The kinetics of cellular recovery in exponential and plateau growth phase human glioma cells following gamma-irradiation

Dose-rate effects in external beam radiotherapy redux

Recent developments in external beam radiotherapy, both in technical advances and in clinical approaches, have prompted renewed discussions on the potential influence of dose-rate on radio-response in certain treatment scenarios. We consider the multiple factors that influence the dose-rate effect, e.g. radical recombination, the kinetics of sublethal damage repair for tumors and normal tissues, the difference in alpha/beta ratio for early and late reacting tissues, and perform a comprehensive literature review. Based on radiobiological considerations and the linear-quadratic (LQ) model we estimate the influence of overall treatment time on radio-response for specific clinical situations. As the influence of dose-rate applies to both the tumor and normal tissues, in oligo-fractionated treatment using large doses per fraction, the influence of delivery prolongation is likely important, with late reacting normal tissues being generally more sensitive to the dose-rate effect than tumors and early reacting tissues. In conventional fractionated treatment using 1.8-2Gy per fraction and treatment times of 2-1 min, the influence of dose-rate is relatively small. Lastly, the dose-rate effect in external beam radiotherapy is governed by the overall beam-on-time, not by the average linac dose-rate, nor by the instantaneous dose-rate within individual linac pulses which could be as high as 3 x 10(6)MU/min.

Image-guided stereotactic radiosurgery using a specially designed high-dose-rate linac

Stereotactic radiosurgery and image-guided radiotherapy (IGRT) place enhanced demands on treatment delivery machines. In this study, we describe a high-dose-rate output accelerator as a part of our stereotactic IGRT delivery system. The linac is a Siemens Oncor without a flattening filter, and enables dose rates to reach 1000 monitor units (MUs) per minute. Even at this high-dose-rate, the linac dosimetry system remains robust; constancy, linearity, and beam energy remain within 1% for 3 to 1000 MU. Dose profiles for larger field sizes are not flat, but they are radially symmetric and, as such, able to be modeled by a treatment planning system. Target localization is performed via optical guidance utilizing a 3-dimensional (3D) ultrasound probe coupled to an array of 4 infrared light-emitting diodes. These diodes are identified by a fixed infrared camera system that determines diode position and, by extension, all objects imaged in the room coordinate system. This system provides sub-millimeter localization accuracy for cranial applications and better than 1.5 mm for extracranial applications. Because stereotactic IGRT can require significantly longer times for treatment delivery, the advantages of the high-dose-rate design and its direct impact on IGRT are discussed.

The effect of high-dose-rate brachytherapy dwell sequence on cell survival

PURPOSE

During a high-dose-rate (HDR) brachytherapy treatment, as the source steps through different dwell positions, the dose rate at any fixed point within the implant varies, because the distance between the point and the source continually changes. The instantaneous dose rate may vary by a factor of 100 or more, in a complex dwell position sequence. Two different points which receive the same total dose may have received that dose with a very different sequence of dose rates. Any effects due to the complex changes in dose rate, including the sequence of dose delivery, are ignored. We investigated the possible effects of the sequence in which dose is delivered at two different dose rates, representative of dose rates that occur during an HDR treatment.

METHODS AND MATERIALS

The target consisted of a tube containing a 1.0 cm(3) suspension of V-79 Chinese hamster cells. Two fixed source dwell positions near and far from the target, representing high and intermediate dose rates, were considered. The experiments compared the survival of V-79 cells exposed to an irradiation sequence consisting of either an HDR component followed by an intermediate-dose-rate component (H-I arm), or the reverse (I-H arm). In either case, the total dose and the dose ratio were the same, only the order in which the high- or intermediate-dose-rate components of the dose were delivered was changed.

RESULTS

When the intermediate-dose-rate component was given before the HDR component, there was increased survival. All data pairs from three experiments showed greater survival for the I-H arm than the H-I arm by amounts ranging from 4% to 24%. Simple linear-quadratic models such as the Lea-Catchside model, which is invariant to time reversal of irradiation sequence, do not predict these results.

