Measurements of Small Field Output Correction Factors on a RefleXion Treatment Machine

The response of various detectors in small fields from a variety of treatment machines has been studied and is summarized in IAEA TRS-483. However, data for the novel RefleXion system remains largely unexplored. This study measured the output correction factors of multiple detectors in small fields for a clinical RefleXion unit.

The RefleXion machine consists of a binary multi-leaf collimator and two pairs of Y-jaws with clinical openings of 1 and 2 cm. The reference dosimetry is applied to the 10 × 2 cm² clinical-reference field, and the output factors of different fields are presented relative to the clinical-reference field. The responses of detectors Edge, Razor, Micro-Diamond, A14SL, CC01 and CC03 in rectangular fields from 1.25 × 1 to 20 × 2 cm² on the RefleXion unit was studied at a depth of 10 cm in an IBA Blue-Phantom-Helix with a 85 cm source-to-surface distance. Gafchromic EBT4 film data in a solid-water phantom were used as the reference to obtain correction factors for the detectors.

In the fields of the 2 cm jaw, all 6 detectors showed similar responses to the film reference within around 0.5% except at the first field width (1.25 cm), where the Edge and Micro-Diamond exhibited over-response and the CC13 showed the volume effect of ion chambers. In the fields of the 1 cm jaw, the Edge and Micro-Diamond had responses close to the film and the same over-response at small field-widths. Significant deviations of the CC13 (∼4%) and the A14SL (∼2.5%) from the film were present over the whole range of field widths.

The small field output correction factors of 6 kinds of detectors were determined for a RefleXion system, conforming to the formalism in TRS-483. All detectors except CC13 fulfil the 5% correction limit recommended by the TRS-483 for output factor measurement.