The effect of very low radiation doses on the human bladder carcinoma cell line RT112

Exploring low-dose radiotherapy to overcome radio-immunotherapy resistance

Immune checkpoint inhibitors (ICIs) have revolutionized the current treatment landscape for cancer, yet the response rates of ICIs remain unmet. Synergistic with immunotherapy, low-dose radiotherapy (LDRT) has been demonstrated to activate anti-tumor immunity - a transition from traditional radiation therapy geared toward local radical treatment to a type of immunological adjuvant. As such, studies utilizing LDRT to enhance the efficacy of immunotherapy have been increasing preclinically and clinically. This paper reviews the recent strategies of using LDRT to overcome the resistance of ICIs, as well as providing potential opportunities in cancer treatment. Despite the potential of LDRT in immunotherapy is recognized, the mechanisms behind this form of treatment remain largely elusive. Thus, we reviewed history, mechanisms and challenges associated with this form of treatment, as well as different modes of its application, to provide relatively accurate practice standards for LDRT as a sensitizing treatment when combined with immunotherapy or radio-immunotherapy.

Low-Dose Radiation Therapy (LDRT) against Cancer and Inflammatory or Degenerative Diseases: Three Parallel Stories with a Common Molecular Mechanism Involving the Nucleoshuttling of the ATM Protein?

Very early after their discovery, X-rays were used in multiple medical applications, such as treatments against cancer, inflammation and pain. Because of technological constraints, such applications involved X-ray doses lower than 1 Gy per session. Progressively, notably in oncology, the dose per session increased. However, the approach of delivering less than 1 Gy per session, now called low-dose radiation therapy (LDRT), was preserved and is still applied in very specific cases. More recently, LDRT has also been applied in some trials to protect against lung inflammation after COVID-19 infection or to treat degenerative syndromes such as Alzheimer's disease. LDRT illustrates well the discontinuity of the dose-response curve and the counterintuitive observation that a low dose may produce a biological effect higher than a certain higher dose. Even if further investigations are needed to document and optimize LDRT, the apparent paradox of some radiobiological effects specific to low dose may be explained by the same mechanistic model based on the radiation-induced nucleoshuttling of the ATM kinase, a protein involved in various stress response pathways.

Pulsed reduced dose-rate radiotherapy for previously irradiated tumors in the brain and spine

Background:

Patients with unresectable locoregional cancer recurrences have limited management options. Reirradiation increases the risk of toxicity, particularly when perilesional dose-volume constraints are exceeded. We present and discuss two cases of previously irradiated tumors in the central nervous system (CNS) that was reirradiated using the pulsed reduced dose-rate radiotherapy (PRDR) technique.

Case Description:

A 58-year-old female with a history of metastatic small cell lung cancer to the brain status post multiple rounds of radiation and chemotherapy presented with increasing weakness in her right arm and leg. Magnetic resonance imaging (MRI) revealed a growly peripherally enhancing 1.2 cm mass in the left precentral gyrus that had previously received prophylactic cranial irradiation and stereotactic radiosurgery. The patient was re-irradiated with 35 Gy in 100 fractions over 3 weeks, using PRDR with improved motor function at 3-month follow-up. A 41-year-old male with recurrent glioblastoma of the thoracic spinal cord presented with worsening neurological symptoms, including inability to ambulate due to bilateral leg weakness, causing wheelchair use. MRI thoracic spine revealed a recurrent thoracic lesion 2.2 × 1 × 0.8 cm. In addition to chronic chemotherapy, the patient was retreated palliatively in the same area at 50 Gy in 250 fractions, over 6 weeks, using PRDR. The treated lesion was stable on follow-up imaging, and the patient was able to walk with the assistance of a walker.

Conclusion:

In our two cases, PRDR proved effective in the treatment of recurrent malignant CNS tumors that were previously irradiated. Prospective studies are needed to delineate the efficacy and toxicity of PRDR.

Differential miRNA expression profiling reveals miR-205-3p to be a potential radiosensitizer for low- dose ionizing radiation in DLD-1 cells

Enhanced radiosensitivity at low doses of ionizing radiation (IR) (0.2 to 0.6 Gy) has been reported in several cell lines. This phenomenon, known as low doses hyper-radiosensitivity (LDHRS), appears as an opportunity to decrease toxicity of radiotherapy and to enhance the effects of chemotherapy. However, the effect of low single doses IR on cell death is subtle and the mechanism underlying LDHRS has not been clearly explained, limiting the utility of LDHRS for clinical applications. To understand the mechanisms responsible for cell death induced by low-dose IR, LDHRS was evaluated in DLD-1 human colorectal cancer cells and the expression of 80 microRNAs (miRNAs) was assessed by qPCR array. Our results show that DLD-1 cells display an early DNA damage response and apoptotic cell death when exposed to 0.6 Gy. miRNA expression profiling identified 3 over-expressed (miR-205-3p, miR-1 and miR-133b) and 2 down-regulated miRNAs (miR-122-5p, and miR-134-5p) upon exposure to 0.6 Gy. This miRNA profile differed from the one in cells exposed to high-dose IR (12 Gy), supporting a distinct low-dose radiation-induced cell death mechanism. Expression of a mimetic miR-205-3p, the most overexpressed miRNA in cells exposed to 0.6 Gy, induced apoptotic cell death and, more importantly, increased LDHRS in DLD-1 cells. Thus, we propose miR-205-3p as a potential radiosensitizer to low-dose IR.

