Determining the optimal block margin on the planning target volume for extracranial stereotactic radiotherapy

Purpose: To determine the block margin that minimizes normal tissue irradiation outside of the planning target volume (PTV) for body stereotactic radiotherapy (Body-SRT) of lung and liver tumors. Methods and Materials: Representative patient cases of lung and liver tumors were chosen for analysis, A PTV was constructed for each case and plans were generated which employed an array of block margins ranging from -2.5 mm to 10 mm at isocenter. Plans were generated for cerrobend blocks and for a multileaf collimator. The prescription isodose coverage was renormalized for each case and dose-volume histograms (DVH) and normal tissue complication probabilities (NTCP) were determined for each plan. Results and Conclusion: For the cases studied, the optimal block margin was in the 0.0 mm range. The ranking of plans was identical for both dose-volume based and biological based criteria. The method of blocking had no significant effect on treatment plans. The use of narrow margins for Body-SRT results in normal tissue sparing and creates significant target dose inhomogeneity which may be beneficial for tumor control. (C) 1999 Elsevier Science Inc.