The relative biological effectiveness (RBE) for animal tumors treated with fractionated doses of 290 MeV/u carbon ions was studied. The growth delay of NFSa fibrosarcoma in mice was investigated following various daily doses given with carbon ions or those given with cesium gamma-rays, and the RBE was determined. Animal tumors were irradiated with carbon ions of various LET (linear energy transfer) in a 6-cm SOBP (spread-out Bragg peak), and the isoeffect doses; i.e. the dose necessary to induce a tumor growth delay of 15 days were studied. The iso-effect dose for carbon ions of 14 and 20 keV/microm increased with an increase in the number of fractions up to 4 fractions. The increase in the isoeffect dose with the fraction number was small for carbon ions of 44 keV/microm, and was not observed for 74 keV/microm. The alpha and beta values of the linear-quadratic model for the radiation dose-cell survival relationship were calculated by the Fe-plot analysis method. The alpha values increased linearly with an increase in the LET, while the beta values were independent of the LET. The alpha/beta ratio was 129 +/- 10 Gy for gamma-rays, and increased with an increase in the LET, reaching 475 +/- 168 Gy for 74 keV/microm carbon ions. The RBE for carbon ions relative to Cs-137 gamma-rays increased with the LET. The RBE values for 14 and 20 keV/microm carbon ions were 1.4 and independent of the number of fractions, while those for 44 and 74 keV/microm increased from 1.8 to 2.3 and from 2.4 to 3.0, respectively, when the number of fractions increased from 1 to 4. Increasing the number of fractions further from 4 to 6 was not associated with an increase in the RBE. These results together with our earlier study on the skin reaction support the use of an RBE of 3.0 in clinical trials of 80 keV/microm carbon beams. The RBE values for low doses of carbon beams were also considered.

Using a mouse model, we investigated the mechanisms of heterogeneity in response to ionizing radiation in this research. C57BL/6J and C3H/HeMs mice were irradiated with gamma rays at 10 and 20 Gy. The animals were sacrificed at times corresponding to the latent period, the pneumonic phase, and the start of the fibrotic phase for histological investigation. Small areas of fibrosis initially appeared in C57BL/6J mice at 4 weeks postirradiation with 20 Gy, whereas small inflammatory lesions appeared at 4 and 8 weeks after 20 and 10 Gy, respectively. The alveoli septa were thickened by an infiltration of inflammatory cells, and alveoli were obliterated in lungs from C57BL/6J mice after 20 Gy irradiation. At 24 hours and from 2 to 4 weeks postirradiation, fourfold more CD44 positive cells had accumulated in the lungs of C3H/HeMs than in C57BL/6J mice. Hyaluronan accumulated 12 hours after irradiation, and the rapid resolution was achieved within 2 weeks in the lungs in both strains of mice. C57BL/6J mice lungs accumulated dense collagen at 8 weeks. Quantitative RT-PCR assay was performed for several genes selected by cDNA microarray analysis. The expression of several genes, such as Cap1, Il18, Mmp12, Per3, Ltf, Ifi202a, and Rad51ap1 showed strain-dependent variances. In conclusion, a histological investigation suggested that C3H/HeMs mice were able to induce a more rapid clearance of matrix after irradiation than C57BL/6J mice. The expression analysis showed that the several genes are potentially involved in interstrain differences in inflammatory response causing radiation-induced lung fibrosis.

Tumor induction in mice legs that were locally irradiated with carbon ions was compared to tumor induction by gamma rays after single and fractionated irradiation. A total of 250 tumors were induced in 1104 mice that received carbon-ion doses of 5 through 65 Gy. A total of 77 tumors were induced in 371 mice that received gamma-ray doses of 45 through 95 Gy. Of 91 carbon-ion induced tumors examined histologically, 97 percent were malignant, and sarcomas such as malignant fibrous histiocytoma (47%) and fibrosarcoma (32%) were most frequently observed. Malignant fibrous histiocytoma was also the most frequently observed tumor (12 out of 20 tumors; 60%) after gamma-ray irradiation, followed by carcinomas (25%) such as adenocarcinoma and squamous cell carcinoma. Neither dose fractionation nor linear energy transfer affected tumor induction for carbon ions and gamma rays. Dose responses were linear for carbon ions and gamma rays, and showed no saturation up to 65 Gy of carbon ions and 95 Gy of gamma rays. The relative biological effectiveness of carbon ions was 2.2 for tumor induction and 1.9 for early skin reaction. We conclude that risk of secondary tumor induction by carbon-ion radiotherapy would not be seriously higher than anticipated.

It is well established that the RBE (relative biological effectiveness) for cell killing depends on LET (linear energy transfer), and that a maximum RBE is observed at approximately 150 keV.micron-1. However, the therapeutic gain depends on the ratio of the RBEs for the effects on the cancer cell population and the effects on normal tissues. The RBE of a given radiation quality depends not only on LET but also on dose, biological system and effect, and irradiation conditions. There is no data available to answer the question: which LET is suitable to improve the biological therapeutic gain of carbon ions? Here, three different LET values of 290 MeV/u carbon ions were selected, and the relative biological effectiveness was compared between tumour-growth retardation and skin damage using a murine transplantable tumour. Larger RBE values for tumours after than the skin type were obtained when carbon ions of intermediate LET were delivered daily for 2 to 5 fractions. The biological therapeutic gain would be high for the carbon ion SOBP if the number of fractions were correctly selected in clinical trials.

The multifraction regimens commonly used in conventional clinical radiotherapy are largely based on radiobiological experiments. However, no experimental reports on skin reactions focusing on inter-strain differences have displayed clinical relevance to the fractionated dose schedule. In this study, mice of inbred strains A/J, C57BL/6J, and C3H/HeMs were used to reveal inter-strain difference after multifractionated irradiation. Irradiation was performed daily at graded doses of 30-60 Gy total doses, with 10 fractions of 3-6 Gy. Acute skin reactions following irradiation were scored for 50 days after irradiation. Dividing a dose into a number of fractions obviously spared skin damage in the three strains of mice. No mouse exhibited a skin damage score more than 1.5, while single dose irradiation resulted in skin damage scores up to 3. The three different strains, however, showed varying susceptibility to fractionated irradiation within the range under 1.5. C3H/HeMs did not display any skin reaction after irradiation with 40 Gy total dose, while C57BL/6J and A/J demonstrated various skin reactions. Different latent periods of damage were also observed among the strains after irradiation at each dose. Our data suggest that genetic factors cause obvious variations in severity of damage and latent period after fractionated irradiation.