ESTRO 37 Abstract book

S1074

ESTRO 37

Material and Methods CT scans of 5 patients were used in this study. The CTV was defined as the 0.5 cm tissue around the applicator (then subtracting the applicator). Total dose was 30Gy delivered in 5 fractions. In HDR-BRT, dose was prescribed at a distance of 0.5 cm from the surface applicator. To take into account set-up uncertainties in external VMAT irradiation, a PTV was obtained from CTV by an expansion of 3 mm. Two VMAT plans were generated using a full arc rotation. The first plan was optimized with an anatomy- based optimization module (AB_VMAT) using a 1mm MLC beam margin to enhance dose heterogeneity and dose fallout. The second plan was generated with a full- inverse planning module (FI_VMAT). CTV and PTV coverage in terms of D99%, mean dose (Dmean) and D1% were used for plan comparison. Conformity indexes (CI100 and CI50), gradient index (GI) and homogeneity index (HI) were calculated. For rectum and bladder, mean dose and doses to 2cc volume were used for evaluation. To account for various dose heterogeneity distributions we calculated the equivalent uniform dose (EUD) using the Niemerko model. A Kruskal-Wallis analysis of variance (ANOVA) followed by Dunn’s-type multiple comparisons between any two of the three techniques was performed. The treatment time was also compared between the three techniques. Results VMAT plans provided targets coverage comparable with HDR (p<0.05). Dose distributions were more heterogeneous with HDR: Dmean were 144.2% for CTV in HDR and 118.0 and 109.1% for PTV in AB_VMAT and FI_VMAT, respectively. The mean values of EUD for CTV were: HDR 40.9 Gy; AB_VMAT 39.0 Gy; FI_VMAT 33.3 Gy. GI indexes were 2.81, 3.37 and 4.15 for HDR, AB_VMAT and FI_VMAT, respectively. Dmean and maximal doses to 2cc for rectum and bladder were lower in HDR-BRT plans compared to AB_VMAT and FI:VMAT (rectum Dmean: 39.8% vs 47.4% and 53.0%; bladder Dmean: 35.4% vs 40.4% and 46.1%,; rectum D2cc: 104.8% vs 108.0% and 116.2%; bladder D2cc: 86.64% vs 94.0% and 100.0%). The mean treatment time for AB_VMAT was approximately 2.5 min on a conventional linac. Conclusion HDR-BRT provided superior dose distribution with respect to VMAT. AB_VMAT plans tend to better mimic the HDR dose distribution, providing a fast, safe and reasonable alternative. AB_VMAT plans showed a successful capability of highly conformal dose distribution, EUD values similar to HDR, steep dose-gradient outside PTV and minimal treatment time. EP-1972 Assessment of multiple treatment planning techniques for intracranial Stereotactic Radiosurgery M. Chu 1 , C.R. Davies 1 , M.G.J. Williams 1 , N.S. Iqbal 2,3 , M.C. Smyth 1 , A.J. Bryant 1 , J.R. Powell 4 , A.E. Millin 1 1 Velindre Cancer Centre, Medical Physics, Cardiff, United Kingdom 2 Cardiff University, Division of Cancer and Genetics, Cardiff, United Kingdom 3 Cardiff University, Cardiff University Brain Research Imaging Centre, Cardiff, United Kingdom 4 Velindre Cancer Centre, Clinical Oncology, Cardiff, United Kingdom Purpose or Objective Brain metastases occur in 20-40% of cancer patients with common primary sites including lung, breast, kidney and melanoma [1]. Locally, Stereotactic Radiosurgery (SRS) is used to treat patients with 1-3 brain metastases and WHO performance status 0-2. The current clinical protocol involves a forward planned, dynamic conformal arc technique using the iPlan Treatment Planning System (TPS) (BrainLAB, Germany). This work investigates whether an inverse planned VMAT technique using the RayStation TPS (RaySearch Laboratories, Sweden) can

produce plans of comparative quality, with the aim of gaining efficiency savings through potential automation and parity with standard planning procedures within the department. Material and Methods VMAT plans were optimised using a 1.5 arc technique for each target volume. This consisted of a 360 o coplanar arc plus a partial arc, close to sagittal but avoiding exit through the entire patient. This technique was applied to a cohort of ten cases previously treated using the dynamic conformal arc iPlan technique. As per local protocol prescriptions were dependent on PTV volume and planned to the 80% isodose. Plan comparison was performed using a range of plan metrics including PTV coverage (D99%), Paddick’s conformity index (PI) [2], gradient index (GI), defined as V50%/V100%, and dose to the normal brain tissue (V12Gy). For consistency these metrics were analysed using the Matlab based package CERR [3] independent of both planning system Results Preliminary results, (Table 1), show that the VMAT technique is capable of producing plans of similar quality to the dynamic Conformal Arc technique in terms of conformality and dose fall-off. The dose homogeneity across the PTV, however, is poorer. The VMAT plans resulted in inferior gradient indices for prescriptions of 21Gy (related to the smallest target volumes), but the reverse is true for other prescriptions.

Conclusion Further development of the VMAT optimisation approach is required to balance the trade-off between dose homogeneity and conformality. References [1] R. Soffietti, P. Cornu, J. Delattre, R. Grant, F. Graus, W. Grisold, J. Heimans, J. Hildebrand, P. Hoskin, M. Kalijo, P. Krauseneck, C. Marosi, T. Siegal and C. Vecht, European Journal of Neurology , 13 : 674-681, 2006. [2] I. Paddick, J Neurosurg (Suppl 3) 93 :219–222, 2000 [3] J. Deasy, A. Blanco and V. Clark, Medical Physics, vol. 30, no. 5, pp. 979-985, 2003. EP-1973 Treatment planning for a 6MV FFF Halcyon linac as a replacement for a 10MV-capable Clinac D. Withers 1 , G. Kidane 1 , D. Farmakidis 1 , L. Crees 1 1 Queen's Hospital, Radiotherapy Physics, Romford, United Kingdom Purpose or Objective To establish that a Halcyon 6MV FFF linac can be used for IMRT and VMAT pelvic cases to replace a 10MV conventional linac. Material and Methods The Halcyon is Varian’s new “ring-design” linac that has features facilitating rapid patient throughput and a more comfortable patient experience. The Halcyon’s technical