CONCLUSIONS

These results suggest that targets receiving the same total dose of radiation during an HDR implant may not experience the same biological effect. This may be related to induced radioresistance or sublethal damage repair.

Correlation between slowly repairable double-strand breaks and thermal radiosensitization in the human HeLa S3 cell line

The effect of heat on double-strand breaks (dsb) repair was compared with thermal radiosensitization using HeLa S3 cells. Cells were exposed to a combined treatment of X-irradiation followed by heat (44 degrees C, 0.5 h) separated by time intervals up to 8 h. DNA dsb were measured by PFGE and survival by the colony forming assay. In non-heated HeLa S3 cells repair of dsb was biphasic with the majority of breaks being repaired fast with a half-time of 14 min and only a minority were repaired slowly with a half-time of 130 min. Heat applied immediately after irradiation was found to cause an increase in both half-times but mainly to result in an increased fraction of slowly repairable dsb. The latter effect was shown to result from the formation of additional dsb. The number of additional dsb declined when irradiation and heat were separated by an interval at 37 degrees C with a half-time of 120 +/- 30 min. This half-time was similar to the half-time of 100 +/- 20 min found for the loss of thermal radiosensitization studied for the same protocol. Both processes were recently found also to correlate in CHO cells but occurred much faster in rodent cells than in the human HeLa S3 cells used in the current study. These results show that in human cells, unlike previously suggested on the basis of rodent cells, thermal radiosensitization is still a substantial contributor to the killing efficacy of a combined treatment even when irradiation and heat are separated by a time internal of 4 h.

The biological effectiveness of intermittent irradiation as a function of overall treatment time: development of correction factors for linac-based stereotactic radiotherapy

PURPOSE

Continuous irradiation of relatively short duration as administered in gamma-ray stereotactic radiosurgery (SRS) is biologically not equivalent to the more protracted intermittent exposures during accelerator-based radiosurgery with multiple arcs. Accelerator-based SRS and fractionated stereotactic radiotherapy (SRT) is currently performed with a high degree of variability in equipment and techniques resulting in highly variable treatment delivery times. The present work is designed to quantify the effects of radiation delivery times on biological effectiveness. For this, the intermittent radiation delivery schemes, typical for linac-based SRS/SRT, have been simulated in vitro to derive biological correction factors.

METHODS AND MATERIALS

The experiments were carried out using U-87MG human glioma cells in suspension at 37 degrees C irradiated with 6 MV X-rays to clinically relevant doses ranging from 6 to 18 Gy, delivered over total irradiation times from 16 min to 3 h. The resulting cell survival data was used to calculate dose correction factors to compensate for wide variations in dose delivery times.

RESULTS

At each total dose level, cell survival increased with increasing total irradiation time. The increase in survival was more pronounced at higher dose levels. At a total dose of 12 Gy, cell survival increased by a factor of 4.7 when irradiation time was increased from 16 to 112 min. Dose correction factors were calculated to allow biologically equivalent irradiations over the range of exposure times. Cells irradiated with corrected total doses of 11.5 Gy delivered incrementally in 16 min up to 13.3 Gy in 112 min were found to exhibit the same survival within the experimental limits of accuracy.

CONCLUSIONS

For a given total dose, variations in dose delivery time typical of SRS/SRT techniques will result in significant changes in cell survival. In the dose range studied, an isoeffect dose correction factor of 2 to 3 cGy/min was shown to compensate for the change in delivery time for U-87 MG human gloma cells in vitro.

Modelling of continuous low dose rate and accelerated fractionated high dose rate irradiation treatments in a human glioma cell line

The Incomplete-Repair model of Thames in its original and in its two-repair processes forms is used to analyse continuous and fractionated irradiation of a glioma cell line treated in vitro. Furthermore, the Incomplete-Repair model is rederived based on the linear-quadratic model bearing a cubic term. Our results show a potentially enhanced but nonsignificantly improved fit to the data when comparing the cubic to the original form of this model. The repair half-time values showed a decrease, followed by an increase, as a function of small doses (< 1.5Gy) per fraction. This is observed using both the original and the cubic forms of the model. We propose this behaviour to be due to an induced resistance of the repair system followed by a saturation process. Two-repair processes were not seen directly due to the large scatter in the fast and the slow components of repair. Repair half-time values are estimated for various dose per fraction protocols using the original and extended forms of the Incomplete-Repair model. Two-repair processes that are consistent as a function of dose per fraction were not detectable in a glioma cell line treated in vitro.

