A comparison of the potential therapeutic gain of p(66)/Be neutrons and d(14)/Be neutrons.
OBJECTIVE
To determine the relationship between photon sensitivity and neutron sensitivity and between neutron RBE and photon resistance for two neutron modalities (with mean energies of 6 and 29 MeV) using human tumor cell lines spanning a wide range of radiosensitivities, the principal objective being whether or not a neutron advantage can be demonstrated.


METHODS
Eleven human tumor cell lines with mean photon inactivation doses of 1.65-4. 35 Gy were irradiated with 0-5.0 Gy of p(66)/Be neutrons (mean energy of 29 MeV) at Faure, S.A. and the same plating was irradiated on the same day with 0-10.0 Gy of Cobalt-gamma-rays. Twelve human tumor cell lines, many of which were identical with the above selection, and spanning mean photon inactivation doses of 1.75-4.08 Gy, were irradiated with 0-4 Gy of d(14)/Be neutrons (mean energy of 6 MeV) and with 0-10 Gy of 240 kVp X-rays at the Essen Klinikum. Cell survival was determined by the clonogenic assay, and data were fitted to the linear quadratic equation.


RESULTS
1. Using the mean inactivation dose, a significant correlation was found to exist between neutron sensitivity and photon sensitivity. However, this correlation was more pronounced in the Faure beam (r(2) = 0.89, p </= 0.0001) than in the Essen beam (r(2) = 0.65, p = 0.0027). 2. No significant relationship could be established between neutron RBE and photon resistance for both modalities (p = 0.69 and p = 0.07, respectively). 3. Using alpha-coefficients as a criterion, the neutron sensitivity for the Faure beam correlated with photon sensitivity (p = 0.001), but this did not apply to the Essen beam (p = 0.27). 4. The neutron RBE for the Essen beam derived from alpha-coefficients showed a steep increase with photon resistance (p = 0.003). In the Faure beam there was no increase of RBE with photon resistance (p = 0.494).


CONCLUSIONS
Radiobiological differences between high-energy and low-energy neutrons are particularly apparent in the dependence of the neutron RBE on photon sensitivity. The increase of RBE with photon resistance is more pronounced in the low-energy Essen neutrons than in the high-energy Faure neutrons. An RBE advantage is indicated for photon-resistant cell lines and this is particularly apparent in the low-dose range using alpha-coefficients as compared to the mean inactivation dose. The clinical application of low-energy neutrons may be more restricted because of poor penetration and lack of skin sparing. However, these neutrons discriminate better between photon-sensitive and photon-resistant cells giving an RBE range of 2-6 and a mean RBE of 4.1, than high-energy neutrons where the RBE range is 1.6-3.5 and the mean RBE is 2.4. From the radiobiological point of view it, therefore, appears that the clinical potential of low-energy neutrons is considerably underrated.
Slabbert
J P
JP
National Accelerator Centre, Faure, South Africa.
Theron
T
T
Zolzer
F
F
Streffer
C
C
Bohm
L
L
D001608:Beryllium / Q000494:pharmacology*; D002470:Cell Survival / Q000528:radiation effects; D003035:Cobalt / Q000494:pharmacology; D009502:Neutrons / Q000627:therapeutic use*; D017785:Photons / Q000627:therapeutic use*; D011836:Radiation Tolerance; D011853:Radiobiology; D012062:Relative Biological Effectiveness; D014407:Tumor Cells, Cultured / Q000528:radiation effects*; D014410:Tumor Stem Cell Assay
D003035:Cobalt; D001608:Beryllium