Background

Practices in planning and delivering radical radiotherapy (RT) for inoperable oesophageal cancers vary. Internal motion, difficulties imaging tumours and patient set-up can compromise coverage. We aimed to assess the adequacy of our planning margins and offline no-action-level (NAL) correction protocol in correcting set-up errors.

Method

Mid-lower oesophageal cancer patients treated with radical RT or chemoradiotherapy (CRT) January 2010-April 2012 were reviewed. All immobilised on a lung board and CT conformally planned. PTV was defined as either CTV+1cm in all planes (institution standard) or CTV+0.5cm axially, 1cm superior-inferior (trial standard). Cone-beam CT and 0.5cm tolerance was used in an offline NAL correction protocol. Set-up data was obtained in superior-inferior, anterior-posterior and lateral planes. Random (σ) and systematic errors (Σ) for the first 3 fractions and overall treatment were calculated. The Van Herk formula (2.5∑+0.7σ) was used to generate margins.

Results

51 patients were identified (30 male). Median age was 71 years. 20 patients had RT, 31 had CRT. Of 451 images taken (mean 8.8/patient), 39.2% were out of tolerance (>5mm), superior-inferior (28.8%), lateral (13.5%) and anterior-posterior (1.7%). 29 patients required ≥1 corrections. 45.7% images from the first 3 fractions were out of tolerance, superior-inferior (35.3%), lateral (13.1%) and anterior-posterior (3.9%).

Overall Σ for the first 3 fractions were superior-inferior=0.46cm, lateral= 0.28cm, anterior-posterior=0.23cm reducing to superior-inferior=0.28cm, lateral=0.21cm, anterior-posterior=0.18cm with our NAL protocol. Overall σ was superior-inferior=0.38cm, lateral=0.25cm, anterior-posterior=0.11cm. The Van Herk formula suggests margins for this population are SI=0.95cm, Lat=0.7cm, AP =0.53cm.

Conclusion

Our institution CTV-PTV margins are adequate for set-up errors but do not account for internal motion or delineation errors. Our NAL protocol identifies and reduces Σ but we would recommend online imaging in the first 3 fractions to further reduce the high Σ and σ.Future use of4-D RTplanning couldallowfurther accuracy in PTV delineation by incorporating tumour movement.