title: The dosimetric and radiobiological impact of calculation grid size on head and neck IMRT.
abstract: OBJECTIVE
  Small-volume structures usually found in the head and neck may be susceptible to dose-volume averaging, which has not been studied. Here, the impact of calculation grid size on dose distribution for tumor control probability (TCP) and normal tissue complication probability (NTCP) is investigated for head and neck (H&N) intensity modulated radiation therapy (IMRT).
  
  
  METHODS
  IMRT plans were generated for H&N patients with different grid sizes (1-5 mm) to calculate dose and related TCP and NTCP. Dose parameters such as D2%, D50%, D98%, and the homogeneity and conformity indices were calculated. The dose distributions were also compared with measured dose for all IMRT plans. A 1-tailed pair t test was used to analyze the data.
  
  
  RESULTS
  The mean dose to planning target volume and TCP decreases with increasing grid size, whereas for organs at risk (OARs), mean dose, and NTCP increase with increasing grid size. The average mean dose to planning target volume decreases linearly with grid size, but for OARs such as cochlea, parotid gland, and the spinal cord, mean dose increases with grid size. IMRT dose verification showed that the number of points meeting the gamma criterion of 3%/3 mm increased with decreasing grid sizes. The homogeneity index for the target increased up to 60% and conformity index decreased on average by 3.5% between 1- to 5-mm grid that resulted in decreased TCP and increased NTCP. A 1-tailed pair t test showed significant statistical differences among various grid size calculations compared with 1-mm grids.
  
  
  CONCLUSIONS
  Based on our findings, the smallest possible grid size should be used for accurate dose calculation in small-volume structures-especially in H&N planning. A smaller calculation grid provides superior dosimetry with improved TCP as well as reduced NTCP, which is more pronounced for smaller OARs.
authors.affiliation: Department of Health Sciences, Purdue University, West Lafayette, Indiana; Department of Radiation Oncology, St. Joseph Hospital and Medical Center, Phoenix, Arizona. Electronic address: srivastavashiv@hotmail.com.
authors.forename: Shiv P
authors.initials: SP
authors.lastname: Srivastava
authors.affiliation: Department of Health Sciences, Purdue University, West Lafayette, Indiana; Department of Radiation Oncology, University Hospitals, Cleveland, Ohio.
authors.forename: Chee-Wai
authors.initials: CW
authors.lastname: Cheng
authors.affiliation: Department of Health Sciences, Purdue University, West Lafayette, Indiana; Department of Radiation Oncology, New York University Medical Center, New York, New York.
authors.forename: Indra J
authors.initials: IJ
authors.lastname: Das
mesh_terms: D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*