Impact of dose reporting modes on dosimetric accuracy in a national end-to-end stereotactic spine radiotherapy audit

Background and purpose Spinal Stereotactic Body Radiotherapy (SBRT) is among the most complex treatments currently delivered. A multi-centre dosimetry audit evaluated the accuracy of spinal SBRT treatment delivery and the impact of different dose reporting modes (DRM).Methods and materials The audit used anthropomorphic phantoms containing alanine in the Planning Target Volume (PTV) and Gafchromic™ EBT3 film in the axial plane, with a combination of onsite and remote audits due to Covid travel restrictions. Centres used their local planning protocol and technique to create a clinically acceptable treatment plan, following national guidance.Results Fifty-nine plans from 44 treatment planning systems at 41 centres were analysed. The mean PTV dose difference was 1.2% [-6.5-8.5%] with alanine and 1.0% [-7.0-8.5%] within the 100% isodose for film. Alanine measurements were, on average, higher than the calculated dose for all plans, but lower for the Dw,m specifically. For alanine and film, the choice of DRM was statistically significant.The mean distance to agreement and average Centre of Dose difference for films was 0.82 mm [0.35-1.7 mm] and 0.78 mm [0.1-1.7 mm] respectively. The choice of DRM was not significant for either metric.Conclusions This audit demonstrated that the majority of centres are delivering spinal SBRT consistently, with acceptable dosimetric and geometric accuracy. However, it also highlighted the challenges of verifying doses reported in Dm,m and Dw,m, as there was a statistically significant difference with the measured dose when stratified by DRM.