Determination of the effect of immobilization systems used in radiotherapy on dose distribution and comparison of these measurements with treatment planning algorithm calculations

OBJECTIVES: We aimed to investigate plexiglass breast board (BB) and vacuum bed (VB) effects on dose distribution and reliability of treatment planning system (TPS) calculations. METHODS: Measurements were performed by placing BBs and VBs of varying thicknesses and air gaps onto water and solid phantoms. CT scans of the solid phantom mechanism were transferred to TPS for dose calculation. RESULTS: 2.6%, 3.9% and 5.5% dose decreases were observed for 10, 15 and 20 mm plexiglass at 5 cm depth. Maximum dose decrease was 1.5% for VB. TPS calculations were within 1% for both systems. Skin dose increased in accordance with thickness. A maximum 6.3% difference was found between calculation and measurements for increasing air gaps, but the differences were not statistically significant. CONCLUSION: BB must be positioned so as to avoid beam entry. Otherwise, it must be included into the CT scan for better calculation. If using VBs, it must be considered that skin doses increase with thickness.