Defining field output factors in small fields based on dose area product measurements: A feasibility study.

Background: Dose area product in water (DAP) in small fields relies on the use of detectors with a sensitive area larger than the irradiation field. This quantity has recently been used to establish primary standards down to 5 mm field size, with an uncertainty smaller than 0.7%. It has the potential to decrease the uncertainty related to field output factors, but is not currently integrated into treatment planning systems.

Purpose: This study aimed to explore the feasibility of converting DAP into a point dose in small fields by determining the volume averaging correction factor. By determining the field output factors, a comparison between the so-called "DAP to point dose" approach and the IAEA TRS483 methodology was performed.

Method: Diodes, microdiamonds, and a micro ionization chamber were used to measure field output factors following the IAEA TRS483 methodology on two similar linacs equipped with circular cones down to 6 mm diameter. For the "DAP to point dose" approach, measurements were performed with a dedicated and built-in-house 3 cm diameter plane-parallel ionization chamber calibrated in terms of DAP in the French Primary Dosimetry Standards Laboratory LNE-LNHB. Beam profile measurements were performed to generate volume averaging correction factors enabling the conversion of an integral DAP measurement into a point dose and the determination of the field output factors. Both sets of field output factors were compared.

Results: According to the IAEA TRS483 methodology, field output factors were within ±3% for all detectors on both linacs. Large variations were observed for the volume averaging correction factors with a maximum spread between the detectors of 26% for the smallest field size. Consequently, deviations of up to 15% between the "IAEA TRS483" and the "DAP to point dose" methodologies were found for the field output factor of the smallest field size. This was attributed to the difficulty in accurately determining beam profiles in small fields.

Conclusion: Although primary standards associated with small uncertainties can be established in terms of DAP in a primary laboratory, the "DAP to point dose" methodology requires volume averaging correction to derive a field output factor from DAP measurements. None of the point detectors studied provided satisfactory results, and additional work using other detectors, such as film, is still required to allow the transfer of a DAP primary standard to users in terms of absorbed point dose.