Cancer induction by radiotherapy: dose dependence and spatial relationship to irradiated volume.

The present study was initiated to identify radiotherapy-related parameters influencing the development of second malignancies. Between 1969 and 1989, about 31000 patients were treated with radiotherapy in Dresden. Of these, 203 were re-admitted for radiotherapy of a newly developed malignancy. Based on completeness of documentation, definition of site of origin and exclusion of recurrent or metastatic tumours, 85 patients were selected for analysis of spatial relation between new tumour and primary treatment fields, and of dose at the site of origin. The majority of second tumours were seen within the margin region of the treatment volume, defined as the volume from 2.5 cm inside to 5 cm outside the margin of the planning target volume (PTV). Fewer than 10% of the second tumours occurred inside the PTV and 11% developed outside the margin region. The majority of second tumours were observed in the volume receiving <6 Gy. Similar results were obtained in a group of 31 patients initially irradiated for benign diseases. This should be considered in radiotherapy, particularly with multiple-field techniques or intensity-modulated radiotherapy, where the volume receiving <6 Gy is substantially increased.