Evaluating and improving patient-specific QA for IMRT delivery

Modern radiation therapy techniques such as intensity-modulated radiation therapy (IMRT) and newly-emerging volumetric modulated arc therapy (VMAT) aim to deliver highly conformal radiation dose to the target volume while sparing nearby critical organs as much as possible with the complex motion of multi-leaf collimator (MLC) leaves. Pre-treatment patient specific quality assurance (QA) has become an essential part of IMRT in making sure the delivered dose distributions agree with the planned ones. This dissertation evaluates the performance of current patient-specific QA process and proposes solutions to improve its sensitivity, accuracy and efficiency.;In step and shoot IMRT, the study on the sensitivity of patient-specific QA to minor MLC errors reveals tighter criterion such as 2%/2mm must be employed to detect systematic MLC positioning errors of 2 mm. However, such criterion results in low average passing rate which leads to excessive false alarms, mainly due to inadequate treatment planning system (TPS) beam modeling on beam penumbra. An analytical deconvolution approach is proposed to recover true photon beam profiles to obtain a true beam model which significantly improves agreement between calculated and measured dose distributions. Thus a tighter criterion could be employed to enhance the sensitivity of patient-specific QA to minor errors in the delivery system.;Measurement based patient-specific IMRT QA is a time-consuming process. A fast and accurate independent planar dose calculation algorithm is proposed to replace measurement based QA. The algorithm analytically models photons coming out from the accelerator and computes dose distribution from first principles. Accuracy of the algorithm is validated against 2D diode array measurements. The algorithm is found to be fast and accurate enough to replace time consuming measurement based QA.;Patient-specific QA for VMAT differs significantly from step and shoot IMRT due to the increased use of dynamic components (dynamic gantry and collimator, dynamic MLC and variable dose rate) in VMAT. A novel four dimensional (4D) diode array is developed by Sun Nuclear Corp for patient-specific VMAT QA. This work develops effective calibration procedures for this novel device by accounting for diode sensitivity and angular response dependence. A real time algorithm to derive gantry angle is developed to interpolate corresponding angular correction factors. Clinical applications of the diode array are demonstrated with IMRT as well as VMAT plans. Excellent agreement (>95% passing rate with 1%/2mm criterion) is achieved between diode array measurement and TPS calculation. The 4D diode array is proved to be a valuable tool for both IMRT and VMAT patient specific QA.