Premium
Comments on shielding for dual energy accelerators
Author(s) -
Rossi M. C.,
Lincoln H. M.,
Quarin D. J.,
Zwicker R. D.
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2911158
Subject(s) - electromagnetic shielding , radiation shielding , dual (grammatical number) , physics , medical physics , linear particle accelerator , dosimetry , nuclear engineering , nuclear physics , optics , nuclear medicine , engineering , medicine , beam (structure) , quantum mechanics , art , literature
Determination of shielding requirements for medical linear accelerators has been greatly facilitated by the publication of the National Council on Radiation Protection and Measurements (NCRP) latest guidelines on this subject in NCRP Report No. 151. In the present report the authors review their own recent experience with patient treatments on conventional dual energy linear accelerators to examine the various input parameters needed to follow the NCRP guidelines. Some discussion is included of workloads, occupancy, use factors, and field size, with the effects of intensity modulated radiotherapy (IMRT) treatments included. Studies of collimator settings showed average values of 13.1 × 16.2cm 2for 6 MV and 14.1 × 16.8cm 2for 18 MV conventional ports, and corresponding average unblocked areas of 228 and 254cm 2 , respectively. With an average of 77 % of the field area unblocked, this gives a mean irradiated area of 196cm 2for the 18 MV beam, which dominates shielding considerations for most dual energy machines. Assuming conservatively small room dimensions, a gantry bin angle of 18 ° was found to represent a reasonable unit for tabulation of use factors. For conventional 18 MV treatments it was found that the usual treatment angles of 0, 90, 180, and 270 ° were still favored, and use factors of 0.25 represent reasonable estimates for these beams. As expected, the IMRT fields (all at 6 MV) showed a high degree of gantry angle randomization, with no bin having a use factor in excess of 0.10. It is concluded that unless a significant number of patients are treated with high energy IMRT, the traditional use factors of 0.25 are appropriate for the dominant high energy beam.