The Estro-equal Results for Photon and Electron Beams Checks in European Radiotherapy Beams*

Background and purpose: European Society for Therapeutic Radiology and Oncology (ESTRO) has set up a Quality Assurance network for radiotherapy (EQUAL) carrying out dosimetry audit. Some of the work is done in cooperation with the IAEA. The network deals with measurements performed with mailed TLD irradiated in reference and non-reference conditions, for on-axis points in photons and electrons beams. Material and methods: The LiF DTL937 (Philitech, France) was used and read with the PCL3 automatic reader (FIMEL-PTW). The participating centres irradiate the TLD capsules to an absorbed dose of 2 Gy determined with the Treatment Planning System used in clinical routine. Results: Statistical data from the participating centres on their radiotherapy structure such as number of machines and beams qualities available, dosimetry protocols and equipment in use were analysed. 23 European and 2 Mediterranean Basin countries participated. Photons beam audit: 282 centres and 757 beams have been checked; 11% Co beams and 89 % of X-ray beams. Compared to the EQUAL reference dosimetry 1.4 % of the reference beam output dose values and 3% of the percentage depth doses are outside the tolerance level (deviation >±5%). The standard deviation for the reference beam output is 1.8 %. Five percent of the rectangular field dose checks and 4 % the wedge transmission factors had deviations >±5%. The analysis of the global results shows deviations >±5% in at least 1 point for 133 out of the 757 beams, mainly for large and rectangular fields and for wedged beams. At least 45 of these centres had one "real dosimetric" problem in one or more parameters, which corresponds to 7% of the checked beams. Electron beam audit: 97 centres and 277 beams have been checked. 1.0 % of the reference beam output values (field size 10 cm x 10 cm) and about 2 % of the beam output for the others field sizes (15 cm x 20 cm, and 7 cm x 7 cm) are outside tolerance level (deviation >+5%). The standard deviation for the reference beam output is 2.1% and for other field sizes about 2.4%. There are about three times more deviations outside the optimal level (> 3%) with electron beams than with photon beams. This indicates that improvements on the electron beam calibration and on the TPS algorithms for the electron beams are needed. Conclusions: The EQUAL programme shows good results for the reference beam output for photon and electron beams, which facilitate exchange of experiences between centres. The largest deviations are for non-reference geometries, which might result in some dose variations between patients in a centre treated with the same prescribed dose but with different set-ups.