MEDRADSC 3S03 Lecture Notes - Lecture 11: Megavoltage X-Rays, Soft Tissue, Pair Production
6 Jan 2017
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Dose distribution will appear close to the same as in water or soft tissue. Tissues with significantly different properties: tha(cid:374) water (cid:894)i. e. (cid:858)i(cid:374)ho(cid:373)oge(cid:374)eities(cid:859)(cid:895) Dose distribution will appear significantly different from that in water or soft tissue. I(cid:374) the prese(cid:374)(cid:272)e of a(cid:374) i(cid:374)ho(cid:373)oge(cid:374)eity the isodose li(cid:374)es (cid:858)shift(cid:859) either away from or towards the skin. Isodose lines are shifted by a shift factor (n) x the amount of inhomogeneity. Lines shift toward the skin for bone (+ve) & away from skin for air, lung (-ve) Inhomogeneity mode is not on on the left, Some isodose lines go deeper and deeper into the patient. Dose will be lower with correction applied, due to loss of electronic equilibrium. Dose will be higher with correction applied. Isodose line is pushed deeper since the dose is not attenuated as much. Dose distribution affected by variations in atomic number of the medium (photoelectric effect) Dose distribution affected primarily by variations in density of the medium (compton effect)
