Investigation of optimal beam margins for LINAC-based stereotactic extracranial radiotherapy (SBRT) with VMAT technique: A "pareto fronts" approach. (September 2016)

• Record Type: Journal Article
• Authors: Cilla, Savino; Ianiro, Anna; Deodato, Francesco; Macchia, Gabriella; Piermattei, Angelo; Valentini, Vincenzo; Morganti, Alessio G.
• Is Part Of: Physica medica. Volume 32(2016)Supplement 3
• Page Start: 330
• Publication Date: 2016-09
• Subjects: Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2016.07.236
• Languages: English
• ISSNs: 1120-1797

Abstract

Abstract : Introduction: In stereotactic body radiotherapy (SBRT), the objectives of ablative high-dose to the tumour and minimal dose to the surrounding normal tissues are inherently in conflict. Pareto fronts are a powerful mathematical strategy to formalize the trade-off between a given set of mutually contradicting objectives. Purpose: To determine the optimal block margin and prescription isodose for both optimal target coverage and normal tissue sparing in SBRT treatments of liver metastases using VMAT technique. Materials and methods: Three spherical-shaped targets (PTVs = 20, 55 and 101cc) were selected. A single fraction dose of 26 Gy was prescribed (PD). VMAT plans were generated with Ergo++ TPS using a 6 MV single arc. Pareto fronts based on (i) different MLC block margin around PTV (ranging from +4 mm to −2 mm) and (ii) different prescription isodose surface (IDS) ranging from 50% to 100% of PD were produced. For each block margin, the greatest IDS fulfilling the two criteria: 95% and 99% of PTV volume reached 100% and 90% of PD, respectively, was considered as providing the optimal clinical plan for PTV coverage. Liver Dmean were used against the PTV coverage (1-V100) to generate the fronts. Conformity (CI = V100/PTV), gradient (GI = V50/V100) and homogeneity (HI = D2%/PD) indexes were calculated to compare different plans Results: About 300 plans were calculated to generate the fronts. The fronts for 1mm MLC margin provided the best plans in terms of minimal liver irradiation; optimal clinical plans were obtained for IDS equal to 77–82%. GI shows a U-shaped behaviour with respect to prescribed IDS, CI and HI indexes, with minimum values at 1mm for all metrics. The location of these minimal points was found independent of tumour dimensions. Minimal GI values were found at HI values approximately equal to 1.3. Conclusion: Pareto fronts provides a rigorous strategy to choice clinical optimal plans in SBRT treatments. We show that a 1 mm MLC block margin provides the best results with regard healthy liver tissue irradiation and steepness of dose fallout.