ESTRO 36 Abstract Book

S984 ESTRO 36 2017 _______________________________________________________________________________________________

risks (OARs: spinal cord, brainstem, optic nerves, parotids and oral cavity) were outlined in Eclipse TPS. The posterior aspect of patient skin was also contoured. Two different approaches were proposed to define the "body structure": 1) the patient outer contour plus the immobilization device (Fig 1); and 2) as previous but also including the air gap between the immobilization device and the patient outer contour (Fig 2). Dose distributions were calculated using identical IMRT plans for each approach. The differences in the minimum (D98%), maximum (D2%) and mean (Dmean) doses to the PTVs and OARs as well as the skin mean doses were compared. Results Differences within ±1% were found in the dosimetric parameters analyzed for PTVs and OARs. Mean skin was up to 2% greater when the gap air between patient surface was considered. Conclusion Little dose differences were observed between the approaches of including or not the air gap existing between the immobilization device and the patient surface. EP-1822 Monitoring of parotid gland changes in radiotherapy of NPC with parapharyngeal space involvement W.C.V. Wu 1 , Y. Zhang 2 , C. Lin 2 , J. Wu 2 1 Hong Kong Polytechnic University, HTI, Kowloon- Hong Kong- SAR, China 2 Sun Yat-sen University Cancer, Cancer Hospital, Guangzhou, China Purpose or Objective Parapharyngeal space (PPS) involvement is present in over 70% of nasopharyngeal carcinoma (NPC) patients. Since PPS is close to parotid gland, a radical course of radiotherapy for this group of patients may deliver high dose to this organ. The purpose of this study was to evaluate the parotid gland changes of NPC patients with PPS involvement during radiotherapy and up to 3 months after treatment. Material and Methods Kilovoltage computed tomography (CT) scans of head and neck region of 39 NPC patients with PPS involvement recruited from Sun Yat-sen University Cancer Center between January 2011 and April 2013 were performed at pre-radiotherapy, 10 th , 20 th and 30 th fractions, and 3 months after treatment. All patients were treated with intensity modulated radiotherapy using 6 MV photons with prescribed doses of 66-70 Gy to the target volume. The parotid glands were contoured in pre-radiotherapy planning CT scan and in subsequent scans. At each time interval, DICE similarity coefficient (DSC), percentage volume change and centroid movement between the planning CT and the subsequent CTs were obtained from the contouring software. In addition, the distance between medial and lateral borders of parotid glands from the midline at various time intervals were also measured. Results The ipsilateral parotid gland received a mean dose of about 5 Gy higher than the contralateral side (56.3±6.2 Gy vs 51.7±9.2 Gy). The mean DSC for ipsilateral parotid gland decreased to 0.63 at 30 th fraction and returned to 0.74 at 3 months after treatment. Partial recovery was observed at 3 months after treatment. All differences between each pair of consecutive measurements (such as between 10 th and 20 th fractions and 20 th and 30 th fractions) were statistically significant (p < 0.05). The mean volume change for ipsilateral parotid gland decreased from - 15.27% at 20 th fraction to -37.49% at 30 th fraction and partially recovered to -23.14% in 3 months. There were no significant differences between ipsilateral and contralateral groups despite the changes in the ipsilateral side being relatively greater. The centroid displacement followed a similar pattern, which moved medially and

superiorly by an average of 0.30 cm and 0.18 cm respectively at 30 th fraction. The changes in ipsilateral gland were slightly greater than the contralateral side. Conclusion In radiotherapy of NPC patients with PPS involvement, the parotid gland shrank by about 1/3 towards the end of the treatment course. DSC and percentage volume changes of both ipsilateral and contralateral parotid glands decreased during the radiotherapy course and partially recovered in 3 months after treatment. This trend was also seen in the displacements of centroids and the medial and lateral borders of the gland. A re-planning was suggested at around 15 th to 20 th fraction so as to reduce the dose to the parotid gland due to the detected movement of this structure during the radiotherapy course. EP-1823 DVH- and NTCP-based dosimetric comparison of different margins for VMAT-IMRT of esophageal cancer S. Münch 1 , M. Oechsner 1 , S.E. Combs 1,2 , D. Habermehl 1 1 Klinikum rechts der Isar- TU München, Radiation Oncology, München, Germany 2 Institute of innovative radiotherapy iRT, Hermholtz Zentrum, Oberschleißheim, Germany Purpose or Objective To cover the microscopic, longitudinal tumor spread in squamous cell carcinoma of the esophagus (SCC), longitudinal margins of 3-4 centimetres are used for neoadjuvant and definitive radiotherapy (RT) protocols. Therefore, RT of SCC is often done with large treatment volumes, which lead to high doses to the organs at risk (OAR). However, while the promising results of the CROSS-Trial, that used longitudinal margins of 4 cm, defined a new standard for neoadjuvant chemoradiation (CRT), a smaller margin of 3 cm might be reasonable, especially for early tumor stages. Purpose of this study was to compare the dose distribution to the organs at risk for different longitudinal margins using a DVH- and NTC-based approach. Material and Methods 10 patients with SCC of the middle or the lower third, who underwent CRT at our institution were retrospective selected. Three planning target volumes (PTV) were created for every patient, with an axial margin of 1.5 cm to the gross target volume (GTV) (primary tumor and PET- positive lymph nodes), analogous to the protocol of the CROSS-trial. The longitudinal margins were 4 cm, 3 cm and 2 cm, respectively. Contouring and treatment planning was performed with the Eclipse 13 planning system (Varian Medical Systems, Palo Alto, CA, USA). For every PTV, volumetric modulated arc therapy (VMAT) plans were optimized. Dose calculation was performed using the AAA algorithm (version 10.0.28) and heterogeneity correction. All plans were normalized to a median prescribed PTV dose of 41.4 Gy with a daily dose of 1.8 Gy. Dose to the lungs, heart and liver were evaluated and compared. Differences of dose parameters were tested for significance with t- test for paired samples. Results Median tumor length was 6 cm with a range of 3 to 10 cm and 8 of the 10 patients (80%) had lymph node metastasis. When using a longitudinal margin of 3 cm instead of 4 cm, all dose parameters (Dmin, Dmax, Dmean, Dmedian and V5-V35), except Dmax could be significantly reduced for the lungs. Regarding the heart, a significant reduction was seen for Dmean and V5, whereas no significant difference was seen for Dmin, Dmax, Dmedian and V10-V35. When comparing a longitudinal margin of 4 cm to a longitudinal margin of 2 cm, not only Dmin, Dmax, Dmean, Dmedian and V5-V35 for the lungs, but also Dmax, Dmin and V5-V35 for the heart were significantly reduced. Nevertheless, no difference was seen for the median heart dose. In addition, the risk of pneumonitis was significantly reduced by a margin reduction of 3 cm and 2 cm.