Is there a role for a biological boost in squamous cell anal cancer?


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Rebecca Muirhead1,Ahmad Sabbagh2,Clare Jacobs2,Rosie Cooke3,Kwun-Ye Chu3,Stasya M Ng4,Victoria Y. Strauss5,Pradeep S. Virdee5,Maria A. Hawkins6,Marianne C. Aznar7,Rebecca Muirhead2
1NHS,2Department of Oncology, Oxford University Hospitals NHS Foundation Trust,3Department of Oncology, Oxford University Hospitals NHS Foundation Trust and CRUK MRC Oxford Institute for Radiation Oncology, University of Oxford,4Oncology Clinical Trials Office, Department of Oncology, University of Oxford,5Centre for Statistics in Medicine, NDORMS, University of Oxford,6CRUK MRC Oxford Institute for Radiation Oncology, University of Oxford,7Manchester Cancer research Centre, Division of Cancer Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, University of Manchester

Abstract

Background

Dose escalation to the whole gross tumour is under investigation in anal cancer chemoradiotherapy (CRT) with a view to improving local control with acceptable increase in toxicity. Targeting a smaller biological boost would likely limit the increased toxicity.

We aimed to investigate the potential role for FDG-PET derived biological boost, by assessing whether the biologically defined volumes identified at outset are spatially consistent during a course of chemoradiotherapy.

  

Method

21 paired FDG-PET scans from patients enrolled into the ART study (NCT02145416) were retrospectively analysed. 29 volumes included primary tumours and involved nodes >2 cm. FDG-PET scans were done prior to treatment and at day 8 or 9 of CRT and deformably registered to the planning CT scan. Volumes were created using thresholds of 34%, 40%, 50% of the maximum SUV on the pre-treatment scans, and at 70% and 80% on the subsequent scans. The overlap fraction (OF), and vector distance (VD) were calculated.

Results

The median (interquartile range (IQR)) OF of the Pre34% (0.97 (0.76, 1.00)), Pre40% (0.92 (0.71, 1.00)) and Pre50% (0.81 (0.60, 1.00)) thresholds with the Sub70% thresholds were each above 0.8. The median (IQR) OF of the Pre34% (1.00 (0.92, 1.00)), Pre40% (1.00 (0.80, 1.00)) and Pre50% (0.92 (0.50, 1.00)) thresholds with the Sub80% thresholds were each above 0.9. The median (range) VD values between Pre34% and Sub80% and Pre40% and the Sub80% were 0.74mm (0.19-2.94) and 0.74mm (0.19-3.39) respectively. Using a mean OF of > 0.7 defined in previous publications as a “good” correlation, 20 out of 29 of the Pre50% subvolumes (69%) achieved that threshold with the relevant Sub80% volumes.

Conclusion

Areas identified at baseline were spatially consistent during chemoradiotherapy treatment. The threshold of 50% of SUVmax on the pre-treatment scan could be considered as a potential target for dose escalation.