A narrow, monoenergetic beam of photons ( per photon) reaches the skin of a patient's…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Radiation physics and diagnostic imaging · Nuclear medicine, dosimetry and radiotherapy

Life at Thermal Energy, Ch Radiation in tissue: interaction, dose and risk

Problem

A narrow, monoenergetic beam of

60\,\mathrm{keV}

photons (

E = 9.61\times10^{-15}\,\mathrm{J}

per photon) reaches the skin of a patient's chest with a fluence of

2.0\times10^{13}\,\mathrm{m^{-2}}

. Take the tissue above the lungs to be

5.0\,\mathrm{cm}

of soft tissue, with

\mu/\rho = 0.2048\,\mathrm{cm^2\,g^{-1}}

and density

1.06\,\mathrm{g\,cm^{-3}}

from the attenuation table, and take its mass energy absorption coefficient to be water's,

\mu_{\rm en}/\rho = 0.0319\,\mathrm{cm^2\,g^{-1}}

. Ignore scattered photons that rejoin the beam. ((a)) Find the fluence reaching lung tissue

5.0\,\mathrm{cm}

deep. ((b)) Find the absorbed dose in the lung tissue at that depth, in milligray. ((c)) Taking that dose as uniform over the lungs, and no significant dose to any other organ, find the effective dose (

w_T = 0.12

for lung) and the nominal added lifetime cancer risk at

5

per cent per sievert. ((d)) A colleague uses

\mu/\rho

in place of

\mu_{\rm en}/\rho

in part (b). By what factor does that overstate the dose, and why is it wrong?

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