Effect of dose-rate and irradiation geometry on the biological response of normal cells and cancer cells under radiotherapeutic conditions

Inverse dose protraction effects of low-LET radiation: Evidence and significance

Biological effects of ionizing radiation vary not merely with total dose but also with temporal dose distribution. Sparing dose protraction effects, in which dose protraction reduces effects of radiation have widely been accepted and generally assumed in radiation protection, particularly for stochastic effects (e.g., solid cancer). In contrast, inverse dose protraction effects (IDPEs) in which dose protraction enhances radiation effects have not been well recognized, nor comprehensively reviewed. Here, we review the current knowledge on IDPEs of low linear energy transfer (LET) radiation. To the best of our knowledge, since 1952, 157 biology, epidemiology or clinical papers have reported IDPEs following external or internal low-LET irradiation with photons (X-rays, γ-rays), β-rays, electrons, protons or helium ions. IDPEs of low-LET radiation have been described for biochemical changes in cell-free macromolecules (DNA, proteins or lipids), DNA damage responses in bacteria and yeasts, DNA damage, cytogenetic changes, neoplastic transformation and cell death in mammalian cell cultures of human, rodent or bovine origin, mutagenesis in silkworms, cytogenetic changes, induction of cancer (solid tumors and leukemia) and non-cancer effects (male sterility, cataracts and diseases of the circulatory system), tumor inactivation and survival in non-human mammals (rodents, rabbits, dogs and pigs), and induction of cancer and non-cancer effects (skin changes and diseases of the circulatory system) in humans. In contrast to a growing body of phenomenological evidence for manifestations of IDPEs, there is limited knowledge on mechanistic underpinnings, but proposed mechanisms involve cell cycle-dependent resensitization and low dose hyper-radiosensitivity. These necessitate continued studies for further mechanistic developments and assessment of implications of scientific evidence for radiation protection (e.g., in terms of a dose rate effectiveness factor).

Intra-fractional dose rate effect in continuous and interrupted irradiation of the MCF-7 cell line: Possible radiobiological implications for breath-hold techniques in breast radiotherapy?

PURPOSE

To investigate the effects of different dose rates (DRs) in continuous and interrupted irradiation on in-vitro survival of the MCF-7 cell line, towards finding possible radiobiological effects of breath-hold techniques in breast radiotherapy (RT), in which intra-fractional beam interruptions and delivery prolongation can occur.

MATERIALS AND METHODS

MCF-7 cells were irradiated continuously or with regular interruptions using 6 MV x-rays at different accelerator DRs (50-400 cGy/min) to deliver a 2 Gy dose. The interrupted irradiation was delivered in a 10 s on, 10 s off manner. Then, cell survival and viability were studied using colony and MTT assays, respectively.

RESULTS

Survival and viability with continuous and interrupted irradiation were similar ( P > 0.5). A significant increase in survival at 50, 100, and 400 cGy/min compared to 200 and 300 cGy/min was observed, also a significant decreasing and then increasing trend from 50 to 200 cGy/min and 200 to 400 cGy/min, respectively ( P < 0.04). Relative to 200 cGy/min, the survival fractions at 50, 100, 300, and 400 cGy/min were 1.24, 1.23, 1.05, and 1.20 times greater, respectively. Cell viability did not show significant differences between the DRs, despite following the same trend as cell survival.

CONCLUSION

Our results suggest that for continuous irradiation of in-vitro MCF-7 cells, with increasing DR within the 50-400 cGy/min range, sensitivity increases and then decreases (inverse effect), also that up to doubling of treatment time in breath-hold techniques does not affect in-vitro radiobiological efficacy with 200-400 cGy/min accelerator DRs. Further confirmatory studies are required.

