Abstract book - ESTRO meets Asia

S114 ESTRO meets Asia 2018

PO-276 Planning study of 6MV FB versus 6MV UF beam in AXB and AAA algorithm in CSI cases using Rapid Arc T. Suresh 1 , S. Madeswaran 2 , Y. Girigesh 3 , S. kothandaraman 3 , G. Munish 3 , D. Abhinav 3 1 Rajiv Gandhi Cancer Institute & Research Centre, Radiotherapy, Delhi, India 2 VIT University, Physics, Vellore, India 3 Rajiv Gandhi Cancer Institute & Research Centre, Radiotherapy, New Delhi, India Purpose or Objective To evaluate the dosimetric advantage of 6MV photon unflattened (UF) beam over 6MV photon flattened beam (FB) in rapid arc modality in craniospinal irradiation (CSI) using Acuros XB (AXB) and Analytical anisotropic algorithm (AAA). Material and Methods Five patients were selected randomly. Patient is positioned under the vac loc in supine position and thermoplastic immobilization device is used to cover the head, thoracic, abdomen and pelvic region. CT scan were acquired for 3D planning and the slice thickness is 3mm. RA plans were planned on Eclipse treatment planning system (V11.0) using true beam linear accelerator in built with high definition MLC. Four isocenters were used to plan the patient, due to large treatment length and the limitation of MLC. The field isocentre locations were brain, cervical, thoracic and lumbar spine. Two full arc (360°) is placed for brain field and two partial arc of 120° directed posterior to each spine fields. The dose prescription to PTV was 36Gy. Rapid arc plans were created using 6MV FB and 6MV UFB photon beam, progressive Resolution Optimizer as inverse planning optimiser is used. Final dose calculated were performed in AXB and AAA with grid size of 2.5mm. The dosimetric parameters were D 98%, D 95%, D 50%, V 110%, conformity index (CI), homogeneity index (HI), low grade dose index, high grade dose index, dose to organ at risks (OAR’s), normal tissue mean dose and noted the effect of low doses on normal tissue were analyzed. Results 6MV flattened and unflattened beam generates similar target coverage (D 98%, D 95%, D 50%, & D 2% ) in both algorithm and significant p value(p<0.05) noted in CI, HI and V 110% . The 6MV UF beam spares Lens, eyes, optic nerves, parotids, thyroid, common lung, heart, kidney, bowel, bladder and body minus PTV region. The p value significant observed in HI, CI, V 110% , common lung, bowel, testis, Treatment MU, low dose of 1Gy, 2Gy, 3Gy, 4Gy and 5Gy on normal tissue. Conclusion 6MV Unflattened beam with arc technique generate highly conformal, homogenous plan and spars OAR’s in CSI cases. UF beam reduce the chance of second cancer risk due to lesser dose on non-targeted tissues. PO-277 Neck skin dose evaluation for optimized nasopharyngeal carcinoma treatments X. Liao 1 , J. Li 1 , Y. Zhang 1 , P. Wang 1 , X. Yan 1 , T. Tan 1 , L. Orlandini 1 , P. Xu 1 , C. Li 1 1 Sichuan Cancer Hospital & Institute, Radiotherapy, ChengDu, China Purpose or Objective Acute skin toxicity observed for head-and-neck cancer radiotherapy is a big concern. Treatments planning optimized using neck skin as a sensitive structure can be a valid option to reduce skin toxicity. The purpose of this study is to evaluate the skin dose reduction obtained with this type of optimization and to confirm the results with MOSFET in vivo dose measurements.

Material and Methods Computed tomography scans of an anthropomorphic phantom set up with a thermoplastic mask were acquired. Three points A,B,C in the IIII, IV, V lymphatic drainage areas were highlighted on the surface of the neck skin.Targets and organs at risk were contoured on MIM and transmitted to Eclipse and Tomotherapy treatment planning systems. The neck skin tissue included 3 mm from the skin surface. For each treatment modality (dIMRT, VMAT and HT) two treatments planning were performed: a standard plan (SP) following the goals and constraints defined in our center and a new plan (NP) taking into consideration also the neck skin as a sensitive structure during inverse planning optimization. Neck skin dose volume histogram cut-off points Dmean and V25Gy, doses in points A, B, C computed by the TPS and measured using MOSFET were used for SP and NP comparisons. Mosfet dose measurements were repeated for NP delivered on the anthropomorphic phantom set up without mask. Results In Table 1, percentage neck skin dose reduction for NP vs SP was reported. A further dose-decrease in point A, B, C of 2.6%, 20.97%, 22.9%was measured for HT, 6.2%, 3.9%, 8.6% for dIMRT and 8.0%, 7.8%, 7.3% for VMAT respectively when the phantom was irradiated without mask. Conclusion Taking into consideration the skin as a sensitive structure during inverse planning optimization, it is possible to reduce the skin dose. Measurements performed with MOSFET in drainage areas III, IV and V confirmed this trend. A further skin dose reduction can be obtained when the mask is removed. PO-278 Dosimetric study of HybridVMAT and IMRT for swallowing-structure sparing in the treatment of NPC M. Siu 1 , M. Cheng 1 , P. Choi 1 , M. Wong 1 , J. Wong 1 , D. Man 2 1 Tung Wah College, Radiation Therapy, Hong Kong, Hong Kong SAR China 2 Queen Elizabeth Hospital, Radiation Therapy, Hong Kong, Hong Kong SAR China Purpose or Objective This study compared the difference in mean dose and the feasibility of sparing pharyngeal constrictor muscles (PCMs) and midline dysphagia /aspiration related structures (DARS) in HybridVMAT and IMRT for Nasopharyngeal cancer (NPC) patients. Material and Methods Thirty nasopharyngeal cancer patients were retrospectively planned with IMRT and HybridVMAT using the same planning CT images. Planned Target Volume (PTV) and Contouring of Organs-at-risk (OARs) were adopted from a clinical radiotherapy centre in Hong Kong. PCMs and DAR structures were delineated based on our modification to the RTOG protocol. For IMRT, nine isocentric, coplanar fields were used and evenly distributed for treatment planning. For HybridVMAT, approximate 50% volume of cervical PTV received 50% of the prescribed dose by AP paired-opposed beam. The residual volume of PTV was optimized by 3 arcs of a Volumetric Modulated Arc therapy (VMAT). At least 95 % of neck volume received 95% prescribed dose as the planning goal. PCMs and DARS were adequately spared Table1 NP vs SP: Percentage skin dose Reduction