An iterative reconstruction (IR) technique in computed tomography (CT) is expected to play an important role in reducing the radiation dose while preserving both spatial resolution and contrast-to-noise ratio. However, images obtained by using the IR technique are known to have different visual appearances from those obtained by using the traditional filtered back-projection (FBP) reconstruction. This appearance is often figuratively described as “blocky,” but it has not been objectively characterized further. In this paper, we propose a novel image quality metric, called “perceptual image dissimilarity” (PID), to characterize the visual dissimilarity between FBP and IR images. The PID was formulated as a grayscale transformation and subsequent structural similarity (SSIM)-based image quality measurement. The PID metric was validated using phantom images with three different modules. Sixty datasets, each consisting of an IR image and its corresponding noise-level-equivalent FBP image, were visually assigned “subjective dissimilarity scores” on a five level scale by six observers. The data sets were then quantitatively analyzed using both the PID and the traditional mean squared error (MSE) metrics. Our results show that the PID is highly consistent with the subjective dissimilarity score and thus delivers superior performance, whereas the MSE fails to quantify the observers’ visual perception.

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Recent advances in the field of radiation therapy (RT) have considerably improved treatment outcomes of various cancers. It is related to not only the technological progress in medical physics but also the analytical progress in radiation biological effectiveness. However, the treatment results of RT, especially in advanced cancer, are still insufficient, therefore it is necessary to establish a safety and more effective method for treating cancer. Understanding the radiation biology is essential to appreciate the effect of RT. Hence, we review the controversial point of RT for radiation biology and introduce the results of basic research.

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DNA double-strand break (DSB) is considered most deleterious among radiation-induced DNA damages and most relevant to the biological effects of radiation. In eukaryotic cells, DSB is repaired mainly through two pathways: non-homologous end-joining (NHEJ) and homologous recombination (HR). These repair pathways seem to play complementary roles. NHEJ is considered less accurate than HR, but HR is available only in late S and G2 phases in vertebrates. Recent studies elucidated how cells choose one from these two pathways depending on the circumstance: cell cycle phase, complexity of DNA damage and chromatin structure.

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The oxygen effect was observed as 1912 by Swartz. The ratio of doses administered under hypoxic to oxic conditions needed to achieve the same biological effect is called the oxygen enhancement ratio (OER). For low-LET radiation, such as photon radiation, the OER at high doses has a value of between 2.5 and 3, and the OER has a smaller value of about 2.5 or less at lower doses. The oxygen effect is large and important in the case of low-LET radiations. Radio-chemical reactions are generally believed to be the fundamental mechanisms underlying oxygen effects. Oxygen fixes the damage produced by free radical. In the absence of oxygen, damage produced by the indirect action may be repaired. The OER has been determined for a wide variety of chemical and biologic systems with different endpoints. For high-LET radiation such as heavy-ions, oxygen effect is very small. The oxygen-in-the-track hypothesis proposed to account for this effect, suggests that cells exposed to high-LET radiation exhibit an oxygenated microenvironment around the particle track, even when they are irradiated under anoxic conditions.

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Since 1994, a Phase I/II clinical study and radiotherapy have carried out using carbon-ion beams produced with the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences. Now we constructed the new treatment facility for the advanced carbon-ion therapy at HIMAC applying a 3D fast spot scanning system with pencil beams. In the field of fundamental biological studies for high-LET heavy ions, there are some reports regarding bystander effects after exposure to alpha particles derived from ²³⁸Pu or He-ion microbeams. However, only limited sets of studies have examined bystander effects after exposure to different ion species heavier than helium, such as carbon ions. We have been investigating bystander cellular responses in both normal human and human tumor cells irradiated with the HIMAC carbon ions. Bystander cell-killing effect was observed in the cells harboring wild-type P53 gene, but not in the P53-mutated cells. Moreover, observed bystander effect was suppressed by treating with a specific inhibitor of gap-junction mediated cell-cell communication. There is clear evidence that the carbon-ion irradiation enables the enhanced cell killing in cells with wild-type P53 gene via gap-junction mediated bystander effect.

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This review describes the secondary cancer after radiotherapy. Secondary cancer is a great concern for cancer survivors, especially for childhood cancer survivors not only because of their intrinsic high susceptibility to radiation but also because of successful achievement of longer survival. Recent advance of molecular biology reveals unique genomic changes, which distinguish radiation-induced tumors from spontaneous or chemically induced tumors.

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