Small field output correction factors at 18 MV.

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Detection

Background:

• Limited data exists for detector response at 18 MV radiotherapy energy, impacting treatment planning accuracy.
• Current guidelines (TRS-483) primarily cover 6 and 10 MV energies for small fields.
• Accurate field output factors are crucial for precise dose distribution calculations in radiotherapy.

Purpose of the Study:

• To address the data gap for 18 MV by determining field output correction factors for various detectors.
• To investigate the energy dependence of detector response at 18 MV.
• To provide essential data for accurate radiotherapy treatment planning at higher energies.

Main Methods:

• Evaluated multiple detectors (ion chambers, diodes, scintillators, film) using an ELEKTA Versa HD accelerator at 18 MV.
• Measured dose response at 10 cm depth for square fields ranging from 0.5 to 10 cm side length.
• Utilized radiochromic film and compatible chamber data as reference for small and large fields, respectively, adhering to TRS-483 protocol.

Main Results:

• Scintillator (W2 1x1) and Razor Chamber showed minimal deviation from reference data.
• Shielded diodes exhibited the highest over-response at small fields, followed by other diodes and microDiamond.
• Ionization chambers displayed volume effects, with significant under-response at small fields (up to 30% for Semiflex).
• Corrections at 18 MV were generally larger than those at 6 and 10 MV, except for the PinPoint 3D Chamber.

Conclusions:

• Field output correction factors were successfully determined for 18 MV energy across various field sizes.
• Most detectors necessitate larger corrections at 18 MV compared to 6 and 10 MV.
• Implementing these correction factors will enhance the accuracy of beam data for 18 MV radiotherapy applications.