ESTRO 38 Abstract book

S372 ESTRO 38

P. De veij mestdagh 1 , W.V. Vogel 2 , M.L. Donswijk 2 , E. Lamers 1 , C. Carbaat 1 , W.H. Schreuder 3 , M.W.M. Van den Brekel 3 , A. Al-Mamgani 1 1 Netherlands Cancer Institute / Antoni van Leeuwenhoek, Radiotherapy, Amsterdam, The Netherlands ; 2 Netherlands Cancer Institute / Antoni van Leeuwenhoek, Nuclear Medicine, Amsterdam, The Netherlands ; 3 Netherlands Cancer Institute / Antoni van Leeuwenhoek, Head and Neck surgery, Amsterdam, The Netherlands Purpose or Objective The great majority of patients with lateralized head and neck squamous cell carcinoma (HNSCC) treated with radiotherapy routinely undergo bilateral elective nodal irradiation (ENI), even though the incidence of contralateral regional failure after unilateral ENI is low. Excluding the contralateral neck from elective irradiation could reduce radiation-related toxicity and improve quality of life. The SUSPECT study (NCT02572661), a prospective fase 2 trial, investigates the feasibility, safety and clinical benefits of a novel, image-guided approach, in which lymph drainage mapping guides the elective irradiation of the neck in HNSCC patients. While we await data maturation for evaluation of oncologic safety, we report on the incidence, severity and duration of acute and late toxicity. Material and Methods Patients with lateralized cT1-3N0-2bM0 HNSCC were eligible for treatment with selective SPECT/CT-guided ENI (SSG-ENI). In case no contralateral drainage was visualized on SPECT/CT, patients received ipsilateral ENI only. If contralateral drainage to one “hot spot” was visible, the elective irradiation to the contralateral neck would be limited to the level containing the hot spot. In case of ≥2 contralateral hot spots on SPECT/CT, the patient was treated with conventional bilateral ENI, and not evaluable for the study endpoints. A matched historical cohort was formed that received conventional bilateral ENI (B-ENI) according to international guidelines. Matching was done based on tumor subsite, T- and N-classification, whether the patient received chemotherapy, and (for oropharyngeal tumors) HPV-status. Planning and treatment technique were identical to the study cohort. Results Fifty patients were treated with SSG-ENI, and were matched to 50 patients treated with B-ENI. Aside from follow-up time and CTV-PTV margin, baseline characteristics between the cohorts did not significantly differ. Mean irradiation doses to organs at risk were lower in the SSG-ENI cohort vs. the B-ENI cohort (contralateral parotid: 5 vs. 19 Gy; contralateral submandibular: 20 vs. 46 Gy; larynx: 39 vs. 52 Gy; constrictor muscles: 39 vs. 53 Gy; all tests p<0.001 [Mann-Whitney-U]). The SSG-ENI cohort had a lower incidence of acute dysphagia, including tube feeding incidence (SSG-ENI: 10%, B-ENI: 50%, p<0.001 [χ²]), and a shorter median duration of acute mucositis and dysphagia. In the first year after treatment, incidences of grade≥2 late xerostomia and grade≥2 late dysphagia were significantly lower in the SSG-ENI cohort, including tube feeding incidence (3% vs. 26%, p=0.002 [χ²]). On multivariate logistic regression, bilateral ENI (OR 7.89, 95% CI 2.17-28.61, p=0.002) and concurrent chemotherapy (OR 4.58, 95% CI 1.33-15.76, p=0.016) were predictive for feeding tube placement.

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PO-0725 Multimodality imaging employing FDG-PET/CT paves the way for de-escalation of the elective dose S. Van den Bosch 1 , P.A.H. Doornaert 2 , C.H.J. Terhaard 2 , J.H.A.M. Kaanders 1 1 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands ; 2 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Modern multimodality imaging has improved the detection threshold of small nodal metastases in head and neck squamous cell carcinoma (HNSCC). This influences the tumorload in elective nodal target volumes and may have consequences for the radiotherapy dose required to control disease in this volume. This study investigates the effects of the introduction of FDG-PET/CT for radiotherapy planning on neck recurrence in HNSCC. Material and Methods A patient cohort that was treated for HNSCC with definitive (chemo)radiotherapy using IMRT/VMAT techniques in two tertiary head and neck clinics in The Netherlands from 2008-2016 was retrospectively analyzed. The focus of this analysis was on regional recurrence in the elective nodal target volume. For this purpose, co- registration was performed of the scans acquired during follow-up demonstrating the neck recurrence and the initial radiotherapy planning (FDG-PET)CT-scan. Actuarial rates of recurrence were calculated using the Kaplan- Meier method and multivariate analyses were performed using the Cox proportional-hazards model. Results A total of 633 patients with all stages HNSCC were included. A pre-treatment FDG-PET/CT-scan was acquired in 46% (290/633) and concomitant chemotherapy was administered in 38% (238/633) of the patients. Median follow-up was 32 months (IQR: 20-41). Two years after treatment, recurrence in the elective nodal target volume was observed in 3.9% (95% CI: 2.3-5.5) (24/633) of all patients. This was only 2.0% (95%CI: 0.2-3.8) in patients that had a FDG-PET/CT-scan for treatment planning ( n =290) versus 5.6% (95% CI: 3.1-8.2) in patients without ( n =343) (p=0.01). The majority of these recurrences occurred synchronously with local recurrence: 17.4% (95% CI: 9.6-25.2) and were isolated in only 1.1% (95% CI: 1.0- 1.2) (p<0.001). For patients free of local recurrence, no recurrences in the elective nodal target volume occurred when a FDG-PET/CT-scan was acquired prior to treatment (p=0.01). No differences in the rate of recurrence in the elective nodal target volume occurred in patients treated with concomitant chemotherapy 3.9% (95% CI: 1.2-6.6) versus 3.9% (95% CI: 1.9-5.9) (p=0.65). In multivariate analyses, both local recurrence (p<0.001) and acquisition of FDG-PET/CT-scan prior to treatment (p=0.03) were significant predictors for recurrence in the elective nodal target volume, concomitant chemotherapy was not (p=0.59). Conclusion The vast majority of nodal recurrences in the elective nodal target volume occur synchronously with local recurrence. Further, concomitant chemotherapy does not reduce this recurrence rate. Both observations indicate that most of these nodal manifestations are new seedings from the local recurrent tumor. Isolated recurrences in the elective nodal target volume are extremely rare (1.1%) and none occurred when FDG-PET/CT-scan was used for radiotherapy planning. This strongly supports the notion that elective neck dose can be safely de-escalated resulting in reduced toxicity of HNSCC radiotherapy. PO-0726 Lower toxicity incidence after SPECT/CT- guided elective nodal irradiation for head and neck cancer