title: Air cavity effects on the radiation dose to the larynx using Co-60, 6 MV, and 10 MV photon beams.
abstract: OBJECTIVE
  To determine the perturbation effect in the surface layers of lesions located in the air-tumor tissue interface of larynx using 60Co, 6 MV, and 10 MV photon beams.
  
  
  METHODS
  Thermoluminescent dosimeters (TLDs), were embedded at 16 measurement locations in slab no. 8 of a humanoid phantom and exposed to two lateral-opposed beams using standard small 7 x 7 cm fields. Similarly, radiographic and radiochromic films were placed between slabs no. 7 and no. 8 of the humanoid phantom and exposed to two lateral-opposed radiation beams. The dosimeters were irradiated with 60Co, 6 MV, and 10 MV photon beams. Computer tomography (CT) treatment planning without inhomogeneity correction was performed.
  
  
  RESULTS
  At the tissue-air interface, the average measured percentage dose (% dosem) is about (108.7 +/- 4.8)% with TLD data, (96.8 +/- 2.5)% with radiographic film data, and (100.8 +/- 4.9)% with radiochromic film data. Similarly, in the central part of the cavity, the % dosem is (98.4 +/- 3.1)% with TLD data, (94.3 +/- 3.3)% with radiographic film data, and (91.7 +/- 5.0)% with radiochromic film data. Using the CT-based generated dose distribution (without inhomogeneity correction), the average calculated percentage dose (% dosec) is (98.7 +/- 1.0)% at the tissue-air interface and 98% in the central part of the air cavity.
  
  
  CONCLUSIONS
  For the beam energies studied, the variation from the % dosem at the tissue-air interface for a given dosimetry technique is relatively small [< 5% (TLD), < 3% (radiographic), and < 5% (radiochromic)] and therefore should not be significant in clinical settings. The variation from the % dosem at the tissue-air interface is more significant for lower energies [8% (60Co), 7.3% (6 MV)]. This variation is about 4.3% for 10 MV photon beam, therefore, while our institutional practice favors lower energy (60Co to 6 MV) for node-negative glottic cancers, physical/dosimetric evidence offers no disadvantage to the use of higher energy photons.
authors.affiliation: Department of Radiation Medicine, Georgetown University Medical Center, Washington, DC 20007-2197.
authors.forename: A
authors.initials: A
authors.lastname: Niroomand-Rad
authors.forename: K W
authors.initials: KW
authors.lastname: Harter
authors.forename: S
authors.initials: S
authors.lastname: Thobejane
authors.forename: K
authors.initials: K
authors.lastname: Bertrand
mesh_terms: D006801:Humans; D007822:Laryngeal Neoplasms / Q000532:radiotherapy; D007830:Larynx / Q000000981:diagnostic imaging / Q000528:radiation effects*; D008954:Models, Biological; D011829:Radiation Dosage; D011867:Radioisotope Teletherapy; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy; D012137:Respiratory System / Q000033:anatomy & histology* / Q000187:drug effects; D012542:Scattering, Radiation; D014057:Tomography, X-Ray Computed / Q000379:methods