The effect of fractionated irradiation on cell kinetics

Linking the history of radiation biology to the hallmarks of cancer

Hanahan and Weinberg recently updated their conceptual framework of the "Hallmarks of Cancer". The original article, published in 2000, is among the most highly cited reviews in the field of oncology. The goal of this review is to highlight important discoveries in radiation biology that pertain to the Hallmarks. We identified early studies that exemplified how ionizing radiation affects the hallmarks or how radiation was used experimentally to advance the understanding of key hallmarks. A literature search was performed to obtain relevant primary research, and topics were assigned to a particular hallmark to allow an organized, chronological account of the radiobiological advancements. The hallmarks are reviewed in an order that flows from cellular to microenvironmental effects.

Comparison of the radiobiological effect of carbon ion beam therapy and conventional radiation therapy on cervical cancer

Little clinical evidence has been provided to show the minimization of radiation resistance of tumors using high linear energy transfer radiation. We therefore investigated the radiobiological and molecular pathological aspects of carbon beam therapy. A total of 27 patients with squamous cell carcinoma (SCC) of the cervix were treated using a carbon beam and 50 control patients with SCC of the cervix using a photon beam. The expression of Ki-67, p53, and p27 proteins before radiotherapy and 5 and 15 days after therapy initiation were investigated using immunohistochemistry. Similar changes were observed in Ki-67 labeling index (LI) and p53 LI during carbon and photon beam therapies. However, for carbon beam therapy, the mean p27 LI significantly decreased from 25.2% before treatment to 18.6% on the 5th day after treatment initiation, followed by a significant increase to 36.1% on the 15th day. In contrast, for photon beam therapy, the p27 LI consistently decreased from the initial 19.9% to 13.7% on the 15th day. Histological effects were observably stronger under carbon than photon beam therapy, though no statistically significant difference was observed (p = 0.07 on the 5th day and p = 0.10 on the 15th day). The changes in p27 LI under carbon beam therapy were significantly different from those under photon beam therapy, which suggests important molecular differences in the radio-biological response between therapies. Further investigation is required to elucidate the clinical relevance of these putative changes and optimize the relative biological effectiveness of carbon beam to X-ray.

Fractionated X-radiation treatment can elicit an inducible-like radioprotective response that is not dependent on the intrinsic cellular X-radiation resistance/sensitivity

Inducible responses are well documented to play a role in the radiation response of cells. However, it is not known whether clinically relevant fractionated X-radiation treatment could elicit an inducible-like radioprotective response and whether there is a direct correlation between the inducible radiation response phenomenon and the intrinsic radiation response of the cell. Therefore, the purpose of this study was to determine whether closely related human colorectal tumor (HCT116) clones treated with fractionated X rays could elicit an inducible-like radiation response to a subsequent acute (i.e. single) X-ray challenge, and whether the magnitude of the inducible-like response correlates with the intrinsic X-ray resistance of the responding clones. After fractionated X irradiation, only the radiosensitive clone showed enhanced clonogenic survival with a subsequent acute X-ray exposure. Cell cycle changes or the selection of subclones with increased intrinsic radiation resistance induced by the fractionated X rays were excluded as the basis of this enhanced tolerance, suggesting the presence of an inducible-like radioprotective response. Using the comet assay, we found similar amounts of intrinsic DNA damage among the clones after acute X irradiation. Our findings demonstrate that fractionated X-ray treatment can elicit an inducible-like radioprotective response and represent the first evidence that this response is independent of the intrinsic radiation resistance/sensitivity of the responding cells.

Relative changes of MIB1 and PC10 labeling indexes in cell kinetics during radiotherapy for cervical squamous cell carcinoma

To assess the changes in cell kinetics during radiotherapy (RT) in terms of the MIB1 and PC10 labeling indexes of cancer cells, a total of 230 biopsy specimens taken from 76 consecutive patients with cervical squamous cell carcinoma treated with RT was investigated. The MIB1 and PC10 indexes represent the growth fraction and proliferative activity, respectively. The MIB1 index ranged from 11 to 67% (mean value: 38%) before RT, from 21 to 86% (50%) at 5.4 Gy, from 15 to 85% (53%) at 9 Gy, and from 0 to 69% (26%) at 27 Gy. The PC10 index ranged from 4 to 46% (23%) before RT, from 5 to 62%, (26%) at 5.4 Gy, from 6 to 63% (23%) at 9 Gy, and from 0 to 35% (11%) at 27 Gy. The correlational relationships between the increment of both indexes during RT (delta index) and the index before RT were analyzed on an individual basis. The delta index was calculated by the index at various doses during RT minus the index before RT. Negative linear relationships were observed between the delta MIB1 index at 9 Gy or delta MIB1 index at 27 Gy and the MIB1 before RT, and also between the delta PC10 index at 9 Gy or delta PC10 index at 27 Gy and the PC10 index before RT. The linear regression coefficient values showed more than 0.5 (0.511 to 0.682) with statistical significance. According to these results, it was suggested that the cervical cancers of lower proliferative activity showed greater increases in growth fraction and proliferative activity by irradiation than those of higher proliferative tumors. These findings suggest that reversal patterns of changes in cell kinetics including growth fraction and proliferative activity took place in the early period of RT for cervical squamous cell carcinomas. The study of changes in cell cycle kinetics in human tumors during RT may have potential impact on the further development of RT.