Radiation survival of two human cervical carcinoma cell lines after multifraction irradiation

PURPOSE

Multifraction irradiation may contribute to radiation therapy treatment failure if selection of radiation resistant subpopulations occurs. We sought to determine whether surviving cells following daily fraction irradiation of two human cervical squamous cell carcinoma lines would express different radiation survival characteristics compared to the unirradiated parent.

METHODS AND MATERIALS

A late-passage line (HTB35) and an early-passage line (RECA) received daily 2 Gy x-irradiation. Two new stable HTB35 cell lines were established after 40 and 60 Gy (HTB35-40 and HTB35-60). A single line was established from RECA after 30 Gy (RECA-30). High dose rate (74 cGy/min) acute radiation survival curves were prepared from the three new lines and the unirradiated parents. Potentially lethal damage repair (PLDR) and sublethal damage repair (SLDR) responses were detailed for HTB35, HTB35-40 and HTB35-60. Low dose rate (1.27 cGy/min) survival was measured for HTB35 and HTB35-60. Clones were derived from HTB35 and from HTB35-60 and the surviving fraction at 2 Gy (SF2) values were determined.

RESULTS

The two parent lines (HTB35 and RECA) differed in acute radiation survival. The surviving lines following multifraction irradiation (HTB35-40, HTB35-60, and RECA-30) showed no change in acute radiation response compared to the appropriate parent. HTB35-40 and HTB35-60 were repair proficient, demonstrating similar PLDR and SLDR recovery ratios as the parent. Likewise, acute, low dose rate survival of HTB35 and HTB35-60 was similar. Nine clones derived from HTB35 lacked a consistent difference in SF2 compared to the original culture. A single clone of seven derived from HTB35-60 was consistently radiation resistant (SF2 = 0.81 +/- 0.06) compared to the original culture (SF2 = 0.50 +/- 0.09).

CONCLUSION

No evidence was obtained that cell lines generated following multiple daily fractions of x-irradiation in vitro possessed acute radiation survival or repair characteristics that were different from the unirradiated parent.

Improving cancer radiotherapy with 2-deoxy-D-glucose: phase I/II clinical trials on human cerebral gliomas

PURPOSE

Evaluation of tolerance, toxicity, and feasibility of combining large fraction (5 Gy) radiotherapy with 2-deoxy-D-glucose (2DG), an inhibitor of glucose transport and glycolysis, which has been shown to differentially inhibit repair of radiation damage in cancer cells.

METHODS AND MATERIALS

Twenty patients with supratentorial glioma (Grade 3/4), following surgery were treated with four weekly fractions of oral 2DG (200 mg/kg body weight) followed by whole brain irradiation (5 Gy). Two weeks later, supplement focal radiation to the tumor (14 Gy/7 fractions) was given. Routine clinical evaluation, x-ray computerized tomography (CT), and magnetic resonance (MR) imaging were carried out to study the acute and late radiation effects.

RESULTS

All the 20 patients completed the treatment without any interruption. The vital parameters were within normal limits during the treatment. None reported headache during the treatment. Mild to moderate nausea and vomiting were observed during the days of combined therapy (2DG + RT) in 10 patients. No significant deterioration of the neurological status was observed during the treatment period. Seven patients were alive at 63, 43, 36, 28, 27, 19, and 18 months of follow-up. In these patients, the clinical and MR imaging studies did not reveal any late radiation effects.

CONCLUSIONS

Feasibility of administering the treatment (2DG + 5 Gy) is demonstrated by the excellent tolerance observed in all 20 patients. Further, the clinical and MR studies also show the absence of any brain parenchymal damage.