ESTRO 37 Abstract book

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ESTRO 37

table. Better planning efficiency was achieved with ECHO. As a result, we have instituted an accelerated planning schedule for paraspinal patients planned with ECHO. Table 1: ECHO plans dosimetric metrics compared with clinical criteria

performed until agreement with histopathological observations is reached. Based on the density distribution resulting from the mathematical model, three targets are defined: the Gross Tumor Volume (GTV), the volume having 100%-60% of the GTV cell density (zone 1), and the volume having 60%-0% of the GTV cell density (zone 2). Eight different treatment plan strategies were considered with different dose prescription levels administered to GTV and peripheral zones. In plan 1, 20 Gy at 50% isodose level were prescribed to the GTV implicitly accounting for the presence of the microscopic cell spread following the judgement of an expert planner (spatial dose distribution resulted as not strictly conformal to the delineated GTV). In plan 2, 20 Gy at 50% isodose were kept strictly conformal to the GTV. In plans 3 to 6 the target was zone 1 and the dose prescribed was 20 Gy at the 50%, 40%, 30%, and 20% isodose level, respectively. Plans 7 and 8 had as target zone 2 and dose prescriptions equal to 20 Gy at 50% and 20% isodose level, respectively. Results Treatment plans were evaluated based both on conformity index and tumor control probability (TCP). Results are shown in Table 1. As seen, treatment plans with similar values of conformity indexes resulted in rather different TCP values. As an example, results for one of the plans considered in this study are presented in Figure 1.

Conclusion We implemented ECHO in our clinic for automated SBRT paraspinal planning. ECHO enhanced efficiency of planning and shortened the time between simulation and treatment in our clinic. PV-0199 Influence of tumor infiltration on treatment outcome in Gamma Knife Radiosurgery: a modeling study M. Lazzeroni 1,2 , Z. Khazraei Manesh 3 , H. Sandström 2 , P. Barsoum 4 , I. Toma-Dasu 1,2 1 Karolinska Institutet, Medical Radiation Physics- Dept. Oncology-Pathology, Stockholm, Sweden 2 Stockholm University, Medical Radiation Physics- Dept. of Physics, Stockholm, Sweden 3 Royal Institute of Technology, School of Technology and Health, Stockholm, Sweden 4 Karolinska University Hospital, Dept. of Medical Physics, Stockholm, Sweden Purpose or Objective High grade gliomas are brain tumors with widespread infiltration into the surrounding tissues. Gamma Knife TM Radiosurgery (GKRS) alone or in conjunction with other techniques has been increasingly used for salvage treatment of recurrent grade III-IV gliomas. The steep dose fall-off of the radiosurgery treatment and the definition of the target as the lesion detectable in the diagnostic images (without margins) make the presence of the microscopic tumor spread a matter of concern when dealing with high grade gliomas. Aim of this work is to investigate the impact of the presence of tumor cells beyond the target boundaries on the treatment outcome of radiosurgery for invasive high grade gliomas. Material and Methods A novel mathematical model able to predict the spatial density distribution of cancerous cells outside the tumor boundary was implemented on a virtual brain tumor. The algorithm calculates the concentration of tumor cells beyond the delineated tumor border by accounting for the motility of cancerous cells through the host tissues relative to white matter. Calculations are iteratively

Conclusion Accounting for the potential infiltration of tumor cells outside the delineated GTV is of key importance in GKRS. Radiobiological parameters (e.g. tumor cell density distribution after treatment and TCP) have the potential to differentiate between otherwise similar plans and could be used for treatment plan evaluation together with well-established conformity indexes.