ESTRO 36 Abstract Book

S998 ESTRO 36 _______________________________________________________________________________________________

EP-1821 Air gap between patient surface and immobilization devices: dosimetric impact on H&N IMRT plans S. Moragues-Femenia 1 , M. Pozo-Massó 2 , J.F. Calvo- Ortega 2 , J. Casals-Farran 2 1 Moragues Femenia Sandra, HOSPITAL QUIRON BARCELONA- Radiotherapy, Barcelona, Spain 2 HOSPITAL QUIRON BARCELONA- Radiotherapy, HOSPITAL QUIRON BARCELONA- Radiotherapy, Barcelona, Spain Purpose or Objective In head and neck (H&N) treatments, there is an avoidable air gap between the typical mask-based immobilization device used and the patient surface ("air gap" from now on). Our aim is to evaluate the dosimetric effect of considering the "air gap" on the patient dose distribution on H&N IMRT plans. Material and Methods A total of 5 H&N patients were selected. The "immobilization" device consisted of a thermoplastic mask covering the head, neck and shoulders, and attached to a board on the linac couch. Targets (PTVs) and organs-at- 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.