Comparison of two radiation techniques for the breast boost in patients undergoing neoadjuvant treatment for breast cancer

OBJECTIVE

After breast conservative surgery (BCS) and whole-breast radiotherapy (WBRT), the use of boost irradiation is recommended especially in patients at high risk. However, the standard technique and the definition of the boost volume have not been well defined.

METHODS

We retrospectively compared an anticipated pre-operative photon boost on the tumour, administered with low-dose fractionated radiotherapy, and neoadjuvant chemotherapy with two different sequential boost techniques, administered after BCS and standard adjuvant WBRT: (1) a standard photon beam (2) and an electron beam technique on the tumour bed of the same patients. The plans were analyzed for the dosimetric coverage of the CT-delineated irradiated volume. The minimal dose received by 95% of the target volume (D95), the minimal dose received by 90% of the target volume (D90) and geographic misses were evaluated.

RESULTS

15 patients were evaluated. The sequential photon and electron boost techniques resulted in inferior target volume coverage compared with the anticipated boost technique, with a median D95 of 96.3% (range 94.7-99.6%) and 0.8% (range 0-30%) and a median D90 of 99.1% (range 90.2-100%) and 54.7% (range 0-84.8%), respectively. We observed a geographic miss in 26.6% of sequential electron plans. The results of the anticipated boost technique were better: 99.4% (range 96.5-100%) and 97.1% (range 86.2-99%) for median D90 and median D95, respectively, and no geographic miss was observed. We observed a dose reduction to the heart, with left-sided breast irradiation, using the anticipated pre-operative boost technique, when analyzed for all dose-volume parameters. When compared with the sequential electron plans, the pre-operative photon technique showed a higher median ipsilateral lung Dmax.

CONCLUSION

Our data show that an anticipated pre-operative photon boost results in a better coverage with respect to the standard sequential boost while also saving the organs at risk and consequently fewer side effects.

ADVANCES IN KNOWLEDGE

This is the first dosimetric study that evaluated the association between an anticipated boost and neoadjuvant chemotherapy treatment.

A concurrent ultra-fractionated radiation therapy and temozolomide treatment: A promising therapy for newly diagnosed, inoperable glioblastoma

We report on a phase II clinical trial to determine the effect of a concurrent ultra-fractionated radiotherapy and temozolomide treatment in inoperable glioblastoma patients. A phase II study opened; patients over 18 years of age who were able to give informed consent and had histologically proven, newly diagnosed inoperable diagnosed and supratentorial glioblastoma were eligible. Three doses of 0.75 Gy spaced apart by at least 4 hr were delivered daily, 5 days a week for six consecutive weeks for a total of 67.5 Gy. Chemotherapy was administered during the same period, which consisted of temozolomide given at a dose of 75 mg/m(2) for 7 days a week. After a 4-week break, chemotherapy was resumed for up to six cycles of adjuvant temozolomide treatment, given every 28 days, according to the standard 5-day regimen. Tolerance and toxicity were the primary endpoints; survival and progression-free survival were the secondary endpoints. In total, 40 patients were enrolled in this study, 29 men and 11 women. The median age was 58 years, and the median Karnofsky performance status was 80. The concomitant ultra-fractionated radiotherapy and temozolomide treatment was well tolerated. Complete responses were seen in four patients, and partial responses were reported in seven patients. The median survival from the initial diagnosis was 16 months. Several long-term survivors were noted. Concurrent ultra-fractionated radiation therapy and temozolomide treatment are well accepted by the patients. The results showed encouraging survival rates for these unfavorable patients.

Low-dose fractionated radiotherapy and concomitant chemotherapy for recurrent or progressive glioblastoma: final report of a pilot study

BACKGROUND

Evaluated in this study were the feasibility and the efficacy of concurrent low dose fractionated radiotherapy (LD-FRT) and chemotherapy as palliative treatment for recurrent/progressive glioblastoma multiforme (GBM).

PATIENTS AND METHODS

Eligible patients had recurrent or progressive GBM, Karnofsky performance status ≥ 70, prior surgery, and standard radiochemotherapy treatment. Recurrence/progression disease during temozolomide (TMZ) received cisplatin (CDDP; 30 mg/m(2) on days 1, 8, 15), fotemustine (FTM; 40 mg/m(2) on days 2, 9, 16), and concurrent LD-FRT (0.3 Gy twice daily); recurrence/progression after 4 months from the end of adjuvant TMZ were treated by TMZ (150/200 mg/m(2) on days 1-5) concomitant with LD-FRT (0.4 Gy twice daily). Primary endpoints were safety and toxicity.

