Is there a role for a biological boost in squamous cell anal cancer?
Session type: Poster / e-Poster / Silent Theatre session
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.
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.
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.
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.