Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Radiationdose~~ANSYS~Dose too---. DEATHHigh/• Replacement of damaged cellsby different cell type (i.e. fibrosis)• Replacement of damaged cellsby same cell type• Organ is not returned to itspreirradiated state• Organ returned to preirradiatedstate• Radioresistant tissues (FPM celltypes) can only effect repair eq.muscle and CNS• Radiosensitive tissues (VIM,DIM, MCT) eq. skin, G.!. tract,bone marrowwhich case the functionality of the organ system is compromised. The types ofresponse and the degree to which they occur are functions of the dose and the relativeradiosensitivity and regenerative capacity of the cells that comprise the organsystem. If the exposures are excessive, the ability of the cells to effect any type ofhealing may be lost, resulting in tissue necrosis.The characterization of an organ system as radioresistant or radiosensitive dependsin large part on the radiosensitivity of cells that comprise the functionalparenchyma. The cells of the supportive stromal tissue consists mainly of cells ofintermediate radiosensitivity. Therefore, when the parenchyma consists of radiosensitivecell types (e.g., stem cells), the initial hypoplasia and concomitant decrease infunctional integrity will be the result of damage to these radiosensitive cell populations.Functional changes are typically apparent within days or weeks after the exposure.However, if the parenchyma is populated by radioresistant cell types (e.g.,nerve or muscle cells), damage to the functional layer occurs indirectly by compromiseof the cells in the vascular stoma. In this case, the hypoplasia of the parenchymalcells is typically delayed several months and is secondary to changes in the supportivevasculature. The principal effect is a gradual proliferation of intimal cells inthe microvasculature that eventually narrows and reduces the blood supply to thepoint that the flow of oxygen and nutrients is insufficient to sustain the functionalparenchyma.