Persistent T2*-hypointensity of the liver parenchyma after irradiation to the SPIO-accumulated liver: An imaging marker for responses to radiotherapy in hepatic malignancies

IONPs-Based Medical Imaging in Cancer Care: Moving Beyond Traditional Diagnosis and Therapeutic Assessment

Cancer-related burden of morbidity and mortality is rapidly rising worldwide. Medical imaging plays an important role in every phase of cancer management, including diagnosis, staging, treatment planning and evaluation. Iron oxide nanoparticles (IONPs) could serve as contrast agents or labeling agents to enhance the identification and visualization of pathological tissues as well as target cells. Multimodal or multifunctional imaging can be easily acquired by modifying IONPs with other imaging agents or functional groups, allowing the accessibility of combined imaging techniques and providing more comprehensive information for cancer care. To date, IONPs-enhanced medical imaging has gained intensive application in early diagnosis, monitoring treatment as well as guiding radio-frequency ablation, sentinel lymph node dissection, radiotherapy and hyperthermia therapy. Besides, IONPs mediated imaging is also capable of promoting the development of anti-cancer nanomedicines through identifying patients potentially sensitive to nanotherapeutics. Based on versatile imaging modes and application fields, this review highlights and summarizes recent research advances of IONPs-based medical imaging in cancer management. Besides, currently existing challenges are also discussed to provide perspectives and advices for the future development of IONPs-based imaging in cancer management.

Treatment margins in radiotherapy for liver tumors visualized as T2*-hypointense areas on SPIO-enhanced MRI at 9.4 T

OBJECTIVE

To investigate whether a SPIO-labeling technique could enable MR visualization of the treatment margin after X-irradiation at a single dose of 30 Gy.

MATERIALS AND METHODS

Fifteen rats bearing N1-S1 hepatoma in either the left (group 1) or right (group 2) liver lobe were examined. Four hours after systemic SPIO administration, the left lobe was selectively irradiated at 30 Gy. Liver T2* maps were acquired 7 days later using a 9.4 T scanner. The livers were excised and examined histologically.

RESULTS

The irradiated area showed T2*-weighted hypointensity with significantly shorter T2* values than those in the non-irradiated area (p < 0.001). Tumors in group 1 completely disappeared, whereas tumors in group 2 had grown outside the T2*-weighted hypointensity by up to ~ 2.3 times that at baseline. Group 1 showed significantly higher probability of tumor regression than group 2 (p = 0.048). Histologically, iron deposition was heavier in irradiated areas than in non-irradiated areas.

DISCUSSION

Even at a single dose of 30 Gy, which is a slightly higher dose than can be used clinically in stereotactic body radiotherapy, MR visualization of the treatment margin was achieved, because tumors showed significant growth outside the T2*-hypointense areas. In contrast, tumors disappeared inside the T2*-hypointense areas.

MR signal changes in superparamagnetic iron oxide nanoparticle-labeled macrophages in response to X irradiation

To investigate whether MR signals associated with macrophages labeled with superparamagnetic iron oxide nanoparticles (SPIONs) change in response to X irradiation, we performed in vitro MRI of SPION-labeled macrophage-like J774A.1 cells that were subsequently X irradiated. We labeled the cells with ferucarbotran at a concentration of 10 μg iron/mL in culture medium for 16 h and subsequently performed X irradiation at doses of 0, 2, 10, or 20 Gy using a low-energy X-ray unit. On Days 3 and 6, we suspended the cells in agar at a concentration of 2 × 10⁶ cells/mL and acquired multi-gradient echo and multi-spin echo images of the cell samples using a 3 T scanner to estimate R₂ * and R₂ . In addition, we microscopically investigated the relationship among the MR signal changes, intracellular SPIONs, and acidic organelles. Our data showed that X irradiation of labeled cells caused increased SPION deposition in lysosomes compared with the non-irradiated control. On Day 3, R₂ * and R₂ values in the 0 to 10 Gy irradiated samples were dose-dependently increased 5.4- and 1.5-fold compared with 17 ± 2 and 13 ± 1/s, respectively, in the non-irradiated control; these values plateaued at more than 10 Gy. Although the increases in R₂ *, R₂ , and SPION deposition were still observed in the 10 and 20 Gy samples on Days 6 and 7, the 2 Gy samples showed recovery in these parameters as cell growth was restored. Acidic organelles were temporarily increased in the irradiated cells, which suggests that the reduction in lysosomal acidity was not attributable to SPION deposition. In conclusion, X irradiation of macrophages can cause SPION deposition and R₂ * and R₂ elevation in a specific dose range. MRI of SPION-labeled and subsequently X-irradiated macrophages may be utilized as a novel technique for investigating macrophage responses to X irradiation.