Does an offline no-action-level correction protocol allow accurate delivery of radical radiotherapy in mid-lower oesophageal cancers?


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Qurrat Mehmood1, Julie Stratford1, Lubna Bhatt1, Alan Jackson1, Hamid Sheikh1
1The Christie NHS Foundation Trust, Manchester, UK

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.