23. FFF 6 MV beam commissioning tests on agility

Abstract

Introduction This work reports some aspects of the FFF 6 MV commissioning on an Elekta Agility accelerator, associated with the TPS Philips Pinnacle 9.8. Documentation manuals and other “instructions for use” give informations on the technical/metrical parameters to be entered and the measurement conditions for the modeling. But there is little information describing a similar installation with clinical conditions at SSD 90 cm. Recently in the literature some data were published [1] , [2] , [3] . Such published informations bring an undeniable support. Modeling could be intrinsically estimated via the fitting but an end-to-end test is necessary to validate the model [4] . Generation of typical clinical plans with different PTV size and their delivery was particularly focused on. Methods FFF beam is matched with 6 FF beam on our both Agility machines. Measurements for the modeling are done with MP3-M PTW water phantom. Data describing linac and relative OF are entered in Pinnacle. Automatic modeling sequences are essentially used to generate a definitive model up to good fitting quality. The agreement between the planned and measured doses was determined using different plastic water phantoms for point dose measurements and with an array or a film for planar dose measurements. Static phantom plans mimic clinical situations: different tissues, field shapes and depth prescriptions. Dynamic plans calculated on patients CT images for 5 cases per localization (breast (DIBH), prostate, lung, and SBRT) are measured with the Octavius 4D on both matched linacs. Results For all the tested square fields, the dose measurement at 10 cm and SSD 90 cm with the Farmer chamber is similar to the prescribed dose (within ±0.13%). For the other configurations, the difference between calculation and measurement is smaller than 1.3%. The QA for VMAT plans of patients are performed with the Octavius 4D. The averaged volume analysis passing the 3 mm-3% criteria for all the localizations and both matched machines is 98 ± 1.5%. Conclusions Verification based on phantom studies offers the possibility to directly compare dose measurement and prescribed dose. Verification based on patient’s treatment plans is not considered to be complete (the QA phantom dose is not really the prescribed patient dose) but mimic all real clinical situations. Moreover, it confirms a beam time 25% to 35% (SBRT) shorter with FFF.