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In the framework of a national dosimetry audit project, a certified audit service is offered to the Italian radiotherapy centres for reference dosimetry in photon beams. Here we report on the audits performed by the Radiotherapy (RT) centres initially enrolled in the audit project.

Audits are provided for reference dosimetry of radiotherapy photon beams in the range 6-18 MV including flattening filter free (FFF) beams and non-standard treatment units, as CyberKnife and TomoTherapy.

Thermoluminescent dosimeters consisting of a set of TLD chips embedded in a PMMA waterproof holder are used. The dosimeter sensitive volume has a length of 2 cm or 1 cm for filtered and FFF beams, respectively. Dosimeters are calibrated in terms of absorbed dose to water in a reference ⁶⁰Co gamma beam at the National Institute of Ionizing Radiation Metrology (ENEA-INMRI). Correction factors accounting for energy dependence, volume averaging (for the FFF beams) and long term stability are applied to the dosimeter signal.

Measurements in radiotherapy beams are performed in reference conditions according to the international dosimetry protocols [IAEA TRS 398 and TRS 483, AAPM TG 51]. For each beam audited, irradiation of two dosimeters with a dose of 2 Gy is required.

Difference between delivered and measured absorbed dose to water, D_w,RT and D_w,TLD, is evaluated in terms of E_n score [1] with a coverage factor k=2:

where u_w,RT and u_w,TLD are the standard uncertainties associated to D_w,RT and D_w,TLD, respectively. If D_w,RT and D_w,TLD are traceable to the same primary standard, u_cp differs from zero and accounts for the standard uncertainty associated to the primary standard.

The audit performance is satisfactory if |E_n| ≤ 1.0 and unsatisfactory if |E_n| > 1.0.

Reference beam output was checked for 64 photon beams across 32 RT centres initially enrolled on a voluntary basis. The beam qualities were 6 MV (43%), 6 MV-FFF (30%), 10 MV (16%), 15-18 MV (11%). Distribution of E_n scores, for all irradiated dosimeters, are reported in Fig.1 showing that 99.2% of values are in the range [-1.0,1.0] and 81.4% of results are of optimal level with E_n in the range [-0.5,0.5]. As for the single unsatisfactory result, data from the form filled in by the RT centre allowed to identify an error in the dosimeter positioning.

The reference dosimetry audit was successfully completed for the 32 RT centres initially enrolled. Feedback from the participant centres was essential to improve the reporting of detailed uncertainties in the audit certificate. Thanks to this first phase, the audit is now offered as a real service in collaboration with the Italian society of medical physics (AIFM). Meanwhile, a technical protocol for audit of VMAT treatments is being developed as advancement of the audit project.

References:

[1] ISO/IEC 17043:2010 Conformity assessment – General requirements for proficiency testing.