SU-E-T-419: MU Verification for Small Lesions in Lung SBRT

To determine if a secondary MU calculation can validate the treatment planning system MU calculation for objects less than 4cm in diameter in small field lung SBRT. To determine the small object modifier to better predict the correct MU.

Four polystyrene spheres were embedded into lung mimicking material. The sphere diameters are 4cm, 3cm, 2cm, and 1.2cm. The spheres are made up of two dome TLD-100 chip holders. Each TLD holder holds two 0.125' square × 0.035' TLDs. The output (cGy/MU) was measured at the center of each sphere for three depth and effective depth combinations. The depth, effective depth, field size, and energy were kept constant. The effective depth was kept constant by adding tissue equivalent water to the top of the phantom as the sphere diameter got smaller. The three depths were 5cm, 10cm, and 15cm, with an effective depth of 5cm, 6cm, and 7cm, respectively. The collimator field size and the effective field size was 3cm × 3cm. The energy was 6MV. The measured results were compared with the output predicted by our treatment planning system.

Dose measurements were made with both TLD and film and the results were averaged. The secondary MU calculation predicted that the output would be the same for each sphere. However, the measured output dropped as the sphere diameter got smaller, especially for the 1.2cm diameter sphere. The output dropped by 5% at a 10cm depth and by 9% at a depth of 15cm.

Secondary MU calculations cannot reliably predict the MU's for small objects less than 3cm in diameter without modifiers. A modifier of 3% is needed for a 2cm sphere and 7% for a 1.2cm diameter sphere.