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OBJECTIVE The aim of this study was to implement a protocol for reference dosimetry in tomotherapy and to validate the beam output measurements with an independent dosimetry system. METHODS Beam output was measured at the reference depth of 10 cm in water for the following three cases: (1) a 5 × 10 cm(2) static machine specific reference field (MSR), (2) a rotational 5 × 10 cm(2) field without modulation and no tabletop in the beam, (3) a plan class specific reference (PCSR) field defined as a rotational homogeneous dose delivery to a cylindrical shaped target volume: plan with modulation and table-top movement. The formalism for reference dosimetry of small and nonstandard fields [Med.Phys.35: 5179-5186, 2008] and QA recommendations [Med.Phys.37: 4817-4853, 2010] were adopted in the dose measurement protocol. All ionization chamber measurements were verified independently using alanine∕EPR dosimetry. As a pilot study, the beam output was measured on tomotherapy Hi-art systems at three other centers and directly compared to the centers specifications and to alanine dosimetry. RESULTS For the four centers, the mean static output at a depth of 10 cm in water and SAD = 85 cm, measured with an A1SL chamber following the TG-148 report was 6.238 Gy∕min ± 0.058 (1 SD); the rotational output was 6.255 Gy∕min ± 0.069 (1 SD). The dose stated by the center was found in good agreement with the measurements of the visiting team: D(center)∕D(visit) = 1.000 ± 0.003 (1 SD). The A1SL chamber measurements were all in good agreement with Alanine∕EPR dosimetry. Going from the static reference field to the rotational ∕ non modulated field the dose rate remains constant within 0.2% except for one center where a deviation of 1.3% was detected. CONCLUSIONS Following the TG-148 report, beam output measurements in water at the reference depth using a local protocol, as developed at different centers, was verified. The measurements were found in good agreement with alanine∕EPR dosimetry. The presented methodology may provide a good concept for reference dosimetry.
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