title: Variation of normal tissue complication probability (NTCP) estimates of radiation-induced hypothyroidism in relation to changes in delineation of the thyroid gland.
abstract: BACKGROUND
  To examine the variations of risk-estimates of radiation-induced hypothyroidism (HT) from our previously developed normal tissue complication probability (NTCP) model in patients with head and neck squamous cell carcinoma (HNSCC) in relation to variability of delineation of the thyroid gland.
  
  
  METHODS
  In a previous study for development of an NTCP model for HT, the thyroid gland was delineated in 246 treatment plans of patients with HNSCC. Fifty of these plans were randomly chosen for re-delineation for a study of the intra- and inter-observer variability of thyroid volume, Dmean and estimated risk of HT. Bland-Altman plots were used for assessment of the systematic (mean) and random [standard deviation (SD)] variability of the three parameters, and a method for displaying the spatial variation in delineation differences was developed.
  
  
  RESULTS
  Intra-observer variability resulted in a mean difference in thyroid volume and Dmean of 0.4 cm(3) (SD ± 1.6) and -0.5 Gy (SD ± 1.0), respectively, and 0.3 cm(3) (SD ± 1.8) and 0.0 Gy (SD ± 1.3) for inter-observer variability. The corresponding mean differences of NTCP values for radiation-induced HT due to intra- and inter-observer variations were insignificantly small, -0.4% (SD ± 6.0) and -0.7% (SD ± 4.8), respectively, but as the SDs show, for some patients the difference in estimated NTCP was large.
  
  
  CONCLUSIONS
  For the entire study population, the variation in predicted risk of radiation-induced HT in head and neck cancer was small and our NTCP model was robust against observer variations in delineation of the thyroid gland. However, for the individual patient, there may be large differences in estimated risk which calls for precise delineation of the thyroid gland to obtain correct dose and NTCP estimates for optimized treatment planning in the individual patient.
authors: a Department of Oncology , Odense University Hospital , Odense , Denmark.
authors: Marianne F
authors: MF
authors: Rønjom
authors: Carsten
authors: C
authors: Brink
authors: Ebbe L
authors: EL
authors: Lorenzen
authors: Laszlo
authors: L
authors: Hegedüs
authors: Jørgen
authors: J
authors: Johansen
mesh_terms: D000328:Adult; D000368:Aged; D002294:Carcinoma, Squamous Cell / Q000532:radiotherapy; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D007037:Hypothyroidism / Q000209:etiology*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D009929:Organ Size; D011832:Radiation Injuries / Q000209:etiology* / Q000473:pathology; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012307:Risk Factors; D000077195:Squamous Cell Carcinoma of Head and Neck; D013961:Thyroid Gland / Q000033:anatomy & histology*