RESULTS

A total of 32 patients were enrolled. Hematologic toxicity G1-2 was observed in 18.7 % of patients and G3-4 in 9.4 %. One patient (3.1 %) had complete response, 3 (9.4 %) had partial response, 8 (25 %) had stable disease for at least 8 weeks, while 20 patients (62.5 %) experienced progressive disease. The clinical benefit was 37.5 %. Median progression-free survival (PFS) and overall survival (OS) were 5 and 8 months, respectively. Survival rate at 12 months was of 27.8 %.

CONCLUSION

LD-FRT and chemotherapy for recurrent/progressive GBM have a good toxicity profile and clinical outcomes, even though further investigation of this novel palliative treatment approach is warranted.

Exposure to low dose ionising radiation: molecular and clinical consequences

This review article provides a comprehensive overview of the experimental data detailing the incidence, mechanism and significance of low dose hyper-radiosensitivity (HRS). Important discoveries gained from past and present studies are mapped and highlighted to illustrate the pathway to our current understanding of HRS and the impact of HRS on the cellular response to radiation in mammalian cells. Particular attention is paid to the balance of evidence suggesting a role for DNA repair processes in the response, evidence suggesting a role for the cell cycle checkpoint processes, and evidence investigating the clinical implications/relevance of the effect.

The importance of bystander effects in radiation therapy in melanoma skin-cancer cells and umbilical-cord stromal stem cells

PURPOSE

To examine direct and bystander radiation-induced effects in normal umbilical-cord stromal stem cell (HCSSC) lines and in human cancer cells.

MATERIALS AND METHODS

The UCSSC lines used in this study were obtained in our laboratory. Two cell lines (UCSSC 35 and UCSSC 37) and two human melanoma skin-cancer cells (A375 and G361) were exposed to ionizing radiation to measure acute radiation-dosage cell-survival curves and radiation-induced bystander cell-death response. Normal cells, although extremely sensitive to ionizing radiation, were resistant to the bystander effect whilst tumor cells were sensitive to irradiated cell-conditioned media, showing a dose-response relationship that became saturated at relatively low doses. We applied a biophysical model to describe bystander cell-death through the binding of a ligand to the cells. This model allowed us to calculate the maximum cell death (χ(max)) produced by the bystander effect together with its association constant (K(By)) in terms of dose equivalence (Gy). The values obtained for K(By) in A375 and G361 cells were 0.23 and 0.29 Gy, respectively.

CONCLUSION

Our findings help to understand how anticancer therapy could have an additional decisive effect in that the response of sub-lethally hit tumor cells to damage might be required for therapy to be successful because the survival of cells communicating with irradiated cells is reduced.

Low-dose fractionated radiotherapy and concomitant chemotherapy in glioblastoma multiforme with poor prognosis: a feasibility study

We explored the feasibility of concurrent palliative chemotherapy and low-dose fractionated radiotherapy (LD-FRT) in glioblastoma multiforme (GBM). Patients with recurrent/progressive GBM at least 3 months after the end of primary radiotherapy received 0.3 Gy twice daily with cisplatin and fotemustine if progressing on temozolomide, or 0.4 Gy twice daily with temozolomide if recurrent 4-6 months later (retreatment group). Newly diagnosed GBM with gross residual mass received 30 Gy with concomitant and adjuvant temozolomide and 0.4 Gy twice daily from the second adjuvant cycle (naive group) for 2-4 cycles. Twenty-six patients were enrolled. In the retreatment group (n = 17; median LD-FRT total dose 7.2 Gy [range 2.4-11.6]), grade 3 or 4 hematological toxicity was observed in 5.9% of patients. Median follow-up time was 20 months (range 4-35). Median progression-free survival (PFS) and overall survival (OS) from the time of recurrence or progression were 4 and 8 months, respectively (OS at 6 months, 69%; at 12 months, 16.7%). In the naive group (n = 9; median LD-FRT total dose 8 Gy [range 3.2-16]), grade 3 or 4 hematological toxicity was observed in 11.1% of patients. Median follow-up time was 17 months (range 8-20)-median PFS was 9 months, with PFS at 6 months and at 1 year of 66.7% and 26.7%, respectively; and median OS was 12 months, with OS at 6 months and at 1 year of 77.8% and 34.6%, respectively. LD-FRT with concurrent chemotherapy was well tolerated.

