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The dose of radiation that can be delivered to a tumor is limited by the damage caused to surrounding normal tissues and the consequent risk of complications. Whether a certain risk of developing complications is regarded as acceptable depends both on the function of the tissue(s) and the severity of the damage involved. This risk must be compared to the probability of benefit (ie, eradicating the tumor) to determine the overall gain from the treatment. This gain can be estimated for an average group of patients, but it may vary for individual patients, depending on the particular characteristics of their tumors and the normal tissues at risk. The balance between the probabilities for tumor control and normal tissue complications gives a measure of the therapeutic ratio of a treatment (see Sec. 16.5.8). The therapeutic ratio can be improved either by increasing the effective radiation dose delivered to the tumor relative to that given to surrounding normal tissues, or by increasing the biological response of the tumor relative to that of the surrounding normal tissues (see Figs. 16–1, 16–2, 16–3).
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External beam radiation therapy is usually delivered in relatively small daily doses over the course of several weeks. The empiric development of such multifractionated treatments, which involve giving fractions of approximately 1.8 to 3 Gy daily for 5 to 8 weeks, is an example of exploiting biological factors to improve the therapeutic ratio. More recently, technical improvements in the physical aspects of radiation therapy have allowed an increase in the effective dose of radiation to deep-seated tumors without increasing the dose to normal tissues. Further improvements are occurring with the use of more sophisticated treatment planning methods, allowing for 3-dimensional (3D) conformal radiotherapy (3D-CRT), intensity-modulated radiation therapy (IMRT), and stereotactic treatments (Fig. 16–1). These new methods limit the volume of normal tissues irradiated to high doses and allow escalated doses to the tumor. Stereotactic body/brain radiation therapy (SBRT) uses a specially designed coordinate system for the exact localization of the tumor in the body so as to treat it with limited, but highly precise, treatment fields. SBRT involves the delivery of a single high-dose radiation treatment or a few (large-dose) fractionated radiation treatments (usually up to 5 treatments). A highly potent biological dose of radiation is delivered to the tumor over this period and has been used for individual brain or vertebral metastases or for small lung tumors (Milano et al, 2008). Finally, low-dose-rate and high-dose-rate brachytherapy can highly conform dose by placing radioactive sources ...