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Med Phys. 1991 Mar-Apr;18(2):292-4.

Depth dose characteristics of 24-MV x-ray beams at extended SSD.

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Division of Radiation Oncology, University of Iowa College of Medicine, Iowa City 52242.


Depth dose characteristics of 24-MV beams from a medical linear accelerator at various SSDs have been measured. The dose buildup, percent depth dose (PDD), and output were measured using ionization chambers and a diode detector under full scatter. Surface dose ranged from 8% to 51% depending upon the collimator setting at 100-cm SSD. It decreased by 10%-15% as distance was increased to 200 cm, but remained unchanged beyond this distance. The dmax migrated rapidly toward the surface with increasing field size at 100-cm SSD. At extended SSD, the dmax occurred at greater depths and shifted a little toward the shallow depths with increasing field size. Large field PDDs measured at extended SSD agreed well with those predicted from 100-cm data. The output followed the 1/(distance)2 relationship quite well. The extended SSD beam data are clinically useful when treating hemibody or performing total body irradiation (TBI) procedures.

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