Prolonged survival for patients with newly diagnosed, inoperable glioblastoma with 3-times daily ultrafractionated radiation therapy

Ultrafractionation of radiation therapy is a novel regimen consisting of irradiating tumors several times daily, delivering low doses (<0.75 Gy) at which hyperradiosensitivity occurs. We recently demonstrated the high efficiency of ultrafractionated radiotherapy (RT) on glioma xenografts and report here on a phase II clinical trial to determine the safety, tolerability, and efficacy of an ultrafractionation regimen in patients with newly and inoperable glioblastoma (GBM). Thirty-one patients with histologically proven, newly diagnosed, and unresectable supratentorial GBM (WHO grade IV) were enrolled. Three daily doses of 0.75 Gy were delivered at least 4 hours apart, 5 days per week over 6-7 consecutive weeks (90 fractions for a total of 67.5 Gy). Conformal irradiation included the tumor bulk with a margin of 2.5 cm. The primary end points were safety, toxicity, and tolerability, and the secondary end points were overall survival (OS) and progression-free survival (PFS). Multivariate analysis was used to compare the OS and PFS with the EORTC-NCIC trial 26981-22981/CE.3 of RT alone vs radiation therapy and temozolomide (TMZ). The ultrafractionation radiation regimen was safe and well tolerated. No acute Grade III and/or IV CNS toxicity was observed. Median PFS and OS from initial diagnosis were 5.1 and 9.5 months, respectively. When comparing with the EORTC/NCIC trial, in both PFS and OS multivariate analysis, ultrafractionation showed superiority over RT alone, but not over RT and TMZ. The ultrafractionation regimen is safe and may prolong the survival of patients with GBM. Further investigation is warranted and a trial associating ultra-fractionation and TMZ is ongoing.

Thresholds and transitions for activation of cellular radioprotective mechanisms – correlations between HRS/IRR and the ‘inverse’ dose-rate effect

PURPOSE

In biophysical modeling for several instances of radiation-induced radioprotection, i.e., adaptive response (AR), hyper-radiosensitivity and induced radioresistance (HRS/IRR), and the inverse dose-rate effect (IDRE), empirical fits are premised for the thresholds and transitions of the radioprotection. We provide realistic model formulations for the observed behaviors, which we apply to both HRS/IRR and IDRE.

MATERIALS AND METHODS

We use homeostatic balance equations, including cell biophysical endogenic adjustments (originating from within the cell), providing a radiation-induced 'trigger' or continuous thresholds and transitions.

RESULTS

A 'trigger' threshold requires an instantaneous, step function. Current HRS/IRR transition model does not provide 'triggered' threshold but continuous progression from high sensitivity to reduced radiosensitivity, although the investigators premise 'trigger' behavior. IDRE data suggest 'triggered' thresholds at discrete dose rates. It appears that HRS/IRR and IDRE at low dose and dose rate intentionally provide protection against potentially carcinogenic mutations.

CONCLUSIONS

The homeostatic formulation shows, when applied to the IRR using a dose and dose rate dependent Linear-Quadratic model (LQ2), that the IRR protection is 'triggered' at a discrete low dose and induced by a transitory increase in the damage repair rate constant in the LQ2 model of the single event, linear response, radiation damage. Since both IDRE and IRR have 'triggered' thresholds and as a result of increased endogenic damage recognition, increased mobilization of repair resources, activation of cell cycle arrest and/or increased repair rate, we premise that both may be from the same endogenic radioprotection biochemical mechanisms.

Combined gamma‐irradiation and subsequent cisplatin treatment in human squamous carcinoma cell lines sensitive and resistant to cisplatin

PURPOSE

To investigate the effects of combined radiation and subsequent cisplatin treatment on the human squamous carcinoma cell line SCC-25 and its cisplatin-resistant derivative SCC-25/CP.

MATERIALS AND METHODS

SCC-25 and SCC-25/CP cells were treated with various gamma-ray doses (5 cGy-7 Gy) followed 60 min later by cisplatin treatment and subsequently assayed for survival using a conventional colony assay. For SCC-25, the subsequent cisplatin treatment was 0.1, 1, 10 and 20 microM for 1 h. For the more cisplatin-resistant SCC-25/CP cells, the subsequent cisplatin treatment was 10 and 50 microM for 1 h.

RESULTS

The cisplatin-resistant SCC-25/CP cells were not cross-resistant to gamma-irradiation. Subsequent treatment with an LD50 concentration of cisplatin (10 and 50 microM for SCC-25 and SCC-25/CP, respectively) resulted in radiosensitization for SCC-25/CP but not for SCC-25 cells. Gamma-irradiation of SCC-25/CP cells followed by treatment with 10 and 50 microM cisplatin for 1 h resulted in radiation survival curves displaying a significant low-dose hypersensitive region followed by increased radioresistance at higher doses. A total of 10 microM cisplatin resulted in radiosensitization confined to the low-dose region (0.05 and 0.25 Gy), whereas the higher cisplatin treatment of 50 microM resulted in the appearance of a hypersensitive region together with a reduction of the increased radioresistance region. In contrast, cisplatin treatment (0.1, 1, 10 and 20 microM for 1 h) of SCC-25 cells had no significant effect on survival following 2.5 or 7.0 Gy and actually resulted in an increased low-dose radiation survival (0.05, 0.25 and 1 Gy) when survival was corrected for cisplatin treatment (p<0.01 for all cisplatin concentrations tested).

