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 cancerweakened 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...
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Chapter 081. Principles of Cancer Treatment (Part 4) Chapter 081. Principles of Cancer Treatment (Part 4) Palliation Surgery is employed in a number of ways for supportive care: insertion ofcentral 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 canalleviate symptoms and prolong survival. Surgical procedures may provide reliefof otherwise intractable pain or reverse neurologic dysfunction (corddecompression). Splenectomy may relieve symptoms and reverse hypersplenism.Intrathecal or intrahepatic therapy relies on surgical placement of appropriateinfusion portals. Surgery may correct other treatment-related toxicities such asadhesions or strictures. Rehabilitation Surgical procedures are also valuable in restoring a cancer patient to fullhealth. Orthopedic procedures may be necessary to assure proper ambulation.Breast reconstruction can make an enormous impact on the patients perception ofsuccessful therapy. Plastic and reconstructive surgery can correct the effects ofdisfiguring primary treatment. Principles of Radiation Therapy Physical Properties and Biologic Effects Exposure to ionizing radiation is constant. Radiation comes from the sunand other cosmic sources, the ground, the air we breathe, the food we ingest, andfrom within our bodies. Radiation therapy uses radiation to treat cancer. Radiationis a physical form of treatment that damages any tissue in its path; its selectivityfor cancer cells may be due to defects in a cancer cells ability to repair sublethalDNA and other damage. Radiation causes breaks in DNA and generates freeradicals 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. Sulfhydrylcompounds interfere with free radical generation and may act as radiationprotectors. Most radiation-induced cell damage is due to the formation of hydroxylradicals: Ionizing radiation + H2O →H2O+ + e– H2O+ + H2O →H3O+ + OH˙ OH˙ →cell damage The dose-response curve for cells has both linear and exponentialcomponents. The linear component is from double-stranded DNA breaks producedby single hits. The exponential component represents breaks produced by multiplehits (Fig. 81-2). Plotting the fraction of surviving cells against doses of x-rays orgamma radiation, the curve has a shoulder that reflects the cells repair of sublethaldamage, 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 thebiologic effects of radiation are not completely defined. Figure 81-2 Shape of survival curve for mammalian cells exposed to radiation. Thefraction of cells surviving is plotted on a logarithmic scale against dose on a linearscale. For alpha particles or low-energy neutrons (said to be densely ionizing), thedose-response curve is a straight line from the origin (i.e., survival is anexponential function of dose). The survival curve can be described by just oneparameter, the slope. For x-rays or gamma rays (said to be sparsely ionizing), thedose-response curve has an initial linear slope, followed by a shoulder; at higherdoses the curve tends to become straight again. A. The experimental data are fittedto a linear-quadratic function. There are two components of cell killing: one isproportional to dose (αD), while the other is proportional to the square of the dose(βD2). The dose at which the linear and quadratic components are equal is theratio α/β. The linear-quadratic curve bends continuously but is a good fit toexperimental data for the first few decades of survival. B. The curve is describedby the initial slope (D1), the final slope (D0), and a parameter that represents thewidth of the shoulder, either n or Dq. (From EJ Hall: Radiobiology for theRadiologist, 5th ed. Philadelphia, Lippincott Williams & Wilkins, 2000; withpermission.)