title: Time-adjusted internal target volume: a novel approach focusing on heterogeneity of tumor motion based on 4-dimensional computed tomography imaging for radiation therapy planning of lung cancer.
abstract: OBJECTIVE
  To consider nonuniform tumor motion within the internal target volume (ITV) by defining time-adjusted ITV (TTV), a volume designed to include heterogeneity of tumor existence on the basis of 4-dimensional computed tomography (4D-CT).
  
  
  METHODS
  We evaluated 30 lung cancer patients. Breath-hold CT (BH-CT) and free-breathing 4D-CT scans were acquired for each patient. The tumors were manually delineated using a lung CT window setting (window, 1600 HU; level, -300 HU). Tumor in BH-CT images was defined as gross tumor volume (GTV), and the sum of tumors in 4D-CT images was defined as ITV-4D. The TTV images were generated from the 4D-CT datasets, and the tumor existence probability within ITV-4D was calculated. We calculated the TTV80 value, which is the percentage of the volume with a tumor existence probability that exceeded 80% on ITV-4D. Several factors that affected the TTV80 value, such as the ITV-4D/GTV ratio or tumor centroid deviation, were evaluated.
  
  
  RESULTS
  Time-adjusted ITV images were acquired for all patients, and tumor respiratory motion heterogeneity was visualized. The median (range) ITV-4D/GTV ratio and median tumor centroid deviation were 1.6 (1.0-4.1) and 6.3 mm (0.1-30.3 mm), respectively. The median TTV80 value was 43.3% (2.9-98.7%). Strong correlations were observed between the TTV80 value and the ITV-4D/GTV ratio (R=-0.71) and tumor centroid deviation (R=-0.72). The TTV images revealed the tumor motion pattern features within ITV.
  
  
  CONCLUSIONS
  The TTV images reflected nonuniform tumor motion, and they revealed the tumor motion pattern features, suggesting that the TTV concept may facilitate various aspects of radiation therapy planning of lung cancer while incorporating respiratory motion in the future.
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan; Department of Radiation Oncology, Hiroshima Prefectural Hospital, Hiroshima, Japan.
authors.forename: Ikuno
authors.initials: I
authors.lastname: Nishibuchi
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan. Electronic address: tkkimura@hiroshima-u.ac.jp.
authors.forename: Tomoki
authors.initials: T
authors.lastname: Kimura
authors.affiliation: Division of Radiation Therapy, Hiroshima University Hospital, Hiroshima, Japan.
authors.forename: Takeo
authors.initials: T
authors.lastname: Nakashima
authors.affiliation: Division of Radiation Therapy, Hiroshima University Hospital, Hiroshima, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Ochi
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Ippei
authors.initials: I
authors.lastname: Takahashi
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Yoshiko
authors.initials: Y
authors.lastname: Doi
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Kenjo
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Kaneyasu
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Syuichi
authors.initials: S
authors.lastname: Ozawa
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Murakami
authors.affiliation: Department of Radiation Oncology, Hiroshima Prefectural Hospital, Hiroshima, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Wadasaki
authors.affiliation: Department of Radiation Oncology, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Nagata
mesh_terms: D056973:Four-Dimensional Computed Tomography / Q000379:methods*; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy* / Q000556:secondary; D009068:Movement; D011336:Probability; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration; D013997:Time Factors; D047368:Tumor Burden