Measurement of the small field output factors for 10 MV photon beam using IAEA TRS-483 dosimetry protocol and implementation in Eclipse TPS commissioning

Abstract

Dosimetry of small fields (SF) is vital for the success of highly conformal techniques. IAEA along with AAPM recently published a code of practice TRS-483 for SF dosimetry. The scope of this paper is to investigate the performance of three different detectors with 10 MV with-flatting-filter (WFF) beam using TRS-483 for SF dosimetry and subsequent commissioning of the Eclipse treatment planning system (TPS version-13.6) for SF data. SF dosimetry data (beam-quality TPR 20,10(10), cross-calibration, beam-profile, and field-output-factor (F.O.F)) measurements were performed for PTW31006-pinpoint, IBA-CC01 and IBA-EFD-3G diode detectors in nominal field size (F.S) range 0.5 × 0.5cm2 to 10 × 10 cm2 with water and solid water medium using Varian Truebeam linac. However, Eclipse-TPS commissioning data was acquired using IBA-EFD-3G diode, and absolute dose calibration was performed with FC-65G detector. The dosimetric performance of the Eclipse-TPS was validated using TLD-LiF chips, IBA-PFD, and IBA-EFD-3G diodes. Dosimetric performance of the PTW31006-pinpoint, IBA-CC01, and IBA-EFD-3G detectors was successfully tested for SF dosimetry. The F.O.Fs were generated and found in close agreement for all F.S except 0.5 × 0.5cm2. It is also found that TPR20,10(10) value can be derived within 0.5% accuracy from a non-reference field using Palmans equation. Cross-calibration can be performed in F.S 6 × 6 cm2 with a maximum variation of 0.5% with respect to 10 × 10cm2. During profile measurement, the full-width half-maxima (FWHM) of F.S 0.5 × 0.5cm2 was found maximum deviated from the geometric F.S. In addition, Eclipse-TPS was commissioned along with some limitations: F.O.F below F.S 1 × 1cm2 was ignored by TPS, PDD and profiles were dropped from configuration below F.S 2 × 2 cm2, and F.O.F which does not satisfy the condition 0.7 < A/B < 1.4 (A and B are FWHM in cross-line and in-line direction) have higher uncertainty than specified in TRS-483. Validation tests for Eclipse-TPS generated plans were also performed. The measured dose was in close agreement (3%) with TPS calculated dose up to F.S 1.5 × 1.5cm2.

A multi‐institutional evaluation of small field output factor determination following the recommendations of IAEA/AAPM TRS‐483

“…This code of practice (TRS-483) provides a guideline on the determination of absorbed dose in small photon fields as well as detector and field size-specific output correction factors for relative dosimetry. 7,8 Since then, a few studies on the application of TRS-483 for determination of SFOFs for standard LINACs [10][11][12][13][14] as well as correction factors for detectors or detector orientations that are not available in TRS-483 have been published. [15][16][17] In 2015, the International Atomic Energy Agency initiated a coordinated research project (Testing of Code of Practice on Small Field Dosimetry-E24021) to test the consistency of the procedures and correction factors published in TRS-483 to find pitfalls, if any, and to guide Member States with its implementation.…”

“…This code of practice (TRS‐483) provides a guideline on the determination of absorbed dose in small photon fields as well as detector and field size‐specific output correction factors for relative dosimetry 7,8 . Since then, a few studies on the application of TRS‐483 for determination of SFOFs for standard LINACs 10–14 as well as correction factors for detectors or detector orientations that are not available in TRS‐483 have been published 15–17 …”