How radiation oncology emerged as a discipline in Finland

The news about the discovery of Röntgen's new rays spread over Finland quite early, and the first Finnish writing about the rays appeared in February 1896. The first x-ray machine was installed in 1897. Proper training of radiologists began, however, at a comparatively late date. For a long time the position of radiologists remained a subordinate one. Radiology had the nature of a spare time hobby, because in most hospitals surgeons treated x-ray machines as a sideline. Because of this, scientific research work was delayed. The specialty of x-ray diagnosis and therapy was, however, established in 1921, and the Finnish Society of Radiology was founded in 1924. The first radiotherapy department with its own beds was opened in 1936 in Helsinki. In 1950, the first professor of medical radiology was appointed at the University of Helsinki. The Finnish Cancer Society played an important role in developing the radiotherapy net in the country by supplying cobalt devises and auxiliary hospital activities in the 1960s. In the early 1960s roentgen diagnosis and radiotherapy were separated into two distinct disciplines. There are now five medical faculties with chairs in radiation oncology. The country is divided into five regions for cancer care. The managing groups in each region are headed by the professor of radiotherapy and oncology. The radiation oncologists in Finland are involved in diagnosis and staging of cancer and are responsible for radiation therapy, chemotherapy and hormones. They are responsible for follow-up of treated patients, their own wards, and the general care of their patients.

G2 arrest following fractionated irradiation in the mouse jejunal crypt

A G2 block is a well-known response to irradiation, showing a delay time that depends on the given single dose. Our question was whether the division delay time is the same after multiple doses as after a single dose. We therefore studied mitosis in the mouse jejunal crypt using a quantitative approach. Three radiation schedules with a fraction size of 2 Gy were compared: a single dose and twice-daily irradiation at a 4-h interval, for 1 or 3 days. With the single dose, mitotic figures disappeared at 0.5 h and reappeared after 2 h. At the end of each multiple-dose regimen, significant numbers of mitoses could be observed at 0.5 and 1 h but a trough was seen at 2 h, with numbers then increasing again. Calculated average division delays after the single dose and repeated doses for 1 and 3 days were 1.5, 1.3 and 1.7 h respectively. In summary, most crypt cells treated with multiple fractions remain susceptible to G2 arrest, showing the same delay time as with the single dose.

Radiosensitivity of mesothelioma cell lines

The present study was carried out in order to examine the radiosensitivity of malignant pleural mesothelioma cell lines. Cell kinetics, radiation-induced delay of the cell cycle and DNA ploidy of the cell lines were also determined. For comparison an HeLa and a human foetal fibroblast cell line were simultaneously explored. Six previously cytogenetically and histologically characterized mesothelioma tumor cell lines were applied. A rapid tiazolyl blue microtiter (MTT) assay was used to analyze radiosensitivity and cell kinetics and DNA ploidy of the cultured cells were determined by flow cytometry. The survival fraction after a dose of 2 Gy (SF2), parameters alpha and beta of the linear quadratic model (LQ-model) and mean inactivation dose (D(MID)) were also estimated. The DNA index of four cell lines equaled 1.0 and two cell lines equaled 1.5 and 1.6. Different mesothelioma cell lines showed a great variation in radiosensitivity. Mean survival fraction after a radiation dose of 2 Gy (SF2) was 0.60 and ranged from 0.36 to 0.81 and mean alpha value was 0.26 (range 0.48 - 0.083). The SF2 of the most sensitive diploid mesothelioma cell line was 0.36: less than that of the foetal fibroblast cell line (0.49). The survival fractions (0.81 and 0.74) of the two most resistant cell lines, which also were aneuploid, were equal to that of the HeLa cell line (0.78). The alpha/beta ratios of the most sensitive cell lines were almost an order of magnitude greater than those of the two most resistant cell lines. Radiation-induced delay of the most resistant aneuploid cell line was similar to that of HeLa cells but in the most sensitive (diploid cells) there was practically no entry into the G1 phase following the 2 Gy radiation dose during 36 h.

