Intensity modulated radiotherapy dose delivery error from radiation field offset inaccuracy.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Intensity Modulated Radiotherapy (IMRT) relies on precise multileaf collimator (MLC) calibration.
• Inaccuracies in MLC calibration, specifically the Radiation Field Offset (RFO), can lead to significant dose errors within target volumes.
• The CORVUS IMRT system's sensitivity to RFO errors requires thorough investigation.

Purpose of the Study:

• To analyze the dosimetric consequences of incorrect RFO values in the CORVUS step-and-shoot IMRT system.
• To quantify the Dose Error Sensitivity Factor (DESF) for various IMRT plans.
• To establish an analytic framework for understanding and predicting RFO-related dose errors.

Main Methods:

• IMRT plans were generated with correct RFO values for a phantom target volume.
• A computer code was developed to simulate delivery with incorrect RFO values by modifying dynamic MLC (DMLC) files.
• Doses were measured using an ionization chamber after irradiating with both correct and incorrect RFO DMLC files.

Main Results:

• Measured DESF values ranged from 0-8% mm⁻¹, indicating varying sensitivity to RFO errors across different plans.
• For the CORVUS system (6 MV, 1 cm x 1 cm, step-and-shoot IMRT), an RFO accuracy of 0.5 mm is required to maintain a fractional dose error below 4%.
• A strong correlation was found between high DESF values and DMLC files with small average leaf openings, with a predicted maximum DESF of 20% mm⁻¹.

Conclusions:

• Precise RFO determination is critical for accurate dose delivery in step-and-shoot IMRT.
• The developed analytic framework and computer code can predict and screen IMRT plans for RFO sensitivity.
• Ensuring sub-millimeter accuracy in RFO calibration is essential to minimize dose errors in IMRT treatments.