Ex Vivo Irradiation of Lung Cancer Stem Cells Identifies the Lowest Therapeutic Dose Needed for Tumor Growth Arrest and Mass Reduction In Vivo

Radiotherapy represents a first-line treatment for many inoperable lung tumors. New technologies offer novel opportunities for the treatment of lung cancer with the administration of higher doses of radiation in smaller volumes. Because both therapeutic and toxic treatment effects are dose-dependent, it is important to identify a minimal dose protocol for each individual patient that maintains efficacy while decreasing toxicity. Cancer stem cells sustain tumor growth, promote metastatic dissemination, and may give rise to secondary resistance. The identification of effective protocols targeting these cells may improve disease-free survival of treated patients. In this work, we evaluated the existence of individual profiles of sensitivity to radiotherapy in patient-derived cancer stem cells (CSCs) using both in vitro and in vivo models. Both CSCs in vitro and mice implanted with CSCs were treated with radiotherapy at different dose intensities and rates. CSC response to different radiation doses greatly varied among patients. In vitro radiation sensitivity of CSCs corresponded to the therapeutic outcome in the corresponding mouse tumor model. On the other side, the dose administration rate did not affect the response. These findings suggest that in vitro evaluation of CSCs may potentially predict patients’ response, thus guiding clinical decision.

Dose Rate Effects on the Selective Radiosensitization of Prostate Cells by GRPR-Targeted Gold Nanoparticles

For a while, gold nanoparticles (AuNPs) have been recognized as potential radiosensitizers in cancer radiation therapy, mainly due to their physical properties, making them appealing for medical applications. Nevertheless, the performance of AuNPs as radiosensitizers still raises important questions that need further investigation. Searching for selective prostate (PCa) radiosensitizing agents, we studied the radiosensitization capability of the target-specific AuNP-BBN in cancer versus non-cancerous prostate cells, including the evaluation of dose rate effects in comparison with non-targeted counterparts (AuNP-TDOTA). PCa cells were found to exhibit increased AuNP uptake when compared to non-tumoral ones, leading to a significant loss of cellular proliferation ability and complex DNA damage, evidenced by the occurrence of multiple micronucleus per binucleated cell, in the case of PC3 cells irradiated with 2 Gy of γ-rays, after incubation with AuNP-BBN. Remarkably, the treatment of the PC3 cells with AuNP-BBN led to a much stronger influence of the dose rate on the cellular survival upon γ-photon irradiation, as well as on their genomic instability. Overall, AuNP-BBN emerged in this study as a very promising nanotool for the efficient and selective radiosensitization of human prostate cancer PC3 cells, therefore deserving further preclinical evaluation in adequate animal models for prostate cancer radiotherapy.

Delivery of Radiation at the Lowest Dose Rate by a Modern Linear Accelerator is Most Effective in Inhibiting Prostate Cancer Growth

PURPOSE

External beam radiotherapy is one of the treatment options for organ-confined prostate cancer. A total dose of 70 to 81 Gray (Gy) is given daily (1.8-2.5 Gy/d), with a dose rate of 3 to 6 Gy/min over 28 to 45 treatments during 8 to 9 weeks. We applied the latest technological development in linear accelerators for enabling a wide range of dose rates (from 0.2-21 Gy/min) to test the effect of different delivery dose rates on prostate tumor growth in an animal xenograft model.

MATERIALS AND METHODS

A prostate cancer xenograft model was established in CD1/nude mice by means of PC-3 and CL-1 cells. The animals were radiated by a TrueBeam linear accelerator that delivered 4 dose rates ranging from 0.6 to 14 Gy/min, and reaching a total dose of 20 Gy. The mice were weighed and monitored for tumor development twice weekly. A 2-way analysis of variance was used to compare statistical differences between the groups.

RESULTS

Tumor growth was inhibited by radiation at all 4 dose rates in the 20 study mice compared to no radiation (n = 5, controls). The most significant reduction in tumor volumes was observed when the same dose of radiation was delivered at a rate of 0.6 Gy/min (P < .01). The animals' weights were not affected by any dose rate.

CONCLUSIONS

Delivery of radiation with a TrueBeam linear accelerator at the lowest possible rate was most effective in prostate cancer growth inhibition and might be considered a preferential treatment mode for localized prostate cancer.

