ESTRO 2021 Abstract Book

S847

ESTRO 2021

therapy (VMAT) photon plan was calculated for the purpose of this study for every patient and compared with the PBSPT delivered plan. The relevant OARs’ doses were extracted to perform the NTCP analysis using the Dutch model based approach for head and neck risk calculation for development of grade ≥2 dysphagia and xerostomia within 6 months after irradiation. Results With a median follow-up time of 46 months (range, 3 – 118 months), 2 (8%) patients died with uncontrolled cancer disease, 2 (8%) experienced local failure (LF) and 5 (19%) suffered from distant failure after PBSPT. Estimated 4-year OS, LC and DC was 90.2%, 90.8% and 77.2%, respectively. Grade 3 acute toxicity (dermatitis, mucositis, otitis media) was observed in 5 (19%) patients. Late Grade 3 otitis media and grade 3 wound dehiscence was observed in 2 (8%) patients. There was no grade 4 or 5 toxicity. With PBSPT, compared to VMAT, a significant reduction of the median dose to the oral cavity (13.5 Gy(RBE) vs. 29.5 Gy, p<000.1), the contralateral parotid (0 Gy(RBE) vs. 10.6 Gy, p<000.1), the contralateral submandibular gland (1.1 Gy(RBE) vs. 18.5 Gy, p<000.1) and the integral dose (10.4 Gy(RBE) vs. 17.8 Gy, p<000.1) was observed. Eleven patients (42%) met the Dutch eligibility criteria qualifying for proton therapy (PT). Conclusion We report excellent oncological outcome and toxicity rates for patients with SGT treated with PBSPT. Significant lower OAR doses were observed with PBSPT plans compared to VMAT plans. NTCP calculation identified 11 (42%) patients with SGT qualifying for PT through a substantial risk reduction of developing grade ≥2 xerostomia or dysphagia within 6 months after irradiation. PO-1018 Dose Escalation in Advanced Floor of the Mouth Cancer Using a Combination of IMRT and SRT Boost T. Blazek 1 1 University Hospital Ostrava, Oncology clinic, Ostrava, Czech Republic Purpose or Objective Treatment of advanced stages of the floor of the mouth tumors requiers a complex approach based on combination of surgery and radiotherapy. For patients who are considered beyond cure and surgery is not feasible, definitive radiochemotherapy is the preferred option. Brachytherapy in combination with external beam radiotherapy (EBRT) and chemotherapy is an effective technique that leads to local dose escalation over the possibilities of up-to-date EBRT technologies. In technically infeasible cases, such as large tumors, patients low performance status, and contraindications of invasive procedures, a new image-guidede stereotactic radiotherapy technique is an alternative option. In addition, preclinical studies suggest that the effect of stereotactic radiotherapy includes the activation of ceramide apoptosis and alteration of the tumor vasculature, which may eventually help overcome tumor radioresistance. We evaluated the efficiency and toxicity of stereotactic hypofractionated boost in combination with conventionally fractionated radiotherapy in the treatment of advanced floor of the mouth cancer. Materials and Methods Thirty-seven patients with histologically confirmed squamous cell carcinoma (p16 negative) ineligible for surgical treatment were indicated for radiochemotherapy or hyperfractionated accelerated radiotherapy (HART). Concurrent chemotherapy cisplatin (40mg/m2) was administered weekly up to a cumulative dose 200mg/m2. The radiotherapy protocol combined external beam radiotherapy (EBRT) and a stereotactic hypofractionated boost to the primary tumor. The dose delivered from EBRT was 70-72.5Gy in 35/50 fractions. The hypofractionated boost followed with 10 Gy in two fractions. For the variables - tumor volume, stage and grade a multivariate analysis was performed to find the relationship between overall survival, local progression and metastasis. Toxicity was evaluated according to CTCAE scale version 4. Results After a median follow-up of 16 months, 23 patients (62%) achieved complete remission. The median time to local progression and metastasis was 7 months. Local control (LC), progression-free, and overall survival at 5 years was 62%, 41%, and 27%, respectively. Statistical analysis revealed that larger tumors had worse overall survival and greater chance of metastasis. Log-Rank GTV >44 ccm (HR = 1.96; [95% CI (0.87; 4.38)]; p = 0.11). The maximum acute treatment toxicity was grade 3 mucositis and grade 3 dysphagia, both of which manifested in 10(27%) patients who required intensification of pain treatment with opioids and nutritional support wia the feeding tube. Late osteonecrosis was observed in 3 patients (8%).