ESTRO 36 Abstract Book

S846 ESTRO 36 _______________________________________________________________________________________________

Results

maximum dose to the left optic nerve (p=0.005) and chiasm (p=0.035). It also lowered the maximum dose and mean dose to the hippocampus (both p values were 0.035). In addition, the volume receiving 5 Gy (V5) values for the temporal lobe and hippocampus were significantly smaller when the dynamic jaw mode was used (p=0.035; p=0.013). The comparison of the 5.0 cm dynamic jaw plan with the 5.0 cm fixed jaw plan also indicated no differences between plan efficiency, HI and CI results, or the dose indices of OARs within the radiation field. However, use of the 5.0 cm dynamic jaw mode significantly improved the maximum dose to the left and right optic nerve (p<0.0005) and chiasm (p=0.005). The mean dose to the temporal lobe was significantly improved using the dynamic jaw mode (p=0.013). It also lowered the maximum dose and mean dose to the hippocampus (both p=0.035). Finally, the dynamic jaw mode also significantly reduced the V5, V10, and V20 values for the temporal lobe and hippocampus. Conclusion When the same field width, the dynamic jaw mode provided better neural structure protection than the fixed jaw mode, in addition to reducing the low-dose volume to the temporal lobe and hippocampus. These advantages were more pronounced when using a larger field width. There were no significant differences, however, in plan quality and efficiency between the two modes for the same field width. The above results indicate that for the treatment of locally advanced NPC patients, use of the 2.5 cm dynamic jaw mode rather than the 2.5 cm fixed jaw mode currently used in clinical practice would provide better neural structure protection and lower low-dose volumes to the temporal lobe and hippocampus with equal plan efficiency. EP-1571 Radiotherapy treatments using a prototype MLC design P. Nitsch 1 , Y. Li 1 , T. Netherton 1 , P. Balter 1 , S. Gao 1 , M. Muruganandham 1 , S. Shaitelman 2 , S. Frank 2 , S. Hahn 2 , A. Klopp 2 , L. Court 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA 2 The University of Texas MD Anderson Cancer Center, Radiation Oncology, Houston, USA Purpose or Objective We are evaluating the clinical efficacy of a prototype multi-leaf collimator (MLC) design which obviates the need for collimating jaws. The new MLCs are 1.0cm wide, potentially giving increased reliability, and have a maximum speed of 5.0cm/sec. The increased leaf width may reduce the achievable intensity modulation, but the impact of this may be mitigated by the increase in MLC speed and by using IMRT/VMAT treatment planning. Here we evaluate (1) whether clinically acceptable plans can be created using such an MLC design, and (2) the agreement between the planned and delivered dose distributions. Material and Methods IMRT, VMAT, field-in-field and electronic compensator plans were created in the Eclipse treatment planning system using the prototype MLC design and a flattening- filter-free 6MV beam, for the following treatment sites: head/neck, lung (standard fractionation, palliative and SBRT plans), cervix (pelvis and extended fields), intact breast (left and right), prostate (SBRT and involved nodes), and whole brain treatments. The planned dose distributions and DVHs reviewed for clinical acceptability by radiation oncologists, and compared with our original clinical plans (120leaf Millennium MLC, 0.5cm MLCs). Delivered dose distributions for IMRT and VMAT plans were evaluated using the ArcCHECK array. Results In most situations, the plan quality (particularly homogeneity in the target) was highest for IMRT (with

In each case the CI is improved using the VMAT solution. The average CI for SCF plans was 0.64, this increased to 0.81 when using VMAT. The GI appeared to increase in several cases, with improvement only shown in 4 out of 10 cases. In each case, NTOF was reduced to close to zero. Perhaps surprisingly for a VMAT solution, the 5Gy volume was reduced for 9 out of 10 cases. The average 5Gy volume for the SCF plans was 36.65cc; this reduced to 26.55cc when using VMAT. Conclusion Paddick CI has proven to be a reliable metric for conformity, while NTOF gives a good indication of how baggy the prescription isodose is. However, Paddick GI was not found to be a reliable indicator of low dose spread, especially for small volumes. In many cases the 40% and 80% volumes had each been reduced and conformity improved but a higher GI was recorded. The Eclipse VMAT solution is adequate for a wide range of tumour shapes, sizes and locations and can be used for all single-target SRS patients. EP-1570 The dynamic jaw mode of tomotherapy: Better neural structure protection for advanced NPC patients? P.T. Fang 1 , T.Y. Lu 1 , M.N. Wu 2 , M.Y. Huang 1 , C.J. Huang 1 1 Kaohsiung Medical University Hospital, Radiation Oncology, Kaohsiung, Taiwan 2 Kaohsiung Medical University Hospital, Neurology, Kaohsiung, Taiwan Purpose or Objective This study investigated the neural structure protection effects of the dynamic jaw mode of tomotherapy for advanced nasopharyngeal cancer (NPC) by comparing use of the dynamic jaw mode and the fixed jaw mode with Twenty patients with locally advanced NPC were selected. All T classifications were T3-4. Plans were using the simultaneous integrated boost technique in 3 dose levels. Four plans were generated utilizing the 2.5 cm dynamic, 2.5 cm fixed, 5.0 cm dynamic, and 5.0 cm fixed jaw modes. Plan efficiency was evaluated in terms of monitor unit and beam-on-time measurements. Plan quality was evaluated using homogeneity index (HI) and conformity index (CI). Dose indices of neural structures such as the optic pathways, temporal lobe, and hippocampus, as well as of other organs at risk (OAR), were compared. Volumetric parameters of the hippocampus and temporal lobe were also assessed. Only plans of the same field width were compared with one another. Results The comparison of the 2.5 cm dynamic jaw plan with the 2.5 cm fixed jaw plan indicated no differences between plan efficiency, HI and CI results, or the dose indices of OARs within the radiation field. However, use of the 2.5 cm dynamic jaw mode significantly improved the different field widths. Material and Methods