ESTRO 2024 - Abstract Book

S4872

Physics - Quality assurance and auditing

ESTRO 2024

International Atomic Energy Agency, Dosimetry and medical radiation physics section, Vienna, Austria

Purpose/Objective:

To report the preliminary results of a small field dosimetry audit for high energy photon beams using a novel holder design conducted by the Dosimetry laboratory of the International Atomic Energy Agency. The audit follows the methodology described previously [1].

Material/Methods:

A novel holder design (Fig 1) has been developed to simplify the small field dosimetry audit procedures by allowing the irradiation of radiophotoluminescent dosimeters (RPLDs, model GD-302M, Asahi Techno Glass Corporation, Japan) and films (RTQA-2, Ashland, USA) in the same geometry. The holder consists of two main parts, the base and the top, which are locked together with thumb screws. Both parts have carvings to accommodate the RPLD capsule, and the width on the contact surface is the same as commercially available RTQA-2 film strips. The base fits Marcus or Roos type parallel plate ion chamber holder for positioning in a water tank. The top of the holder has 5 pins for marking the film as well as an engraving of 1×1 cm 2 and 2×2 cm 2 square corners with additional marks 1 mm away. These allow the use of the film for correcting the position of the holder prior to irradiation of RPLDs as well as for analyzing the field profiles at different dose levels. The main outcome of the audit is the verification of the treatment planning system calculated doses as delivered to RPLDs in four field sizes: 1×1 cm 2 , 2×2 cm 2 , 4×4 cm 2 (or cones of the corresponding diameters) and machine-specific reference field size (usually 10×10 cm 2 ). The uncertainty of the dose determination is 2.30% (k=1 level), and the acceptance limit adopted is 5% for all field sizes. The secondary result of the audit is the comparison of TPS calculated and film measured profile widths at 20, 50 and 80% dose levels as normalized to the central axis for the two smallest fields, with 3 mm acceptance limit. The multicentre pilot included five institutions from Vietnam, Trinidad and Tobago, Tunisia, Peru and Australia, participants of the IAEA coordinated research project E24022. Altogether, fourteen photon beams were checked, ten 6 MV with flattening filter (WFF), three 6 MV without flattening filter (FFF) and one 10 MV WFF beam. Ten beams were collimated with multileaf collimators, two with large cones, and two with stereotactic cones. Results of irradiations with stereotactic cones were excluded from the analysis due to very small field sizes used (0.4 cm diameter).

Figure 1. Design of the novel holder allowing to irradiate RPLDs and films in the same geometry. Left: configuration with an RPLD capsule in the irradiation position. Right: configuration with a film strip.