Patient-specific IMRT verification using independent fluence-based dose calculation software: experimental benchmarking and initial clinical experience

Abstract

Experimental methods are commonly used for patient-specific intensity-modulated radiotherapy (IMRT) verification. The purpose of this study was to investigate the accuracy and performance of independent dose calculation software (denoted as 'MUV' (monitor unit verification)) for patient-specific quality assurance (QA). 52 patients receiving step-and-shoot IMRT were considered. IMRT plans were recalculated by the treatment planning systems (TPS) in a dedicated QA phantom, in which an experimental 1D and 2D verification (0.3 cm(3) ionization chamber; films) was performed. Additionally, an independent dose calculation was performed. The fluence-based algorithm of MUV accounts for collimator transmission, rounded leaf ends, tongue-and-groove effect, backscatter to the monitor chamber and scatter from the flattening filter. The dose calculation utilizes a pencil beam model based on a beam quality index. DICOM RT files from patient plans, exported from the TPS, were directly used as patient-specific input data in MUV. For composite IMRT plans, average deviations in the high dose region between ionization chamber measurements and point dose calculations performed with the TPS and MUV were 1.6 +/- 1.2% and 0.5 +/- 1.1% (1 S.D.). The dose deviations between MUV and TPS slightly depended on the distance from the isocentre position. For individual intensity-modulated beams (total 367), an average deviation of 1.1 +/- 2.9% was determined between calculations performed with the TPS and with MUV, with maximum deviations up to 14%. However, absolute dose deviations were mostly less than 3 cGy. Based on the current results, we aim to apply a confidence limit of 3% (with respect to the prescribed dose) or 6 cGy for routine IMRT verification. For off-axis points at distances larger than 5 cm and for low dose regions, we consider 5% dose deviation or 10 cGy acceptable. The time needed for an independent calculation compares very favourably with the net time for an experimental approach. The physical effects modelled in the dose calculation software MUV allow accurate dose calculations in individual verification points. Independent calculations may be used to replace experimental dose verification once the IMRT programme is mature.

Effect of Photon-Beam Energy on VMAT and IMRT Treatment Plan Quality and Dosimetric Accuracy for Advanced Prostate Cancer

“…This shows that further improvement of the optimization and dose calculation process is necessary to improve the planning process. Besides experimental verification of IMRT for patient-specific quality assurance, independent monitor unit verification is an attractive tool for clinical routine and large scale implementation [9,10]. For VMAT, existing independent dose calculation tools need to be further developed.…”

Monte Carlo simulations to replace film dosimetry in IMRT verification

“…Based on film measurement results obtained in this study, an aimed confidence limit of 3 % dose deviation (with respect to the prescribed dose) or 6 cGy, as suggested in Ref. [27], was only fulfilled for 16 out of 25 IMRT cases in the high-dose region. A significant lower uncertainty and dose deviation of less than 3 % for all plans compared to the PB based TPS could be achieved when applying verification calculations with XVMC/VEFM; however, there were still slight differences.…”

A method to reconstruct and apply 3D primary fluence for treatment delivery verification

“…Two‐dimensional beam fluence can be directly measured with various dosimeters including diode or ion chamber arrays, e.g., MapCheck (Sun Nuclear, Melbourne, FL, USA) or MatriXX (IBA, Bartlett, TN, USA), or even with onboard electronic portal imaging devices (EPID) 4, 24, 25. Beam 2D fluence can also be digitally and proximately reconstructed from treatment plan parameters or LINAC machine log files by integrating across a beam aperture multiplied by the per‐segment beam MU 10, 26, 27, 28.…”