CONCLUSIONS

The significant radiosensitization for SCC-25/CP given subsequent treatment with 50 microM cisplatin indicates cisplatin can inhibit the increased radioresistance response in SCC-25/CP cells. In contrast, the subsequent cisplatin treatment of SCC-25 cells can enhance their survival following low radiation doses.

The impact of the bystander effect on the low-dose hypersensitivity phenomenon

The aim of this study was to investigate the possible relationship between the bystander effect and the low-dose hypersensitivity/increased radio-resistance phenomenon in BJ fibroblast cells taking as response criteria different end points of radiation damage such as cell survival, chromosomal damage (as detected by using micronucleus assay) and double strand breaks (DSBs) of the DNA. Although gamma-H2AX foci were observed in confluent bystander BJ cells, our data suggest that X-irradiation does not lead to a significant rate of DSBs in bystander cells. Thus, neither bystander effect induced unstable chromosomal aberrations nor bystander effect induced DSBs are sufficiently pronounced to explain the apparent relationship between bystander effect and low-dose hypersensitivity. The experiments described here suggest that the hyper-radiosensitivity phenomenon might be related to bystander factor induced cell inactivation in the low-dose region (lower than 1 Gy).

Ultrafractionation in A7 human malignant glioma in nude mice

PURPOSE

Low-dose hyperradiosensitivity (HRS) has been demonstrated in numerous cell lines in vitro, including a number of radioresistant human malignant glioma cell lines such as A7. The aim of our experiment was to show whether HRS can be exploited by using ultrafractionated irradiation (UF) to improve local control of A7 tumours growing in nude mice. Extrapolation of the in vitro results predict a 3.7-fold difference in the efficacy of UF compared with conventional fractionation (CF).

MATERIAL AND METHODS

Subcutaneuously growing A7 tumours were irradiated either with UF (126 fractions in 6 weeks, 0.4 Gy per fraction) or CF (30 fractions in 6 weeks, 1.68 Gy per fraction). The total dose was 50.4 Gy in both experimental arms. Fractionated irradiations were given under ambient conditions and followed by graded top-up doses under clamp hypoxia. Endpoints were tumour growth delay and local tumour control 180 days after the end of treatment.

RESULTS

UF resulted in a significant decrease of tumour growth delay and in a significant increase of the top-up TCD(50) compared with CF (40.0 Gy [95% CI 29; 61 Gy] versus 28.3 Gy [24; 35 Gy], p=0.047).

CONCLUSIONS

Despite a pronounced HRS phenomenon in vitro, UF was significantly less effective than CF in A7 human malignant glioma in nude mice. These results neither disprove the existence of HRS nor do they exclude a possible clinical value of UF. The findings rather indicate that simplistic extrapolation from results obtained after single-dose exposure or few fractions in vitro is not sufficient to predict outcome of UF in vivo and that comprehensive evaluation of novel treatment options in animal models continues to be an essential requirement for clinical translation.

Effects of exposure to low-dose-rate (60)co gamma rays on human tumor cells in vitro

Cells of three asynchronously growing human tumor cell lines, PC3 (human prostate carcinoma), T98G and A7 (human glioblastomas), which have been shown previously to demonstrate low-dose hyper-radiosensitivity to low acute single doses, were irradiated with (60)Co gamma rays at low dose rates (2 cGy-1 Gy h(-1)). Instead of a dose-rate sparing response, these cell lines demonstrated an inverse dose-rate effect on cell survival at dose rates below 1 Gy h(-1), whereby a decrease in dose rate resulted in an increase in cell killing per unit dose. A hyper-radiosensitivity-negative cell line, U373MG, did not demonstrate an inverse dose-rate effect. Analysis of the cell cycle indicated that this inverse dose-rate effect was not due to accumulation of cells in G(2)/M phase or to other cell cycle perturbations. T98G cells in reversible G(1)-phase arrest also showed an inverse dose-rate effect at dose rates below 30 cGy h(-1) but a sparing effect as the dose rate was reduced from 60 to 30 cGy h(-1). We conclude that this inverse dose-rate effect in continuous exposures reflects the hyper-radiosensitivity seen in the same cell lines in response to very small acute single doses.

