SU-E-T-567: Improve Dose Conformity with IMRT Using Shorter Source to Tumor Distance.

PURPOSE To reduce the potential of late toxicity (tissue necrosis and cognitive function loss) from brain radiotherapy, we studied a novel planning technique for MLC-equipped linear accelerators (LINACS) that reduces the high doses delivered outside the PTV, enhancing radiation dose conformity. METHODS The feasibility of improving dose conformity by using variable source-to-tumor distances was tested on brain CT datasets with tumor contours representing solitary lesions and an example with two lesions. For the single tumor cases, different tumor volumes from 2 to 83 cc were explored, while an example double lesion had a total volume of 13 cc. Three different source-to-axis distances (SADs), 100, 80 and 65 cm, were used to create separate 5-field IMRT plans with the same beam angles for all SADs. For each example, the PTV dose coverage and critical organ doses were constrained to be the same, except for the normal brain doses that were determined by the dose conformity. High dose spillage outside the PTV was quantified by R50 (the ratio between the 50% isodose volume and the PTV), and the V12 (volume of normal brain receiving 12Gy or higher). RESULTS R50 decreased monotonically with shorter SAD for all examples. Compared with 100 cm SAD, average R50 reductions of 13% and 19% were observed with 80 and 65 cm SAD respectively. Improved conformity was more notable on smaller lesions and the multiple lesion case. V12 also decreases significantly with shorter SAD. CONCLUSIONS Improvement in dose conformity can be achieved on existing LINACS by reducing the treatment distance for each IMRT field. Sharper beam penumbra and smaller projected MLC leaf width contribute to the improvement. Precise delivery of non- isocentric beams can be challenging on traditional LINACS, but the problem is much more manageable on machines with precise robotic gantry and couch capabilities.