Accuracy limits of the equivalent field method for irregular photon fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• The equivalent field method is used in radiation therapy to simplify dose calculations for irregular fields.
• Accuracy of this method can be affected by field shape and depth variations.
• Understanding dose errors is crucial for precise treatment delivery.

Purpose of the Study:

• To mathematically evaluate the accuracy of the equivalent field method for clinical photon beams.
• To quantify dose errors when field equivalencies are applied across different depths.
• To investigate the impact of irregular field shapes (uniform vs. multiform) on dose calculation accuracy.

Main Methods:

• Developed a mathematical approach using basic clinical photon beam data.
• Performed analytical calculations of dose errors.
• Expressed phantom scatter summation as a Riemann-Stieltjes integral.
• Categorized irregular fields into uniform and multiform types.

Main Results:

• Multiform fields generate nearly twice the dose errors compared to uniform fields in extreme cases.
• For uniform fields, maximum local relative dose errors were 3.8% (6 MV) and 8.8% (cobalt-60) when shifting from 10 cm to 30 cm depth.
• These errors represent 1-2% of the maximum dose, supporting the applicability of equivalencies for rectangular fields across energies.

Conclusions:

• The accuracy of the equivalent field method is influenced by irregular field shapes.
• Dose errors, though small in relative terms for uniform fields, can be significant for exit dose monitoring.
• This study enhances understanding of how field geometry affects the reliability of the equivalent field method in radiotherapy.