiation therapy, and miscellaneous. The highest absolute deviation was -0.8 ± 1.5% corresponding to the prostate. A linear fit between doses calculated by RIDC and by TPS produced a correlation coefficient of 0.9991 and a slope of 1.0023. These results are very close to those obtained in the validation process. This agreement led us to consider this RIDC software as a valuable tool for QA in VMAT plans.