The dosimetric and clinical advantages of the GTV-CTV-PTV margins reduction by 6mm in head and neck squamous cell carcinoma: significant acute and late toxicity reduction.

We aim to identify the dosimetric and clinical impact of reducing the total GTV-CTV-PTV margins in head-and-neck squamous cell carcinoma (HNSCC) treated with definitive (chemo)radiation.The acute and late toxicity and outcomes of 155 consecutive patients treated between February 2017 and March 2019 with GTV-CTV-PTV margins of 6mm were compared to those of 155 consecutive patients treated with total margin of 9mm margin, before April 2015. All patients were treated with VMAT with daily-image guidance using CBCT.Reducing the GTV-CTV-PTV by 6mm resulted in significant reduction of total irradiated volume (PTV-total) by a median of 28.1% and significant reduction of doses to all salivary glands (largest reduction ipsilateral parotid gland; median -9.6 Gy) and constrictor muscle (-6.1 Gy) with subsequent reduction of the incidence of overall acute grade 3 toxicity (47.7% for 9mm and 66.5% for 15 mm groups, p=0.001), grade 3 mucositis (18.1% vs. 35.5%, p<0.001) and feeding tube-dependency at the end of treatment (24.5% vs. 40%, p=0.005). The incidence of late grade ≥2 xerostomia and dysphagia were also significantly lower in the 9mm group (31.7% vs. 58.6% p<0.001, and 15.4% vs. 26.7%, p=0.04). The 2-year rates of loco-regional control, disease-free and overall survival were 78.8% vs.75.8%, 70.9% vs. 64.4%, and 83.8% vs. 67.6%, (p>0.05, all).Reduction of the total GTV-CTV-PTV margins from 15 to 9mm in HNSCC significantly reduced the irradiated volumes and the dose to salivary glands and constrictor muscle with significant reduction of radiation-related toxicity. The loco-regional control rates of both groups were comparable.