title: Dose distribution in the thyroid gland following radiation therapy of breast cancer--a retrospective study.
abstract: OBJECTIVE
  To relate the development of post-treatment hypothyroidism with the dose distribution within the thyroid gland in breast cancer (BC) patients treated with loco-regional radiotherapy (RT).
  
  
  METHODS
  In two groups of BC patients postoperatively irradiated by computer tomography (CT)-based RT, the individual dose distributions in the thyroid gland were compared with each other; Cases developed post-treatment hypothyroidism after multimodal treatment including 4-field RT technique. Matched patients in Controls remained free for hypothyroidism. Based on each patient's dose volume histogram (DVH) the volume percentages of the thyroid absorbing respectively 20, 30, 40 and 50 Gy were then estimated (V20, V30, V40 and V50) together with the individual mean thyroid dose over the whole gland (MeanTotGy). The mean and median thyroid dose for the included patients was about 30 Gy, subsequently the total volume of the thyroid gland (VolTotGy) and the absolute volumes (cm3) receiving respectively <30 Gy and ≥30 Gy were calculated (Vol<30 and Vol≥30) and analyzed.
  
  
  RESULTS
  No statistically significant inter-group differences were found between V20, V30, V40 and V50Gy or the median of MeanTotGy. The median VolTotGy in Controls was 2.3 times above VolTotGy in Cases (ρ=0.003), with large inter-individual variations in both groups. The volume of the thyroid gland receiving<30 Gy in Controls was almost 2.5 times greater than the comparable figure in Cases.
  
  
  CONCLUSIONS
  We concluded that in patients with small thyroid glands after loco-radiotherapy of BC, the risk of post-treatment hypothyroidism depends on the volume of the thyroid gland.
authors.affiliation: Institute for Cancer Research, Oslo University Hospital-Radiumhospitalet, N-0310 Oslo, Norway. safora.johansen@radiumhospitalet.no
authors.forename: S
authors.initials: S
authors.lastname: Johansen
authors.forename: K V
authors.initials: KV
authors.lastname: Reinertsen
authors.forename: K
authors.initials: K
authors.lastname: Knutstad
authors.forename: D R
authors.initials: DR
authors.lastname: Olsen
authors.forename: S D
authors.initials: SD
authors.lastname: Fosså
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000532:radiotherapy*; D016022:Case-Control Studies; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000209:etiology; D008875:Middle Aged; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D012189:Retrospective Studies; D013961:Thyroid Gland / Q000528:radiation effects*; D014018:Tissue Distribution; D014057:Tomography, X-Ray Computed