title: Robustness of 4D-optimized scanned carbon ion beam therapy against interfractional changes in lung cancer.
abstract: Moving targets could be conformally treated with actively scanned carbon ion beams using 4D-optimization. As this heavily exploits 4D-CTs, an important question is whether the conformity also upholds in the context of interfractional changes, i.e. variable positioning, anatomy and breathing patterns.
  
  
  
  In 4 lung cancer patients, 6 weekly 4D-CTs were available. 4D-CTs and their phases were non-rigidly registered to propagate contours and 4D-doses. On the first 4D-CT, a 4D-optimized plan delivering a uniform dose to each motion phase (total dose 9.4Gy(RBE)) was simulated, as well as an ITV plan for comparison. On the five following 4D-CTs, 4D-dose was forward calculated and evaluated for target coverage and conformity. Variable uniform (3-7mm) and range margins (2mm/%) were investigated.
  
  
  
  For all patients, target coverage (V95>95% accumulated over 5 fractions) could be achieved, but with variable margin size weakly depending on motion amplitude and range changes. The same margins were also necessary for ITV plans, which lead to lower conformity and higher integral doses.
  
  
  
  4D-optimization appears feasible also under interfractional changes and maintains a dosimetric advantage over less conformal ITV irradiations. Further studies are needed to identify patients benefiting most from the technically more complex 4D-optimization.
authors.affiliation: GSI Helmholzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany. Electronic address: c.graeff@gsi.de.
authors.forename: Christian
authors.initials: C
authors.lastname: Graeff
mesh_terms: D019583:Dose Fractionation, Radiation; D056973:Four-Dimensional Computed Tomography / Q000379:methods*; D063193:Heavy Ion Radiotherapy / Q000379:methods*; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000528:radiation effects; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D009038:Motion; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000379:methods*; D012119:Respiration; D016896:Treatment Outcome
keywords: 4D-optimization; Charged particle therapy; Interfractional changes; Ion beam therapy; Lung cancer; Moving targets