Abstract book - ESTRO meets Asia

S117 ESTRO meets Asia 2018

used for statistical analysis and the P-value of 0.05 was set as the level of significance. Results While the conformity number (CN) and the homogeneity index (HI) of both VMAT plans were comparable (showing similar target coverage), the four dose parameters (i.e. Dmean, D05%, D95%, Dmax) of the left and right cochleae in plans including the cochleae as dose objective for plan optimization was significantly lower than those plans without including the cochleae as dose objective, especially in the advanced stage NPC patients. Conclusion Including the cochlea as dose objective in plan optimization is an effective method to minimize the cochleae dose in NPC without compromising the target coverage. Therefore, this planning technique is recommended to preserve patients’ hearing functions. PO-284 A simple technique for treatment of the posterior cervical lymph nodes in resource limited settings N. Joseph 1 , A. Ramalingam 1 , S. Sureranjan 1 , L. Alagiyawanna 2 , C. Akurana 2 , A. Choudhury 3 1 Teaching Hospital, Clinical Oncology, Jaffna, Sri Lanka 2 National Cancer Institute, Clinical Oncology, Maharagama, Sri Lanka 3 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom Purpose or Objective Intensity modulated radiotherapy (IMRT) is now standard of care in the delivery of radical radiotherapy for squamous cell carcinoma of the head and neck. However, in many parts of the world where head and neck cancer is highly prevalent, access to linear accelerators capable of delivering inverse planned IMRT is limited. These centres are often equipped only with Cobalt teletherapy machines or single energy linear accelerators without inverse planned IMRT. In such a background, delivery more than 40-44 Gy to the posterior cervical nodes is not possible due to spinal cord tolerance. We performed a planning to study to evaluate a simple 3D conformal technique which permits the delivery of a higher dose to the posterior cervical lymph nodes in resource limited settings Material and Methods Twenty patients with SCCHN where the high dose planning target volume (PTV 70Gy) included the posterior cervical lymph nodes bilaterally were included in the study. The gross tumour volume, clinical target volume and planning target volumes were contoured as per institutional protocol. The posterior cervical lymph nodes defined as the nodal target volume posterior to the sternocleidomastoid muscle was contoured separately. Phase I treatment comprised two parallel opposed lateral fields with wedges treated to a dose of 40 Gy. For the second phasee, our novel technique involves treatment with 6 pairs of equispaced parallel opposed fields with conformal shielding of the spinal cord. The prescribed dose to the PTV for phase II was 30 Gy. The dose delivered to the PTV, posterior cervical lymph nodes and spinal cord with the novel technique was compared with conventional treatment which comprised two lateral parallel opposed fields with the posterior border adjusted to avoid the irradiation of the spinal cord. Results Using conventional treatment the mean minimum dose to PTV and posterior cervical lymph nodes was 44 Gy (range, 43-47). The novel technique resulted in a significantly higher mean minimum dose to the PTV at 59 Gy (range, 57-61; p<0.01) as well as a higher minimum dose to the posterior cervical lymph nodes at 61 Gy (range, 60-62Gy; p<0.01). There was no significant difference in the maximum dose to the spinal cord with the two techniques, with conventional treatment delivering a mean maximum

Figure 2. % average gamma pass-rate drop from original plan for all scenarios Conclusion EPID-based dosimetry showed the most sensitive QA tool to detect the realistic and complex errors in patient- specific HN VMAT QA. However, the threshold of patient specificVMAT QA should be determined for specific QA tool and specific treatment site due to detector sensitivity and treatment filed characteristic difference. PO-283 A dosimetric comparison on the cochleae dose in VMAT for radiotherapy treatment for NPC patients W.S.A. Mak 1 , K.H. Fong 1 , H.C. Shum 1 , Y.N. Wong 1 , Y.Y. Law 1 , Y.W.E. Cheung 1 , H.S. Ho 2 1 Tung Wah College, School of Medical and Health Sciences, Kowloon, Hong Kong SAR China 2 Tuen Mun Hospital, Oncology Center New Territories West, Hong Kong SAR China Purpose or Objective In radiation therapy (RT) of Nasopharyngeal Carcinoma (NPC), the cochleae would receive significant dose which would lead to hearing loss of the patients. This study aimed to evaluate whether including the cochleae as dose objective in plan optimization would minimize the dose to the cochleae in Volumetric Modulated Arc Therapy (VMAT) for NPC. Material and Methods This was a retrospective study with prior ethics approval obtained. Between July and October 2017, the treatment planning computed tomography (CT) images to the head and neck region of fifteen patients with stage I-IVb non- metastatic NPC were recruited for this study. Radiation therapy to head and neck region of 54 Gy, 60 Gy and 70 Gy in 1.6Gy/fr, 1.8Gy/fr and 2.12Gy/fr was prescribed to the three planning target volumes (PTVs) simultaneously: including the primary tumor, microscopic disease and the neck lymph nodes for the squamous cell carcinoma NPC patients. Two plans were generated for each patient: (1) VMAT plan including the cochleae as dose objective in plan optimization (VMAT+op), and (2) VMAT plan without including the cochleae as dose objective in plan optimization (VMAT-op). They were then compared for evaluation of their performance. The cochleae were chosen as they were related to hearing ability. The cochleae delineation was based on the CT images with the use of bone window level. MRI fusion would be used for assisting the cochlea delineation. The cochleae dose objective for plan optimization were standardized - less than 0% of the cochlea receives more than 40Gy with priority 50. VMAT-op plans were generated first. Hence, VMAT+op plans were generated after adding the cochleae dose objectives. Minor adjustments were needed according to the PTVs coverage and the dose to the OARs. Four dose parameters: mean dose (Dmean), dose received in 5% volume (D05%) and 95% volume (D95%), maximum dose (Dmax) to both left and right cochleae were used to analyze the cochleae dose. Wilcoxon Signed Rank test was