Low-dose radiation hypersensitivity in human tumor cell lines: effects of cell-cell contact and nutritional deprivation

The hyper-radiosensitivity at low doses recently observed in vitro in a number of cell lines is thought to have important implications for improving tumor radiotherapy. However, cell-cell contact and the cellular environment influence cellular radiosensitivity at higher doses, and they may alter hyper-radiosensitivity in vivo. To confirm this supposition, we investigated the effects of cell density, multiplicity and nutritional deprivation on low-dose hypersensitivity in vitro. Cell survival in the low-dose range (3 cGy to 2 Gy) was studied in cells of two human glioma (BMG-1 and U-87) and two human oral squamous carcinoma (PECA-4451 and PECA-4197) lines using a conventional macrocolony assay. The effects of cell density, multiplicity and nutritional deprivation on hyper-radiosensitivity/induced radioresistance were studied in cells of the BMG-1 cell line, which showed prominent hypersensitivity and induced radioresistance. The induction of growth inhibition, cell cycle delay, micronuclei and apoptosis was also studied at the hyper-radiosensitivity-inducing low doses. Hyper-radiosensitivity/induced radioresistance was evident in the cells of all four cell lines to varying extents, with maximum sensitivity at 10-30 cGy, followed by an increase in survival up to 50 cGy-1 Gy. Both the glioma cell lines had more prominent hyper-radiosensitivity than the two squamous carcinoma cell lines. Low doses inducing maximum hyper-radiosensitivity did not cause significant growth inhibition, micronucleation or apoptosis in BMG-1 cells, but a transient G(1)/S-phase block was evident. Irradiating and incubating BMG-1 cells at high density for 0 or 4 h before plating, as well as irradiating cells as microcolonies, reduced hyper-radiosensitivity significantly, indicating the role of cell-cell contact-mediated processes. Liquid holding of BMG-1 cells in HBSS + 1% serum during and after irradiation for 4 h significantly reduced hyper-radiosensitivity, suggesting that hyper-radiosensitivity may be due partly to active damage fixation processes at low doses. Therefore, our findings suggest that the damage-induced signaling mechanisms influenced by (or mediated through) cell-cell contact or the cellular environment, as well as the lesion fixation processes, play an important role in hyper-radiosensitivity. Further studies are required to determine the exact nature of the damage that triggers these responses as well as for evaluating the potential of low-dose therapy.

Individualizing cancer treatment: biological optimization models in treatment planning and delivery

PURPOSE

During the last 30 years radiation therapy has developed from classical rectangular beams via conformation therapy with largely uniform dose delivery, but irregular field shapes, to fully intensity modulated dose delivery where the total dose distribution in the tumor can be fully controlled in three dimensions. This last step has been developed during the last 15-20 years and has opened up the possibilities for truly optimized radiation therapy.

METHODS AND MATERIALS

Today it is not only possible to produce almost any desired dose distribution in the tumor volume. It is also possible to deliver the dose distribution, which has the highest probability to cure the patient without inducing severe complications in normal tissues. To fully exploit the advantages of intensity-modulated radiation therapy, quality of life or radiobiologic objectives have to be used, preferably combined with predictive assay of radiation sensitivity.

RESULTS

This article will briefly discuss the biologic objective functions and the associated advantages in the treatment outcome using new approaches such as consideration of stochastic variations in sensitivity and optimization of the angle of incidence and fractionation schedule with intensity-modulated beams. Finally, different possibilities for realizing general three-dimensional intensity-modulated dose delivery will be discussed.

CONCLUSIONS

Once accurate genetically and/or cell survival based predictive assays become available, radiation therapy will become an exact science allowing truly individual optimization considering also the panorama of side-effects that the patient is willing to accept.

Low-dose hypersensitivity: current status and possible mechanisms

PURPOSE

To retain cell viability, mammalian cells can increase damage repair in response to excessive radiation-induced injury. The adaptive response to small radiation doses is an example of this induced resistance and has been studied for many years, particularly in human lymphocytes. This review focuses on another manifestation of actively increased resistance that is of potential interest for developing improved radiotherapy, specifically the phenomenon in which cells die from excessive sensitivity to small single doses of ionizing radiation but remain more resistant (per unit dose) to larger single doses. In this paper, we propose possible mechanisms to explain this phenomenon based on our data accumulated over the last decade and a review of the literature.

CONCLUSION

Typically, most cell lines exhibit hyper-radiosensitivity (HRS) to very low radiation doses (<10 cGy) that is not predicted by back-extrapolating the cell survival response from higher doses. As the dose is increased above about 30 cGy, there is increased radioresistance (IRR) until at doses beyond about 1 Gy, radioresistance is maximal, and the cell survival follows the usual downward-bending curve with increasing dose. The precise operational and activational mechanism of the process is still unclear, but we propose two hypotheses. The greater amount of injury produced by larger doses either (1) is above a putative damage-sensing threshold for triggering faster or more efficient DNA repair or (2) causes changes in DNA structure or organization that facilitates constitutive repair. In both scenarios, this enhanced repair ability is decreased again on a similar time scale to the rate of removal of DNA damage.

