ESTRO 2025 - Abstract Book

S3636

Physics - Quality assurance and auditing

ESTRO 2025

Results:

Figure 1. Jaw defined small-field OF comparison

The OF comparisons show: 1) the EPID-derived and measured OFs are in close agreement apart from at the 1x1 cm 2 field where EPID is over-estimating; 2) OFs for both measured and EPID show small variation between the centres for above 0.5x0.5 cm 2 field size with ~1.5% (1 SD) variation for jaw and ~1.0% for MLC at 1x1 cm 2 field size ; 3) variations in the TPS beam-models between the centres are apparent for both jaw and MLC defined field, with models over-predicting the smallest field OF for jaw and under-predicting the smallest OF for MLC. TPS calculated OFs at 1x1 cm 2 vary more than measured OFs between the centres.

Figure 2. MLC defined small-field OF comparison

Conclusion: These data suggest that an EPID method can reproduce measured doses for very small fields used in MTSI SRS without the need for specialised equipment, resource-intensive measurement techniques or an onsite auditor. These data will also enable standardisation between institutions to consolidate both MLC setup and TPS dose algorithms for MTSI SRS.