SU‐E‐T‐674: Development and Validation of a Monte Carlo Model of a Kilovoltage X‐Ray Therapy Unit for Chest Wall Irradiation

Purpose: Given the high local control rates observed in breast cancer patients undergoing chest‐wall irradiation by kilovoltage x‐rays, we aimed to revisit this treatment modality by accurate calculation of dose distributions using Monte Carlo simulation. We report on the development of a Monte Carlo model of the Siemens Stabilipan kilovoltage unit and its validation using various experimental measurements for chest wall irradiation. Methods: The machine components including the housing, anode, filter, collimators, etc were simulated using the MCNP4C code. To validate the 120 kVp beam model, the 1st and 2nd half‐value layers (HVLs) were first measured using high‐purity Al foils and an ion chamber. The percentage depth doses (PDDs) and off‐axis beam profiles (depths 2 and 5 cm) for beams normally incident on a water phantom were also measured using diode and ion chamber dosimeters. Finally, dose measurements were performed for a realistic situation of oblique incidence on the chest wall using 10 thermoluminescence dosimeters (TLDs) placed within the chest wall, heart and lung of an inhomogeneous Rando phantom. The results of the experimental measurements and MCNP simulations were then compared. Results: The 1st and 2nd HVLs were 3.8 and 10.3 mmAl respectively, which were in close agreement with the values obtained by MCNP (3.8 and 10.6 mmAl). The differences between the measured and calculated PDDs and beam profiles in water were less than 2mm/2% for all data points. MCNP‐calculated relative doses agreed with TLD measurements in the Rando phantom to within 4.5% of the prescription dose, the differences for majority of the data points being within 2%. Conclusions: The acceptable level of agreement between the results of the simulations and the measurements validate the accuracy of the MCNP model for use in treatment planning studies of chest‐wall irradiation using this kilovoltage beam.