Study on the Effect of Low-Dose Exposed Neighbor Cells on the Survival of High-Dose Targeted Cells

Radiation therapy has been improved with the goal of local control of targeted cancer cells and maximum preservation of normal tissue. The advantage of radiation therapy is that it minimizes the burden on a patient by shortening the procedure time and recovery time compared with the surgical operation. In conventional radiotherapy, where the fractionated irradiation method is employed, a dose is delivered daily for several weeks. Daily fractionated irradiation is expected to provide normal cells a chance to recover from damage. However, the opportunity to recover may not be taken only by normal cells but also by the targeted tumor cells. Intensity modulated radiotherapy (IMRT) is advanced modulated beam therapy, which is the most widely used treatment modality. IMRT has achieved the significant reduction in normal tissue damage by dividing a single high-dose fraction into several smaller fields with steep dose gradients. In addition, the multi-leaf collimator (MLC) that can move each tiny tungsten collimator independently is enabled to deliver the precise doses to the targeted area. However, the total treatment time is increased compared with the conventional method. For more successful IMRT, it is necessary to improve the radiological imaging technology for precise diagnosis of cancer and to develop physical devices for more precise dose delivery. IMRT inevitably generates local regions of gradient doses between targeted and shielded cells throughout the treatment volume. Therefore, the complete understanding of the radiation-induced effect in modulated beam fields is necessary to suggest advanced treatment planning. However, the classical theory explaining the radiation-induced effect does not give an adequate answer to the cellular response in dose-gradient regions. In this study, the cell surviving fractions under non-uniform beam fields were observed with rat gliosarcoma cells, rat diencephalon cells, and mouse endothelial cells. The cells were irradiated with 200 kVp X-rays in two different beam field. The one is whole-beam exposure that all the cells in the flask were exposed to the X-ray beam. The other is half-beam exposure that the one-half of the cells (targeted cells) in the flask were exposed to the beam while the other half (neighbor cells) were shielded from the beam and were exposed to the penumbra dose. Targeted cells were exposed to 1, 2, 4, 6, 8, and 10 Gy, and the penumbra dose was approximately 10 ~ 20% of the target dose. The dose delivered to the cells in the flask was assessed by measuring the optical density of Gafchromic EBT films that were exposed to the X-ray beam in the same way as the cells in the flask. The surviving fraction of high-dose ( > 6 Gy) targeted cells increased when they have penumbra-dose exposed neighbor cell. The surviving fraction of the targeted cells under half-beam exposure was reduced when the radiation self-conditioned medium was replaced with a fresh one within 5 min following exposure. It was further reduced when the targeted cells were harvested immediately after irradiation and incubated in new dishes with fresh culture media. In the last decade, the new types of radiation-induced bystander effect (RIBE) have been suggested by few research groups. We investigated the new types of bystander effect (bystander effect Type 2 and Type 3), which had been defined in previous studies. Cells near the region of non-uniform doses might respond differently from the cells of uniform dose. In this study, we verified the response of high-dose targeted cells to the neighbor cells under low penumbra-dose exposure. The low penumbra-dose exposed neighbor cells benefited the survival of the high-dose targeted cells. The low penumbra-dose exposed neighbor cells exhibited the effect of increasing the survival of the high-dose targeted cells by secretion or activation of factors during and after irradiation. We named this phenomenon as the "Neighbor effect." Further studies are required to understand the biological mechanism of the neighbor effect fully. Until then, careful assessment is suggested on the therapeutic efficacy of IMRT therapy, which most likely generates local regions of gradient doses in the treatment volume