Evaluating equivalent square field size definitions for rectangular small fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• The IAEA TRS-483 Code of Practice (CoP) uses the Cranmer-Sargison definition to approximate rectangular small fields to equivalent square fields.
• The CoP's uncertainty estimates for this approximation are limited to rectangular fields with aspect ratios between 0.7 and 1.4.

Purpose of the Study:

• To compare the accuracy of the Cranmer-Sargison definition with alternative formulas for equivalent square fields.
• To assess these formulas for measuring field output factors (FOFs) of rectangular small fields, both within and outside the CoP's specified range.

Main Methods:

• Radiochromic films (Gafchromic EBT4) were used for measurements.
• Compared definitions included Cranmer-Sargison, Sterling, Superellipse, Sterling-Partial Superellipse, Sterling-Superellipse, Vadash and Bjärngard, and Fogliata.
• Accuracy was determined by the lowest discrepancy between measured and analytical FOF values, using log-likelihood as the metric.

Main Results:

• The Fogliata model demonstrated the highest accuracy, followed by Sterling-Partial Superellipse.
• Sterling-Superellipse and Vadash and Bjärngard models also performed well.
• Sterling-Partial Superellipse and Sterling-Superellipse models depend only on field geometry, while Fogliata and Vadash and Bjärngard include a fitting parameter.
• Sterling definition improved upon Cranmer-Sargison but was less accurate.
• Cranmer-Sargison and Superellipse models showed the largest discrepancies.

Conclusions:

• Several equivalent square small field size definitions can enhance the IAEA TRS-483 CoP for rectangular fields.
• The Fogliata model provides the most accurate results for field output correction factors in rectangular small fields.