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The probability of control of a primary head and neck tumor by radiation therapy depends in part on the

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Figure 21-13. Comparisons of three treatment planning approaches for thyroid cancer. Note the respective isodose curves that show the differing dose distributions to the thyroid region and to the adjacent vital structures. A, Two dimensional treatment plan consisting of AP/PA and off-spinal cord oblique fields. B, Three dimensional conformed radiation therapy (3DCRT) plan. C, Intensity modulated radiation therapy (IMRT). D, Dose-volume histogram (DVH) comparing the three treatment approaches.

Figure 21-13. Comparisons of three treatment planning approaches for thyroid cancer. Note the respective isodose curves that show the differing dose distributions to the thyroid region and to the adjacent vital structures. A, Two dimensional treatment plan consisting of AP/PA and off-spinal cord oblique fields. B, Three dimensional conformed radiation therapy (3DCRT) plan. C, Intensity modulated radiation therapy (IMRT). D, Dose-volume histogram (DVH) comparing the three treatment approaches.

number of tumor clonogens present which is related to the volume of the mass. Since radiation cell kill is exponential, each fraction of radiation administered results in a set proportion of cells that are killed which is in contradistinction to an absolute number of cells killed. The larger the tumor mass, the greater the total cumulative dosage of radiation that is required to achieve control. The total dosage administered is based on the initial tumor volume and should not be decreased based on the rate of regression of the lesion, as this can result in an under-dosage of the tumor with an increased risk of recurrence.

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