Evaluating the effect of low dose rate of gamma rays in germ cells of Drosophila melanogaster

Purpose: Evaluation of genetic risk in germ cells is still matter of research, mainly due to their role in the transmission of genetic information from one generation to another. Although numerous experiments have been carried out in Drosophila in order to study the effect of radiation on germ cells, the role of dose rate (DR) has not been fully explored. The purpose of this study was to evaluate the action of DR on the radioprotection induction on male germ cell of D. melanogaster.Material and method: The productivity and the sex-linked recessive lethal (SLRL) tests were used to evaluate the radio-sensitivity of different states of the germ line of males. Two-day-old males of Canton-S wild type strain were pretreated with 0.2 Gy at 5.4 or 34.3 Gy/h of gamma rays from a ⁶⁰Co source, three hours later, they were irradiated with 20 Gy at 907.7 Gy/h. Thereafter, each single male was crossed with 3 five-day old Basc virgin females, that were replaced every other day by new females. This procedure was conducted three times, to test the whole germ cell stages.Results: Females crossed with males irradiated with 0.2 Gy at both DR tested, laid a higher number of eggs than control, but egg-viability was reduced. On the other hand, in the group of 0.2 Gy + 20 Gy -combined treatments- the total number of eggs laid decreased only when 0.2 Gy were delivered at 34.3 Gy/h however, the egg-viability increased. The dose of 0.2 Gy at both DR did not modify the baseline frequency of SLRL. A tendency to decrease in the frequency of lethals in brood III was found in combined treatments at both DR.Conclusion: The fact that 0.2 Gy at 5.4 or 34.3 Gy/h induced an increase in the egg-viability and a tendency to decrease the genetic damage in pre-meiotic cells provoked by 20 Gy, might indicate the induction of any mechanism that could be interpreted as radioprotection in male germ cells of D. melanogaster. Results emphasize the need to carry out more studies on the effect of the DR on the induction of genetic damage in germ cells.

Comparison of Biological Effects of γ-Radiation of Low and Ultra-High Dose Rate on Lymphocytes and Cultured Human Malignant Lymphoma Cells

We studied the effect of low and high-dose rate photon radiation on activation of cell death by apoptosis and necrosis in malignant cell lines of lymphocytic origin Raji and Jurkat (human B and T-cell lymphomas) and normal human lymphocytes from healthy volunteers. It was shown that photon radiation with ultra-high dose rate induced significantly higher levels of "early" apoptosis and lower levels of necrosis compared to γ-radiation with dose rate used for radiation therapy.

Investigation of the level of DNA double-strand breaks and mechanisms of cell death under irradiation of lung cancer and melanoma cells with ultra-high dose rate photon radiation

Research into the effects of radiation delivered at ultrahigh dose rates > 1 × 107 Gy/min to biological objects is a new promising area of radiobiology. The unique characteristics of the high-current nanosecond electron accelerator Mir-M enable its use in medical and biological research, specifically in the experiments aimed at investigating the effect of therapeutic doses at a dose rate up to 100 MGy/s. In this work we study the effects of ultrahigh dose rate photon radiation on human lung carcinoma (A549) and melanoma (MelMtp-x) cells lines and compare them with those of the therapeutic gamma unit Rokus-AM. We show that ultrahigh dose rates induce more significant damage in the studied cell lines at doses between 2 and 7 Gy, radioresistant melanoma being more sensitive to photon radiation delivered at ultrahigh dose rates.

