SU-E-T-09: Evaluation of Depth Dose Distribution with Limit Scatter Condition and Comparing with Treatment Planning System

Abstract

Purpose: Beam commissioning data are acquired in full scatter condition but some body parts such as fingers, hands and legs are treated with limited scatter. An evaluation of depth dose distribution in carried out to study limited scatter situation and compared with treatment planning system.Material and Methods: Radiation scatter is considered as an important contribution of dose in patients. This experiment evaluated the depth dose distribution under different size of water phantoms and compared the dose calculated by Eclipse treatment planning software (AAA Algorithm). Measurements were made specially designed limited scatter phantom that were stackable with 6 and 16MV photon beams. The phantom was 3.5cmX12cmX5cm (deep) paper container filled with water was used as a water phantom unit cell that were added to achieve variability in scatter. For different scatter condition purpose, measurement was made by adding different number of phantom unit. An ion chamber reading is used to compare the dose. Results: The measurements show that the depth dose distribution at central axis has weakly dependent on lateral scatter condition. The dose from 6 and 16 MV beams at 1.0 cm depth has 1.02% and 2.70% difference from single unit (considered as 3.5cmX5cmX12cm water phantom) to 9 units (considered as 10.5cmX15cmX12cm water phantom). The difference at dmax for both 6x and 16x are less than 0.5%. Conclusion: The lateral scatter contributes relatively small on central axis doses when the phantom size is comparable to the field size. Advance algorithm used in TPS provides reasonable (<1%) good dose information evaluation in limited scatter condition.