1101 poster COMPARISON OF SURFACE AND PERIPHERAL RADIOTHERAPY DOSES USING ENHANCED DYNAMIC AND PHYSICAL WEDGES

Purpose: The wedge fields are often used in various clinical situations to obtain homogeneous dose distribution around target volume. Differences in surface and peripheral dose between Enhanced dynamic wedge and physical wedges are important for choosing a wedge system for clinical application. The aim of this study is to measure and compare surface and peripheral dose in Enhanced dynamic wedge and upper Physical wedge using Gafchromic EBT film and Advanced Markus chamber. Materials: Measurements were performed using 6 and 18 MV X-rays from Linac. Following exposure parameters were applied: 100 cm SSD, 5*5 cm2, 10*10 cm2 and 20*20 cm2 field sizes for 15◦ and 30◦ wedge angles and 5*5 cm2, 10*10 cm2 and 15*15 cm2 for 60◦ wedge angle. A 2*2 cm2 film was placed at the center and four pieces outside field on the surface of the solid water phantom and at the depth of maximum dose deposition. All measurements were carried out and repeated by Advanced Markus Plane-Parallel chamber (PTW). Measurements of peripheral doses were performed for field sizes of 5*5 cm2 and 10*10 cm2 at the distance of 5cm and 10cm outside the f ields. For field sizes of 15*15 cm2 and 20*20 cm2, measurements were done only at 5cm distance away from outside the fields. We used water-equivalent RW3slab phantoms (PTW) for all measurements. Results: Surface doses on the central axis under dynamic wedge are more than that of the physical wedge using the same exposure conditions. The maximum surface dose produced by the dynamic wedge is 27±2cGy and by the physical wedge is 24.5±0.5cGy for 30 wedge in 20*20 cm2 field size. Peripheral doses of dynamic and upper wedges are similar (PV<0.002). Surface and peripheral doses for 18MV beams are less than that for 6MV. The percentage discrepancies between 18 and 6MV beams for 5*5 cm2, 10*10 cm2 and 20*20 cm2 for 15◦ wedge angle are 53.01, 43.56 and 35.36 ,respectively, this discrepancy reduces by increment of wedge angle. Our results showed a direct relationship between the eld size and surface/peripheral doses. GafChromic EBT lm provides precise measurements for surface dose in the high energy photons. Agreement between lm and plane-parallel chamber measurements was found to be within ±0.8% for doses in depth of maximum dose. There was 7% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all eld sizes in photon beams of all energies. Conclusions: The two wedge systems produce significantly different surface and peripheral doses that should be considered when choosing a wedge system for clinical use. Dynamic wedge and Physical wedge systems deliver peripheral doses similar to those of open fields.