225. Low modulated 6 MV Flattening Filter Free Intensity Modulated Radiation Therapy (FFF-IMRT) for left breast treatments with Active Breath Coordinator™ (ABC): A feasibility study

Purpose The Flattening Filter Free (FFF) modality of the LINAC generates an unflattened X-ray beam with a dose rate up to 5 times (1200 MU/min) the maximum dose rate of a standard flattened beam. Aim of this study is to take advantage of high dose rate to spare treatment time for left breast breath-holding patients and to deliver an equal or better treatment in dose distribution. Here a 6 MV three-fields FFF-IMRT technique is proposed and compared to the standard two tangential 3D-CR conformal radiotherapy (3D-CR). Methods Our 6 MV three-fields IMRT technique adds a slightly tilted beam to the usual two tangential beams. The low modulation allows the maximum dose rate. For a group of 22 patients undergoing left breast treatment with ABC, a comparison was made between 3D-CR standard treatments and FFF-IMRT. 3D-CR plans were calculated by CMS XIO® (v.5.11 convolution superposition algorithm) and Monaco® (v.5.11.02 collapsed cone algorithm), FFF-IMRT plans by Monaco® (v.5.11.02 Monte Carlo algorithm). For plan comparison, 14 indicators were examined by a paired Student T-test to assess statistical significance of differences (p  • Left lung: V 20 Gy , V 10 Gy • Heart: Mean Dose, V 25 Gy • Right breast: V 2 Gy • Patient: Maximum Dose ( D 1 % ) • PTV: V 95 % , Maximum Dose ( D 1 % ) , V 105 % (cm 3 ), V 105 % (%), Conformity Index (as defined in Monaco® v.5.11.02) • CTV: V 98 % • Apnoeas number • Treatment time Overall treatment times were evaluated for 3D-CR and FFF-IMRT plans by simulating a patient with a 25 s breath hold phase and a breath recover phase of 30 s. Times for patient set-up, portal imaging and treatment room leaving were also taken into account (total 600 s). Results Significant differences were found for the indicators listed in Table 1 . FFF-IMRT plans showed better CTV coverage, high doses (hot spots) control and conformity. All the FFF-IMRT plans were faster and achieved a mean time gain of 15.1% (2%–18%) for 50 Gy, 2 Gy/fraction prescriptions and 14.9% (4%–18%) for 42.40 Gy, 2.65 Gy/fraction prescriptions, and a mean reduction of apnoeas number by two. Conclusions The FFF-IMRT technique delivers an equal or better dose distribution with a significant reduction of treatment time and number of apnoeas, substantially improving patient comfort.