A purpose-built iodine-125 irradiation plaque for low dose rate low energy irradiation of cell lines in vitro

The phenomenon of hyper-radiosensitivity (HRS) to very low acute single doses of radiation has been demonstrated in several cell lines in vitro and in vivo, and has been studied in theory and in practice. The theory suggests a similar hypersensitivity when cells are continuously exposed to radiation at very low dose rates. These low dose rates are used when radioactive seed (iodine-125 or palladium-103) implants of the prostate are used as an alternative to surgery or external beam radiotherapy. To investigate the radiobiology of hypersensitivity of this type on various cell lines in vitro, an iodine-125 seed irradiator has been designed and built for safe use in the Gray Laboratory. In practice, the calculated dose rate has been used for consistency. Discrepancies between calculated and measured dose rates are discussed.

Increased repair and cell survival in cells treated with DIR1 antisense oligonucleotides: implications for induced radioresistance

PURPOSE

To determine whether repression of a recently isolated, X-ray-responsive gene, DIR1, using antisense oligonucleotides could affect clonogenic cell survival and repair of DNA strand breaks and have a possible role in the mechanism underlying the phenomenon of 'induced radioresistance' (IRR).

MATERIALS AND METHODS

Three cell lines, V79, RT112 and UM-UC-3, which are known to exhibit low-dose hypersensitivity (HRS) and induced radioresistance (IRR), and the radiosensitive cell line ATBIVA, were transfected with antisense oligonucleotides directed towards the DIR1 gene. Scrambled oligonucleotides were used as controls. DNA single-strand break (ssb) repair, using the alkaline comet assay, and cell survival using a standard clonogenic assay was measured after exposure to X-rays.

RESULTS

Following treatment with 4Gy X-rays, the V79, RT112 and UM-UC-3 cell lines all exhibited significantly increased rates of ssb repair after transfection with DIR1 antisense oligonucleotides compared with cells transfected with scrambled oligonucleotides. They also demonstrated significantly enhanced survival after exposure to 2 Gy X-rays; the radiosensitive ATBIVA cells did not show these effects.

CONCLUSIONS

Repression of the DIR1 gene product leads to an increase in the rate of repair and cell survival in three radioresistant cells lines but not in the radiosensitive ATBIVA cell line. Because DIR1 is repressed by X-rays in the dose range where IRR is observed, it may represent a candidate gene involved in the IRR phenomenon.

Inducible repair and intrinsic radiosensitivity: a complex but predictable relationship?

Two groups have proposed a simple linear relationship between inducible radioresistance in a variety of mammalian cells and their intrinsic radiosensitivity at 2 Gy (Lambin et al., Int.J. Radiat. Biol. 69, 279-290, 1996; Alsbeih and Raaphorst, unpublished results, 1997). The inducible repair response (IRR) is quantified as a ratio, alpha(S)/alpha(R), i.e. the slope in the hypersensitive low-dose region, alpha(S), relative to the alpha(R) term of the classical linear-quadratic formula. These proposals imply that the intrinsic radiosensitivity at clinically relevant doses is directly linked to the cell's ability to mount an adaptive response as a result of exposure to very low doses of radiation. We have re-examined this correlation and found that the more extensive data set now available in the literature does not support the contention of a simple linear relationship. The two parameters are correlated, but by a much more complex relationship. A more logical fit is obtained with a log-linear equation. A series of log-linear curves are needed to describe the correlation between IRR and SF2, because of the spectrum of alpha/beta ratios among the cell lines and hence the confounding effect of the beta term at a dose of 2 Gy. The degree of repair competence before irradiation starts could also be a major factor in the apparent magnitude of the amount of repair induced. There appears to be a systematic difference in the data sets from different series of cell lines that have been obtained using flow cytometry techniques in the laboratory in Vancouver and using dynamic microscope imaging at the Gray Laboratory. We suggest that the use of a brief exposure to a laser beam in flow cytometry before the cells are irradiated might itself partially induce a stress response and change the DNA repair capacity of the cells. The clinical consequences of the relationship for predicting the benefits of altered fractionation schedules are discussed. [ru5]

Investigation of hypersensitivity to fractionated low-dose radiation exposure

PURPOSE

Hypersensitivity to cell killing of exponentially growing cells exposed to X-rays and gamma rays has been reported for doses below about 0.5 Gy. The reported results have been interpreted to suggest that a dose of 0.5 Gy or less is not sufficient to trigger an inducible repair mechanism. The purpose of this study was to examine this suggested hypersensitivity after multiple low doses (0.3 Gy) of gamma rays where a) the effect would be expected to be significantly magnified, and b) the effect might be of clinical relevance.

METHODS AND MATERIALS

C3H 10T1/2 mouse embryo cells were grown to confluence in culture vessels. While in plateau phase of growth, cells were exposed to 6 Gy of gamma rays, delivered in either 6 Gy, 3 Gy, 2 Gy, 1 Gy, or 0.3 Gy well-separated fractions. Corresponding experiments were performed with V-79 and C3H 10T1/2 cells in exponential growth. Cells were replated at low density and assayed for clonogenicity.

