Clinical study of relative biological effectiveness for cervical carcinoma treated by ²⁵²Cf neutrons and assessed by histological tumour eradication

Clinical data of the University of Kentucky trial using californium (²⁵²Cf), or caesium (¹³⁷Cs), are reviewed for dose-response based on the endpoint of tumour eradication estimated from hysterectomy specimens obtained 4-6 weeks after preoperative irradiation. These data are used to assess the relative biological effectiveness (RBE) for ²⁵²Cf neutrons compared with ¹³⁷Cs γ radiation. Tumours treated were of common stage but were of bulky or barrel shape suitable for 'radiosurgical' therapy. Dose-repsonse curves were constructed, and additional data from the literature used to analyse the curve shape. The photon dose-response curve is complex on a logarithmic plot, whereas the ²⁵²Cf neutron curve is exponential. This indicates that the RBE can be different depending on the number of implants, schedule and size of dose delivered per session. The RBE values were approximately 8.0 at low doses or for multiple implants but they may rise to approximately 16 at larger doses or for single ²⁵²Cf implants.