Abstract
A retrospective study has been undertaken to test the NSD formula as a predictor of isoeffective schedules in women treated after mastectomy by parasternal radiotherapy using fraction sizes of 2-3 Gy. Forty three women were identified who have been treated on the same orthovoltage unit 8-18 years previously and the severity of overall skin changes and telangiectasia were assessed by three observers using a 4-point categorical scale. At each level of skin damage, radiotherapy schedules delivered using fraction sizes of 250 R or 300 R were associated with lower TDF values than schedules using 225 R fraction. The difference between TDF factors for isoeffective regimes using small (225 R) and large (250 R and 300 R) fraction sizes is highly significant for both overall skin damage and telangiectasia (p = 0.004 by rank sum test). Care should be exercised when using current empirical formulae to calculate regimes isoeffective for late skin damage, even when modest changes in fraction size from 2 to 3 Gy are contemplated.
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