SU-E-T-421: 6MV Radiation Small Field Dose: Off Axis and Penumbra Effects, a Study with Radiochromic Film.

Purpose: To accurately determine relative dose factor scaling, tissue depth relation, and penumbra details of small 6MV radiation fields with an emphasis on far-off-central-axis fields for various MLC designs -subject matter important in the context of SRS, FSRT, SBRT, and IMRT.

Methods: Radiochromic film designed for quantitative dosimetry, EBT3™ film (Ashland Inc.) can be reliably used as a relative dosimeter to within 1% accuracy if one controls for both film and flatbed scanner variation. This enabled a study of the fine detail of small radiation fields and the impact upon these small dose profiles by different MLC designs both on central axis and off. In particular, the impact of leaf-end penumbra from bifocal MLC Optifocus (Siemens) and Synergy Beam Modulator (Elekta) is examined.EBT3 films were placed at 100cm SAD in solid water phantom and irradiated to 200 MU at various depths 1.5cm - 20cm and for various off-axis locations. The films were analysed to give the detailed dosimetric profile of each small field and relate them to both a standard reference field (10 × 10)cm and to a broadfield flatness/symmetry irradiation (20 × 20)cm . Films were scanned at ∼20 hours post irradiation on an Epson 2000 flatbed scanner and analysed using FilmQAPro and in-house methods.

Results: As small field size decreases, the field center falls within the penumbra of each MLC edge for megavoltage photon energies. The penumbral effects cause the conventional dosimetric quantity SP to sharply depart from the smooth fieldsize dependence for larger conventional fields.

Conclusions: Careful characterization of small field dose and leaf end modeling within a TPS is crucial in both predicting accurate small field dosimetry and off-axis dosimetry.