Output factors and dose calculations for blocked x-ray fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Accurate output factor determination is crucial for precise radiation dose delivery in radiotherapy.
• Blocked fields in radiation therapy introduce complexities in dose calculation compared to open fields.

Purpose of the Study:

• To measure output factors for blocked fields using 6-MV x-rays in a polystyrene phantom.
• To investigate the relationship between output factors, collimator field size, and blocked field surface size.
• To develop and validate a method for calculating output factors for any collimator and blocked field size.

Main Methods:

• Measurements were conducted in a polystyrene phantom at the calibration source-surface distance (SSD).
• Output factors were measured for four collimator field sizes and two blocking schemes.
• A novel method was proposed and validated for calculating output factors of blocked fields.

Main Results:

• Output factors for blocked fields are dependent on both the surface field size of the blocked field and the collimator field size.
• Output factors for blocked fields were found to be lower than the collimator field size but higher than unblocked fields of the same surface area.
• The proposed calculation method demonstrated validity when compared to measured doses.

Conclusions:

• The study provides essential data for understanding and calculating output factors in the presence of blocking.
• The proposed method offers a reliable approach for determining output factors for complex blocked fields, enhancing treatment planning accuracy.
• Accurate dosimetry for blocked fields is critical for optimizing radiation therapy efficacy and patient safety.