The probability of a photon interacting with tissue via the photoelectric effect or the Compton effect depends on the photon energy. Use Fig. P38.44 to determine the best description of how the photons from the linear accelerator described in the passage interact with a tumor.

(a) Via the Compton effect only;

(b) Mostly via the photoelectric effect until they have lost most of their energy, and then mostly via the Compton effect;

(c) Mostly via the Compton effect until they have lost most of their energy, and then mostly via the photoelectric effect;

(d) Via the Compton effect and the photoelectric effect equally.

Figure P38.44

Malignant tumors are commonly treated with targeted x-ray radiation therapy. To generate these medical x rays, a linear accelerator directs a highenergy beam of electrons toward a metal target€”typically tungsten. As they near the tungsten nuclei, the electrons are deflected and accelerated, emitting high-energy photons via bremsstrahlung. The resulting x rays are collimated into a beam that is directed at the tumor. The photons can deposit energy in the tumor through Compton and photoelectric interactions. A typical tumor has 10⁸ cells/cm³, and in a full treatment, 4-MeV photons may produce a dose of 70 Gy in 35 fractional exposures on different days. The gray (Gy) is a measure of the absorbed energy dose of radiation per unit mass of tissue: 1 Gy = 1 J/kg.

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1.00 x 104 1.00 x 10 Compton 1.00 x 10-2 1.00 x 105 Photoelectric 1.00 x 108 1.00 x 10-!! 0.001 0.010 0.100 1.000 10.000 100.000 Photon energy (MeV) Relative probability