Evidence that the radioprotector effect of ascorbic acid depends on the radiation dose rate

Many studies have revealed that ascorbic acid (Aa) acts as a powerful inhibitor of genetic damage. The objetive of the present study was to evaluate the radioprotector effect of Aa at two diferent radiation dose rates. The somatic mutation and recombination test in Drosophila melanogaster was used. 48 h larvae were treated for 24 h with 25, 50 and 100 mM of Aa. After pretreatment, larvae were irradiated with 20 Gy of gamma rays administered at 36 or 960 Gy/h. Toxicity, development rate and frequency of mutant spots were recorded. Results provide evidence of a radioprotective effect for all tested concentrations of Aa only when 20 Gy were delivered at 36 Gy/h and only with 25 mM using the 960 Gy/h. To consider the use of Aa as radioprotector or therapeutic agent, it is necessary to know its potential under different situations to avoid unwanted injuries.

Dependence of micronuclei assay on the depth of absorbed dose

AIM

The purpose of the present study is to investigate the dependence of micronuclei response on the depth of absorbed dose.

BACKGROUND

One of the most common cytogenetic methods used for radiation dosimetry is micronuclei (MN). Being less complex and faster than other methods are two remarkable advantages of the MN method which make it suitable for monitoring of population. In biological dosimetry based on the micronuclei method, the investigation into the dependence of response on the depth in which dose is absorbed is significant, though has received less attention so far.

MATERIALS AND METHODS

Blood samples were poured in separate vials to be irradiated at different depths using a linear accelerator system.

RESULTS

According to the results, MN, as a function of the absorbed dose, had the best fitness with the linear-quadratic model at all depths. Furthermore, the results showed the dependence of MN response on the depth of absorbed dose. For doses up to 2 Gy, the maximum difference from the reference depth of 1.5 cm was related to the depth of 10 cm; however, by increasing the absorbed dose, the response associated with the depth of 20 cm showed the maximum deviation from the reference depth.

CONCLUSIONS

Consequently, it is necessary to apply a correction factor to the biological dosimetry. The correction factor is dependent on the depth and the absorbed dose.

Radioprotective effects of lentil sprouts against X-ray radiation

The present study investigated the radioprotective efficacy of lentil (Lens culinaris) sprouts against X-ray radiation-induced cellular damage. Lentil seeds were dark germinated at low temperature and the sprout extract was prepared in PBS. Free radical scavenging of extract was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay and then the radioprotective potency of extract (0 to 1000 μg/mL) on the lymphocyte cells was determined by lactate dehydrogenases assay. Moreover, micronuclei assay was assessed using the cytokinesis-block technique. The irradiations were performed using 6 MV X-ray beam. The value of IC₅₀ for DPPH assay was 250 μg/mL. The median lethal dose for radiation was determinate at 5.37 Gy. Pretreatment with lentil sprout extract at 1000 μg/mL reduced cytotoxicity at 6 Gy total concentration from 70% to 50%. The results of micronuclei assay indicated that cells were resistant to radiation at concentrations of 500–1000 μg/mL of exogenous lentil sprout extract. The value of median effective concentration for micronuclei assay was 500 μg/mL. The results indicated that lentil sprout extract showed actually somewhat radioprotective effect on lymphocyte cell. In addition, the obtained results suggest that extract of total lentil sprout have more antioxidant activity than radicle part.

Can high dose rates used in cancer radiotherapy change therapeutic effectiveness?

Current cancer radiotherapy relies on increasingly high dose rates of ionising radiation (100-2400 cGy/min). It is possible that changing dose rates is not paralleled by treatment effectiveness. Irradiating cancer cells is assumed to induce molecular alterations that ultimately lead to apoptotic death. Studies comparing the efficacy of radiation-induced DNA damage and apoptotic death in relation to varying dose rates do not provide unequivocal data. Whereas some have demonstrated higher dose rates (single dose) to effectively kill cancer cells, others claim the opposite. Recent gene expression studies in cells subject to variable dose rates stress alterations in molecular signalling, especially in the expression of genes linked to cell survival, immune response, and tumour progression. Novel irradiation techniques of modern cancer treatment do not rely anymore on maintaining absolute constancy of dose rates during radiation emission: instead, timing and exposure areas are regulated temporally and spatially by modulating the dose rate and beam shape. Such conditions may be reflected in tumour cells' response to irradiation, and this is supported by the references provided.