**9. Results of dosimetry audit with RPLDs**

Bulgarian radiotherapy centres participated in the IAEA/WHO Postal Dose Audit Service with (RPLD) in last three years. The new Varian and Elekta therapy treatment machines have been installed in 2011–2017. The energy of the photon beams is in the range of 6 MV-15 MV. The total number of 34 beams were cheked. The results are given on **Figure 13**. The 33 beams (97%) in reference conditions are in the tolerance of +/− 5%. Follow up have been organized for the beam exceed the tolerance and successfully is clarified the reason. The results of the dosimetry audits despite the fact, that the radiotherapy equipment in Bulgaria was in long-term technology stagnation, show the ability of Bulgarian medical physicists to provide quality dosimetric control at the current world criteria.

The results show, that all measured values of the applied dose are within ±5%. There is a tendency to improve the accuracy, which we attribute to the in-depth

#### **Figure 13.**

*The results of IAEA/WHO RPLD audit 2017–2019. Ratios of IAEA mean dose/stated dose. Each point in the graph represent averaged dose of 2(two) capsules.*

knowledge, experience and skills of the staff of medical physicists due to their regularly participation in the dosimetry audits.

Independent dosimetry audits play an important role in patient treatment quality, radiation protection and safety. Audits have the potential to identify issues and resolve them, reducing the probability of harmful errors to occur. They also support the safe implementation of new techniques and technologies, and promote knowledge sharing at a national and/or international level by benchmarking centres with similar equipment [28]. Indeed, the IAEA stresses the importance of every radiotherapy centre equipped with new machines and those that are going to introduce new treatment techniques in clinical practice, participate in dosimetry audits before starting treating patients, and regularly after that [29]. Moreover, a recent European Directive (2013/59 Euratom) recommends that new radiological procedures should be audited. Independent dose audits are also mandatory in many multi-institutional clinical trials in radiotherapy to ensure that participants deliver accurate doses and so the reported results are not biased [30–32].
