Backscattered dose perturbation effects at metallic interfaces irradiated by high-energy X- and gamma-ray therapeutic beams.
OBJECTIVE
To analyze backscattered dose enhancements near different metallic interfaces for cobalt-60 ((60)Co) gamma rays and 6- and 18-MV photon beams.


METHODS
Measurements were carried out with a PTW thin-window, parallel-plate ionization chamber and an RDM-1F electrometer. Thin sheets of aluminum, mild steel, copper, cadmium and lead were used as inhomogeneities. The chamber was positioned below the inhomogenities with the gantry maintained under the couch.


RESULTS
It can be noticed that the backscatter dose factor (BSDF) reaches the saturation value within few millimeters of all inhomogeneities and the thickness at which the saturation value is reached depends on the atomic number of the inhomogeneity. The amount of backscattered radiation was noticed to be greater with lesser-energy photons ((60)Co) compared to the high erenergy photons. The BSDF varies across the beam when the inhomogeneity is present due to the change in beam quality. The backscattered electrons from lead inhomogeneity have a range in the order of 5-7 mm.


CONCLUSIONS
Higher atomic number inhomogeneities result in an increase in BSDF, as they have higher scattering cross section for the secondary electrons. The increase in dose was noticed for few millimeters upstream from the metallic inhomogeneity, which suggests that the range of backscattered electrons is very small. Since the factors affecting the BSDF at the interface are energy dependent, it is expected that the variation in BSDF will also be sensitive to the beam energy.
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bangalore, India. kidwai@kar.nic.in
Manickam
M
Ravikumar
Ramamoorthy
R
Ravichandran
Saminathan
S
Sathiyan
Sanjay Sudhakar
SS
Supe
D000535:Aluminum; D002104:Cadmium; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D003300:Copper; D004583:Electrons; D005720:Gamma Rays; D006801:Humans; D007854:Lead; D008670:Metals; D008962:Models, Theoretical; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011137:Polystyrenes; D011867:Radioisotope Teletherapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation; D013232:Steel
D003037:Cobalt Radioisotopes; D008670:Metals; D011137:Polystyrenes; D002104:Cadmium; D013232:Steel; D007854:Lead; D003300:Copper; D000535:Aluminum