Introduction/Background Our aim was to analyse the PTV used for cervical external beam radiotherapy and to determine whether smaller margins could be used without affecting clinical target volume (CTV) coverage to reduce toxicity.

The significant organ motion during pelvic radiotherapy is well recognised and locally the INTERLACE protocol for IGRT using intensity modulated radiotherapy (IMRT) has been adopted with 2cm or 3cm PTV set-up margin with a ‘plan of the day’ model.

Methodology All patients receiving radical definitive radiotherapy for cervical cancer at the Royal Devon and Exeter Hospital between 1/3/2021 and 31/12/2021 were included (n=13). They received 45 gray (6/13) or 55 gray (7/13) in 25 fractions. The radiographer-led choice between 2cm or 3cm margins with daily on-set cone beam computed tomography (CBCT) was reviewed. Based on CBCT, a margin calculation was performed to determine what margin was required to cover the disease.

Results Results showed 23.1% (3/13) of patients were adequately treated with 2cm margin throughout, described as non-movers. These 3 patients could have been adequately treated with a 1.5cm margin; a 1cm margin would cover 77.3% of fractions. The remaining 10 patients required the 3cm margin for 15.9% of fractions (mean 3.9/25, range 2–8). For these patients a 1cm margin would cover 31.8% of fractions and 1.5cm 66.6%.

Conclusion In conclusion a smaller set-up margin can be utilised, particularly in ‘non-movers’, without compromising disease coverage. Reducing the PTV allows decreased dose to organs at risk, reducing likelihood of toxicity but further analysis of dosimetry and radiographer plan selection is required

Disclosures Nil