Differences between planned and delivered dose for head and neck cancer, and their consequences for normal tissue complication probability and treatment adaptation

Background and purpose: Anatomical changes induce differences between planned and delivered dose. Adaptive radiotherapy (ART) may reduce these differences but the optimal implementation is insufficiently clear. The aims of this study were to quantify the difference between planned and delivered dose in HNC patients, assess the consequential difference in normal tissue complication probability (Delta NTCP) and to explore the value of Delta NTCP as an objective selection strategy for ART.Materials and methods: For 52 patients, daily doses were accumulated to estimate the delivered dose. The difference from planned dose was analyzed for CTVs and 9 organs-at-risk (OAR). Delta NTCP was calculated for xerostomia, dysphagia, parotid gland dysfunction and tube feeding dependency at 6 months. ART was deemed necessary if Delta NTCP was >5%. The positive predicted value (PPV) was calculated for identification of ART-patients by clinical judgement, and Delta NTCP at fraction 10 and 15.Results: Delta NTCP >5% was seen five times for dysphagia and twice for the other toxicities. Only 5/9 patients with any Delta NTCP >5% clinically received ART, although ART had been done for 13/52 patients (PPV: 0.38). PPV was 0.86 and 0.75 for accumulated dose at fraction 10 and 15, respectively, using a Delta NTCP cut-off for the allocation of ART of 5%. Using other Delta NTCP cut-offs did not substantially improve PPV. With this cutoff the negative predictive value was 0.93 for Delta NTCP method of fraction 10 and fraction 15, and 0.90 for clinical judgement.Conclusion: To identify patients accurately for ART, NTCP calculations based on the dose differences between planned and delivered dose at fraction 10 are superior to clinical judgement. (C) 2019 Published by Elsevier B.V.