ESTRO 2022 - Abstract Book

S1333

Abstract book

ESTRO 2022

seem appropriate for a good control dosimetry of our future UHDR experiments at the ARRONAX cyclotron. Users should however observe a four days delay before film reading and analysis.

PO-1550 An Italian dosimetry audit service in radiotherapy

P. Martucci 1,2 , M. Pimpinella 1 , S. Russo 3 , C. Fiandra 4 , A. Embriaco 1 , V. De Coste 1 , P. De Felice 1 , M. Stasi 5

1 ENEA-INMRI, National Institute of Ionizing Radiation Metrology, Rome, Italy; 2 Tor Vergata University, Biomedicine and Prevention, Graduate School of Medical Physics, Rome, Italy; 3 AUSL Toscana Centro, Health Physics, Florence, Italy; 4 University of Turin, Oncology, Turin, Italy; 5 AO Ordine Mauriziano, Health Physics, Turin, Italy Purpose or Objective 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. Materials and Methods 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 60 Co 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. Results 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.

Conclusion 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