SU-E-T-200: IMRT Patient Specific QA for On-Line Adaptive Radiotherapy

On-line adaptive IMRT requires a highly efficient radiotherapy team and an intensive amount of work, as well as significant amount of Quality Assurance (QA). With regard to patient specific QA, it is clinically unrealistic to perform fluence measurements via a QA phantom while patient is setup awaiting treatment. We therefore developed an alternative way to check point dose and the IMRT fluence right before beam delivery.

In this study, 28 IMRT plans were generated with Prowess Panther 5.1 for three prostate cancer patients on 28 CT datasets with CTV and critical organs contoured by treating physicians. The corresponding QA plans were generated by Prowess, and then transfered to Pinnacle 9.2 for fluence recalculation on a flat surface phantom, In addition 28 QA plans were delivered on a Siemens Artiste accelerator and fluence were measured with Matrixx IMRT Device. We analyzed both point dose and fluence for these 28 samples.

Of all 28 IMRT plans, the point dose difference between Prowess and Pinnacle are well within 1%. The point dose difference between measurements and Prowess calculation are all within 3%. The passing rates using gamma criteria (3%3mm) for the fluence comparison between Prowess and Pinnacle calculation are at least 98.5% while the passing rates of the gamma analysis between fluence measurement and Prowess calculation are all better than 98.5%. The passing rate of gamma difference between Prowess vs Pinnacle and Prowess vs QA measurement is less than 1.5% for all 28 samples. Therefore, second TPS (e.g. Pinnacle) can be used to verify planned fluencies and can serve as a valuable patient specific QA when conventional IMRT QA measurement is not feasible.

A new IMRT plan is desired if significant anatomy change from the date of original CT scan is observed before the radiation delivery. It is not clinically feasible to check the fluence on the machine before radiation delivery while the patient is in the treatment suite awaiting radiation delivery. Our method showed here is an alternative way to verify the planned fluence with a second TPS and can serve as a valuable IMRT patient specific QA in online adaptive radiotherapy.