Pions for radiotherapy at TRIUMF.

Beams of negative pi-mesons (pions) have depth-dose characteristics which are favorable for radiation therapy. As well, the densely ionizing radiations resulting from pion capture give pion beams an increased RBE and reduced OER, largely confined to the treatment volume. Three facilities capable of testing pion beams for therapy now exist: TRIUMF in Vancouver, LAMPF in New Mexico and SIN in Villigen, Switzerland. Patient treatment is already underway at LAMPF and will commence at TRIUMF in November, 1979, and at SIN in 1980. Pre-clinical studies of the pion beam at TRIUMF have confirmed the predicted physical and radiobiologic properties of this new type of radiation. Fractionated irradiation of mouse skin suggests an RBE of 1.4 -- 1.6 for small treatment volumes receiving 10 dose fractions of peak pions. Single dose studies with cultured cells give an RBE for peak pions of 1.2 -- 1.5 and an oxygen gain factor of 1.2. The results are generally in good agreement with those obtained at LAMPF and SIN. Techniques have been developed which facilitate the delivery of well-defined dose distributions with uniform biologic effect throughout the treatment volume.