title: Stereotactic ablative radiation therapy for subcentimeter lung tumors: clinical, dosimetric, and image guidance considerations.
abstract: OBJECTIVE
  Use of stereotactic ablative radiation therapy (SABR) for subcentimeter lung tumors is controversial. We report our outcomes for tumors with diameter ≤1 cm and their visibility on cone beam computed tomography (CBCT) scans and retrospectively evaluate the planned dose using a deterministic dose calculation algorithm (Acuros XB [AXB]).
  
  
  METHODS
  We identified subcentimeter tumors from our institutional SABR database. Tumor size was remeasured on an artifact-free phase of the planning 4-dimensional (4D)-CT. Clinical plan doses were generated using either a pencil beam convolution or an anisotropic analytic algorithm (AAA). All AAA plans were recalculated using AXB, and differences among D95 and mean dose for internal target volume (ITV) and planning target volume (PTV) on the average intensity CT dataset, as well as for gross tumor volume (GTV) on the end respiratory phases were reported. For all AAA patients, CBCT scans acquired during each treatment fraction were evaluated for target visibility. Progression-free and overall survival rates were calculated using the Kaplan-Meier method.
  
  
  RESULTS
  Thirty-five patients with 37 subcentimeter tumors were eligible for analysis. For the 22 AAA plans recalculated using AXB, Mean D95 ± SD values were 2.2 ± 4.4% (ITV) and 2.5 ± 4.8% (PTV) lower using AXB; whereas mean doses were 2.9 ± 4.9% (ITV) and 3.7 ± 5.1% (PTV) lower. Calculated AXB doses were significantly lower in one patient (difference in mean ITV and PTV doses, as well as in mean ITV and PTV D95 ranged from 22%-24%). However, the end respiratory phase GTV received at least 95% of the prescription dose. Review of 92 CBCT scans from all AAA patients revealed that the tumor was visualized in 82 images, and its position could be inferred in other images. The 2-year local progression-free survival was 100%.
  
  
  CONCLUSIONS
  Patients with subcentimeter lung tumors are good candidates for SABR, given the dosimetry, ability to localize tumors with image guidance, and excellent local control.
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, the Netherlands; Department of Radiation Oncology, London Regional Cancer Program, University of Western Ontario, London, Canada; Department of Epidemiology, Harvard School of Public Health, Harvard University, Boston, Massachusetts. Electronic address: Dr.alexlouie@gmail.com.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Louie
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, the Netherlands.
authors.forename: Suresh
authors.initials: S
authors.lastname: Senan
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, the Netherlands.
authors.forename: Max
authors.initials: M
authors.lastname: Dahele
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, the Netherlands.
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Slotman
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, the Netherlands.
authors.forename: Wilko F A R
authors.initials: WF
authors.lastname: Verbakel
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000601:surgery*; D008297:Male; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided; D012119:Respiration; D012189:Retrospective Studies; D047368:Tumor Burden