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NuTeC is performing remote dosimetry audits for lung SBRT in Belgium using alanine/EPR and radiochromic film dosimetry. The goal is to provide an independent assessment of the dosimetric accuracy for the SBRT practice of the hospitals in Belgium.

The centres receive the audit material and the DICOM data beforehand. The audit consists of a series of basic tests in water and solid water and an E2E test for a lung SBRT case on a commercial lung phantom preloaded with detectors. The alanine detectors had an uncertainty of 1 %. Both detectors are calibrated in dose-to-water (Dw). Consequently the centres using Acuros had to provide the dose calculations in Dw instead of dose-to-medium (Dm). The tumour volume is simulated using a 3D printed mould filled with a silicone mixture. An alanine pellet is placed in the centre of this tumour volume. Large film detectors (EBT-XD) are placed through the target in the lung equivalent material and also on top of the lungs. The comparisons of film to plans were done using gamma analysis with global normalisation and 10 % threshold. Our alanine/EPR dosimetry was also validated by the IAEA for multiple photon beams.

25 beams have been audited at the time of writing. For the basic tests we observe that 23/25 beams are within 2 % for the beam output with 14/25 beams being within 1 %. For 1 centre the difference between measurements and TPS calculations was higher than 3 %, which is under investigation. The differences are within 3 % for the other basic tests normalised to the measured beam output.

Figure 1 and Figure 2 show the alanine and films results for the E2E SBRT test respectively. For the E2E delivery the alanine results for 15/25 beams are within 3 % of the calculated doses, with 4 beams being within 1 %. Only for 1 beam did the measured dose deviated by more than 5 % from the calculations.

For the film through the tumour the constraints of the gamma analysis were 5%/1mm. 23/25 beams had a passing rate > 95 % with 20/25 beams having > 98 %. 2 beams had a passing rate between 90 % - 95 %. The film op top of the lungs was analysed with 3%/2mm as criteria. 20/25 beams had a passing rate > 95 % with 12/25 having > 98 %. 3 beams had a passing rate < 90 %.

The comparison of the alanine/EPR dosimetry with IAEA was within 0.5 % for all tested beams.

Figure 1: Alanine/EPR results for the E2E SBRT delivery for 25 beams.

Figure 2: Gamma analysis results for the E2E SBRT delivery for 25 beams.

Although dose-to-medium is mainly used in clinic, Acuros Dw had to be used here to respect the calibration of the detectors. The difference is only 2%. The results of the basic dosimetry and the complex dosimetry for the SBRT delivery are very satisfactory.