Radiation effects on S-phase duration, labelling index, potential doubling time and DNA distribution in head and neck cancer xenografts

The effect of irradiation on S-phase duration (Ts), labelling index (LI), potential doubling time (Tpot), and cell cycle phase distributions was determined by DNA flow cytometry in xenografted human squamous cell carcinoma of the head and neck (SCCHN). Tumours were treated with a single dose of 3 Gy, and excised at intervals over a 90-h period. Six hours before each excision the tumours were labelled in vivo with bromodeoxyuridine (BrdUrd). Although the growth rate of irradiated tumours was comparable with that of untreated controls, analysis of BrdUrd uptake revealed a transient reduction of LI and a prolongation of Ts in irradiated tumours. Maximum mean Tpot was 931 days in irradiated tumours as compared to 13 days in untreated controls. The variations in Ts, LI and Tpot all occurred within the first hours after irradiation; during the remainder of the observation time, the values of the variables did not differ from those of untreated controls. In irradiated tumours the distribution of cells according to DNA content changed significantly on three occasions during the observation period: 1) Parallel to the initial lowering of LI and prolongation of Ts there was a transient increase in the proportion of cells in G0/G1 and a decrease in the proportion of cells in S and G2; 2) At 18 h, the most pronounced cell cycle phase redistribution occurred when the G0/G1 fraction decreased and the S and G2 phase fractions increased; 3) At 66 h (i.e., approximately one cell cycle later), the pattern was the same as that after 18 h. The findings suggest that the transient prolongation of DNA replication seen in SCCHN cells immediately after a single radiation dose is a symptom of DNA damage inflicted during late G1 or early S-phase, and that this disturbance in DNA synthesis is associated with the subsequent accumulation of cells in G2 phase.

Radiotherapy of the rhabdomyosarcoma R1H of the rat: influence of the time interval between two daily fractions during hyperfractionated radiotherapy

The response of the rhabdomyosarcoma R1H of the rat to hyperfractionated irradiation with different time intervals between the two daily fractions has been investigated. All tumours were exposed to irradiation 5 days per week over 6 weeks. A standard treatment of 30 fractions, i.e. one fraction per day, of 1.83-2.75 Gy (200 kVp X-rays) was compared with a hyperfractionated schedule of 60 fractions, i.e. two fractions per day, of 0.92-1.38 Gy with time intervals of either 1, 2, 3, 5 or 6 h between the two daily fractions. Tumour response has been assessed by (a) net growth delay and (b) local tumour control. Compared with standard treatment (one fraction per day) significant reduction (p < 0.005) of net growth delay was observed for the tumours treated with two daily fractions separated by 2 h. However, at a time interval of 5 and 6 h between the two daily fractions net growth delay increased considerably (p < 0.0001 and p < 0.01) as compared with the standard treatment and an increased rate of local tumour control was observed. The major findings is that tumour response is not as would be predicted by repair if the interval between the two daily fractions exceeds 2 h. The competing or overriding mechanisms cannot be identified ultimately, but the data, though based on small animal numbers and collected over an extended time period, reflect a significant effect.

Transient increases of growth fraction during fractionated radiation therapy for cervical carcinoma. Ki-67 and PC10 immunostaining

BACKGROUND

The Ki-67 and PC10 indexes are consistent with the growth and S-phase fractions, respectively.

METHODS

Eighty-three biopsy specimens from 18 patients with cervical squamous cell carcinoma were examined before and during fractionated radiation therapy (RT), using an immunohistochemical method.

RESULTS

The 18 patients were divided into two groups according to the Ki-67 index before RT; those with values of less than 42% (Group A; n = 11) and those with values of 42% or more (Group B; n = 7). The Ki-67 index in Group A increased approximately twofold at radiation doses of 5.4-9 Gy compared with values before RT. The Ki-67 index in Group B, which increased slightly at doses of 5.4-9 Gy, decreased by approximately one-fifth at a dose of 27 Gy, compared with the value before RT. The PC10 indexes in both groups showed little change to doses of 5.4-9 Gy. The mitotic indexes were less than 1% before RT and decreased to a minimum with doses of 18-27 Gy. There was no correlation between the Ki-67 and PC10 indexes before or during RT.

CONCLUSION

In cancer tissues with a low growth fraction before RT, radiation doses of 5.4-9 Gy produced increases in the numbers of cycling cancer cells in the G1 or G2 phases. This transient increase possibly was attributable to the combined effects of the recruitment phenomenon and G1 or G2 block.