Chapter 081. Principles of Cancer Treatment (Part 6)

Application to PatientsTeletherapyRadiation therapy can be used alone or together with chemotherapy to produce cure of localized tumors and control of the primary site of disease in tumors that have disseminated. Therapy is planned based on the use of a simulator with the treatment field or fields designed to accommodate an individual patients anatomic features. Individualized treatment planning employs lead shielding tailored to shape the field and limit the radiation exposure of normal tissue. Often the radiation is delivered from two or three different positions. Conformal three-dimensional treatment planning permits the delivery of higher doses of radiation to the...
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Chapter 081. Principles of Cancer Treatment (Part 6) Chapter 081. Principles of Cancer Treatment (Part 6) Application to Patients Teletherapy Radiation therapy can be used alone or together with chemotherapy toproduce cure of localized tumors and control of the primary site of disease intumors that have disseminated. Therapy is planned based on the use of a simulatorwith the treatment field or fields designed to accommodate an individual patientsanatomic features. Individualized treatment planning employs lead shieldingtailored to shape the field and limit the radiation exposure of normal tissue. Oftenthe radiation is delivered from two or three different positions. Conformal three-dimensional treatment planning permits the delivery of higher doses of radiation tothe target volume without increasing complications in the transit volume. Radiation therapy is a component of curative therapy for a number ofdiseases, including breast cancer, Hodgkins disease, head and neck cancer,prostate cancer, and gynecologic cancers. Radiation therapy can also palliatedisease symptoms in a variety of settings: relief of bone pain from metastaticdisease, control of brain metastases, reversal of spinal cord compression andsuperior vena caval obstruction, shrinkage of painful masses, and opening ofthreatened airways. In high-risk settings, radiation therapy can prevent thedevelopment of leptomeningeal disease and brain metastases in acute leukemiaand lung cancer. Brachytherapy Brachytherapy involves placing a sealed source of radiation into or adjacentto the tumor and withdrawing the radiation source after a period of time preciselycalculated to deliver a chosen dose of radiation to the tumor. This approach isoften used to treat brain tumors and cervical cancer. The difficulty withbrachytherapy is the short range of radiation effects (the inverse square law) andthe inability to shape the radiation to fit the target volume. Normal tissue mayreceive toxic exposure to the radiation, with attendant radiation enteritis or cystitisin cervix cancer or brain injury in brain tumors. Radionuclides and Radioimmunotherapy Nuclear medicine physicians or radiation oncologists may administerradionuclides with therapeutic effects. Iodine 131 is used to treat thyroid cancersince iodine is naturally taken up preferentially by the thyroid; it emits gammarays that destroy the normal thyroid as well as the tumor. Strontium 89 andsamarium 153 are two radionuclides that are preferentially taken up in bone,particularly sites of new bone formation. Both are capable of controlling bonemetastases and the pain associated with them, but the dose-limiting toxicity ismyelosuppression. Monoclonal antibodies and other ligands can be attached to radioisotopesby conjugation (for nonmetal isotopes) or by chelation (for metal isotopes), andthe targeting moiety can result in the accumulation of the radionuclidepreferentially in tumor. Iodine 131–labeled anti-CD20 and yttrium 90–labeledanti-CD20 are active in B cell lymphoma, and other labeled antibodies are beingevaluated. Thyroid uptake of labeled iodine is blocked by cold iodine. Dose-limiting toxicity is myelosuppression. Photodynamic Therapy Some chemical structures (porphyrins, phthalocyanines) are selectivelytaken up by cancer cells by mechanisms not fully defined. When light, usuallydelivered by a laser, is shone on cells containing these compounds, free radicalsare generated and the cells die. Hematoporphyrins and light are being used withincreasing frequency to treat skin cancer; ovarian cancer; and cancers of the lung,colon, rectum, and esophagus. Palliation of recurrent locally advanced disease cansometimes be dramatic and last many months. Toxicity Though radiation therapy is most often administered to a local region,systemic effects, including fatigue, anorexia, nausea, and vomiting, may developthat are related in part to the volume of tissue irradiated, dose fractionation,radiation fields, and individual susceptibility. Bone is among the mostradioresistant organs, radiation effects being manifested mainly in childrenthrough premature fusion of the epiphyseal growth plate. By contrast, the maletestis, female ovary, and bone marrow are the most sensitive organs. Any bonemarrow in a radiation field will be eradicated by therapeutic irradiation. Organswith less need for cell renewal, such as heart, skeletal muscle, and nerves, aremore resistant to radiation effects. In radiation-resistant organs, the vascularendothelium is the most sensitive component. Organs with more self-renewal as apart of normal homeostasis, such as the hematopoietic system and mucosal liningof the intestinal tract, ...