Chapter 081. Principles of Cancer Treatment (Part 4) Palliation Surgery is employed in a number of ways for supportive care: insertion of central venous catheters, control of pleural and pericardial effusions and ascites, caval interruption for recurrent pulmonary emboli, stabilization of cancer- weakened weight-bearing bones, and control of hemorrhage, among others. Surgical bypass of gastrointestinal, urinary tract, or biliary tree obstruction can alleviate symptoms and prolong survival. Surgical procedures may provide relief of otherwise intractable pain or reverse neurologic dysfunction (cord decompression). Splenectomy may relieve symptoms and reverse hypersplenism. Intrathecal or intrahepatic therapy relies on surgical placement of appropriate infusion portals. Surgery may correct other treatment-related toxicities such as adhesions or strictures. Rehabilitation Surgical procedures are also valuable in restoring a cancer patient to full health. Orthopedic procedures may be necessary to assure proper ambulation. Breast reconstruction can make an enormous impact on the patient's perception of successful therapy. Plastic and reconstructive surgery can correct the effects of disfiguring primary treatment. Principles of Radiation Therapy Physical Properties and Biologic Effects Exposure to ionizing radiation is constant. Radiation comes from the sun and other cosmic sources, the ground, the air we breathe, the food we ingest, and from within our bodies. Radiation therapy uses radiation to treat cancer. Radiation is a physical form of treatment that damages any tissue in its path; its selectivity for cancer cells may be due to defects in a cancer cell's ability to repair sublethal DNA and other damage. Radiation causes breaks in DNA and generates free radicals from cell water that may damage cell membranes, proteins and organelles. Radiation damage is dependent on oxygen; hypoxic cells are more resistant. Augmentation of oxygen is the basis for radiation sensitization. Sulfhydryl compounds interfere with free radical generation and may act as radiation protectors. Most radiation-induced cell damage is due to the formation of hydroxyl radicals: Ionizing radiation + H 2 O →H 2 O + + e – H 2 O + + H 2 O →H 3 O + + OH˙ OH˙ →cell damage The dose-response curve for cells has both linear and exponential components. The linear component is from double-stranded DNA breaks produced by single hits. The exponential component represents breaks produced by multiple hits (Fig. 81-2). Plotting the fraction of surviving cells against doses of x-rays or gamma radiation, the curve has a shoulder that reflects the cell's repair of sublethal damage, followed by a linear portion reflecting greater cell kill with larger doses. The features that make a particular cell more sensitive or more resistant to the biologic effects of radiation are not completely defined. Figure 81-2 Shape of survival curve for mammalian cells exposed to radiation. The fraction of ce lls surviving is plotted on a logarithmic scale against dose on a linear scale. For alpha particles or low- energy neutrons (said to be densely ionizing), the dose- response curve is a straight line from the origin (i.e., survival is an exponential function of dose). The survival curve can be described by just one parameter, the slope. For x- rays or gamma rays (said to be sparsely ionizing), the dose- response curve has an initial linear slope, followed by a shoulder; at higher doses the curve tends to become straight again. A. The experimental data are fitted to a linear- quadratic function. There are two components of cell killing: one is proportional to dose (αD), while the other is proportional to the square of the dose (βD 2 ). The dose at which the linear and quadratic components are equal is the ratio α/β. The linear- quadratic curve bends continuously but is a good fit to experimental data for the first few decades of survival. B. The curve is described by the initial slope (D 1 ), the final slope (D 0 ), and a parameter that represents the width of the shoulder, either n or D q . (From EJ Hall: Radiobiology for the Radiologist, 5th ed. Philadelphia, Lippincott Williams & Wilkins, 2000; with permission.) . Chapter 081. Principles of Cancer Treatment (Part 4) Palliation Surgery is employed in a number of ways for supportive care: insertion of central venous catheters, control of pleural. uses radiation to treat cancer. Radiation is a physical form of treatment that damages any tissue in its path; its selectivity for cancer cells may be due to defects in a cancer cell's ability. successful therapy. Plastic and reconstructive surgery can correct the effects of disfiguring primary treatment. Principles of Radiation Therapy Physical Properties and Biologic Effects Exposure