International journal of radiation oncology, biology, physics
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Int. J. Radiat. Oncol. Biol. Phys. · Mar 1989
Dose fractionation and regeneration in radiotherapy for cancer of the oral cavity and oropharynx: tumor dose-response and repopulation.
In a retrospective study, local control of the primary tumor in 498 squamous cell carcinomas of the oral cavity and oropharynx was analyzed with respect to initial tumor volume, total dose after normalization for variations in fraction size, and to overall treatment time. Primary tumors were grouped into 4 sites, tongue (175), oral cavity including floor of mouth, faucial pillar, soft and hard palate and gingiva (210), tonsil (72) and buccal mucosa (41). Total doses of 60Co irradiation ranged from 30 Gy to 72 Gy, overall treatment times from 15 to 80 days and dose per fraction from 1.8 to 6 Gy. ⋯ Over the interval of about 30-55 days used in treating most of this series of patients, an increase of 60 cGy per day, on average, was required for a constant control rate. (2) The increase in dose was attributed to accelerated tumor clonogen growth rate. Such accelerated growth could be a major determinant of failure in protracted regimens. (3) The accelerated rate of regrowth was similar for all tumor sites and stages. (4) The dose for tumor control was relatively independent of variations in fraction size within a range of about 1.6 Gy to 3 Gy: the alpha/beta value in the linear quadratic isoeffect equation was at least 15 Gy. (5) Local control at the primary site required an average of about 3 Gy more for each increase in T stage. This increase most likely reflected an increased number of tumor clonogens, not a decreased tumor cell radiosensitivity. (6) The probability of control at the primary site was less likely if lymph nodes were positive, but this association was only shown to be statistically significant for primaries classified here as oral cavity and oropharynx, not tonsil, tongue or buccal mucosa. (7) After allowing for differences in treatment parameters, especially for heterogeneity in overall treatment times, tumor control probability increased steeply with increase in total dose. (8) A general principle of radiotherapy, at least for squamous carcinomas of head and neck, should be to deliver the desired fractionated dose regimen without unnecessary interruptions and in the shortest time compatible with no reduction in dose below that tolerated by the late-responding normal tissues.
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The incidence of permanent injury to the spinal cord as a complication of radiation therapy generally correlates positively with total radiation dosage. However, several reports in the literature have indicated that fraction size is also an important factor in the development or nondevelopment of late injuries in normal tissue. To determine the effect of fraction size on the incidence of radiation-induced spinal cord injuries, we reviewed 144 cases of head and neck cancer treated at our institution between 1971 and 1980 with radiation greater than 5600 cGy to a portion of the cervical spinal cord. ⋯ Nearly half of these (26 patients) received greater than 6000 cGy with fraction sizes of 133 cGy to 180 cGy. Only 1 of the 53 (1.9%) has sustained permanent spinal cord injury; 20 months after completion of radiation treatments he developed Brown-Séquard syndrome. Our experience suggests that radiation injuries to the spinal cord correlate not only with total radiation dosage, but also with fraction size; low fraction sizes appear to decrease the incidence of such injuries.