Calculation of dose in off-central axis planes using off-axis ratios

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Accurate dose calculation is critical for effective radiation therapy.
• Treatment planning systems (TPS) rely on algorithms to predict radiation dose distribution.
• Off-central axis calculations can introduce significant errors in dose prediction.

Purpose of the Study:

• To investigate the accuracy of dose calculations for regular fields in off-central axis planes.
• To evaluate the performance of the RAD-8 treatment planning computer for Varian linear accelerators.
• To identify potential sources of error in dose calculation algorithms.

Main Methods:

• Investigated dose calculation accuracy on a RAD-8 TPS.
• Used 4-MV and 10-MV X-ray beams from Varian Clinac-4 and Clinac-18 linear accelerators.
• Compared calculated doses with measured doses in off-central axis planes.

Main Results:

• 4-MV X-ray beam calculations showed errors up to 25%-30% within the irradiated volume.
• Errors for the 4-MV beam were attributed to the lead flattening filter's effect on dose falloff.
• 10-MV X-ray beam calculations agreed with measurements within 4%-6%.

Conclusions:

• The RAD-8 TPS exhibits significant inaccuracies for off-central axis dose calculations with 4-MV X-ray beams.
• The limitations stem from the TPS's inability to accurately model dose falloff caused by beam-flattening filters.
• 10-MV X-ray beams demonstrate reliable dose calculation accuracy in off-central planes with this system.