Treatment planning for negative pi-meson radiation therapy: Simultaneous multi-port irradiation with the Stanford Medical Pion Generator (SMPG)

The cylindrical geometry of the Stanford Medical Pion Generator (SMPG) offers promise in achieving pion star fractions from 0.57 to 0.67 within the tumor volume and tumor to entrance region dose ratios of greater than 25:1 for tumor volumes up to 10 cm dia. The star fractions and tumor to entrance region dose ratios decrease slightly with increasing tumor diameters in the same water phantom but do not change for the same tumor diameter in water phantoms with radii from 10 to 20 cm. Although the radial pseudo-penumbrae (distance between the 90% and 20% isodose lines) for 60 beams with a ±2% momentum spread are 2.6 and 3.1 cm for 15 and 20 cm range (in water) pion beams, the dose distributions are superior to those achievable with X-rays and neutrons. In addition, the sparing of normal tissues can be further improved by selectively omitting some of the 60 pion beams, by using a smaller momentum spread or by using pious of lower mean momentum. The tailoring of the irradiation volume along the axis of symmetry of the SMPG is influenced primarily by the length of the pion production target and will require a compromise between the ideal tumor volume configuration and the inherent focal properties of the SMPG. Thus each treatment plan will require careful consideration of the numerous interrelationships between the physical characteristics of the patient and tumor volume and the treatment parameters of the SMPG.