RESULTS

The results of this study were not inconsistent with some hypersensitivity at low doses, in that 20 fractions each of 0.3 Gy produced a slightly lower (though nonsignificant) surviving fraction compared with the same dose given in 2-Gy fractions. However, the results of the 20 x 0.3 Gy exposures also agreed well with the standard linear-quadratic (LQ) model predictions based on high dose per fraction (1-6 Gy) data. In addition, effects of cellular redistribution were seen which were explained quantitatively with an extended version of the LQ model.

CONCLUSIONS

These experiments were specifically designed to magnify and probe possible clinical implications of proposed "low-dose hypersensitivity" effects, in which significant deviations at low doses from the LQ model have been suggested. In fact, the results at low doses per fraction were consistent with LQ predictions based on higher dose per fraction data. This finding is in agreement with the well-documented utility of the LQ approach in estimating isoeffect doses for alternative fractionation schemes, and for brachytherapy.

Adaptive response and induced resistance

Cellular stress responses are upregulated following exposure to radiation and other DNA-damaging agents. Therefore radiation response can be dose dependent so that small acute exposures (and possibly exposures at very low dose rates?) are more lethal per unit dose than larger exposures above a threshold (typically 10-40 cGy) where induced radioprotection is triggered. We have termed these interlinked phenomena low-dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. HRS/IRR has been recorded in cell-survival studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal normal-tissue models in vivo. There is indirect evidence that cell survival-related HRS/IRR in response to single doses is a manifestation of the same underlying mechanism that determines the well-known adaptive response in the two-dose case and that it can be triggered by high- and low-LET radiations as well as a variety of other stress-inducing agents such as hydrogen peroxide and chemotherapeutic agents. Little is currently known about the precise nature of this underlying mechanism, but there is evidence that it operates by increasing the amount and rate of DNA repair, rather than by indirect mechanisms such as modulation of cell-cycle progression or apoptosis. Changed expression of some genes, only in response to low and not high doses, may occur within a few hours of irradiation and this would be rapid enough to explain the phenomenon of induced radioresistance although its specific molecular components have yet to be identified. Net cancer risk is a balance between cell transformation and cell kill. Our known low-dose cell-survival responses suggest that lethality may more than compensate for transformation at low radiation doses. However, adaptive reduction in sensitivity to radio-mutation has also been reported, which implies the existence also of enhanced mutation following very low single doses. So far this has not been confirmed, but provided the trigger dose for mutational protection was lower than the trigger dose for protection against cytotoxicity, cell killing would still dominate over at least the first 10 cGy of low-LET exposure. This would lead to a non-linear, threshold, dose-risk relationship and even provide some explanation for anecdotal reports of apparent 'health promoting' effects and lowered cancer risk from very low exposure to ionising radiation.

Hypersensitivity to very-low single radiation doses: its relationship to the adaptive response and induced radioresistance

There is now little doubt of the existence of radioprotective mechanisms, or stress responses, that are upregulated in response to exposure to small doses of ionizing radiation and other DNA-damaging agents. Phenomenologically, there are two ways in which these induced mechanisms operate. First, a small conditioning dose (generally below 30 cGy) may protect against a subsequent, separate, exposure to radiation that may be substantially larger than the initial dose. This has been termed the adaptive response. Second, the response to single doses may itself be dose-dependent so that small acute radiation exposures, or exposures at very low dose rates, are more effective per unit dose than larger exposures above the threshold where the induced radioprotection is triggered. This combination has been termed low-dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. Both the adaptive response and HRS/IRR have been well documented in studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal models in vivo. There is indirect evidence that the HRS/IRR phenomenon in response to single doses is a manifestation of the same underlying mechanism that determines the adaptive response in the two-dose case and that it can be triggered by high and low LET radiations as well as a variety of other stress-inducing agents such as hydrogen peroxide and chemotherapeutic agents although exact homology remains to be tested. Little is currently known about the precise nature of this underlying mechanism, but there is evidence that it operates by increasing the amount and rate of DNA repair, rather than by indirect mechanisms such as modulation of cell-cycle progression or apoptosis. Changed expression of some genes, only in response to low and not high doses, may occur within a few hours of irradiation and this would be rapid enough to explain the phenomenon of induced radioresistance although its specific molecular components have yet to be identified.

Might intrinsic radioresistance of human tumour cells be induced by radiation?

Survival measurements were made on six human tumour cell lines in vitro after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are radioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. However, HT29, Be11, and RT112 cells showed a significant increase in X-ray radiosensitivity at doses below < 1 Gy compared with the prediction extrapolated from a LQ model fitted to the data at higher doses. The LQ model also slightly underpredicted the effect of low-dose X rays in MeWo cells, but the response of SW48 and HX142 cells was well described by the LQ model at all doses, with no evidence of increased low-dose effectiveness. The most plausible explanation for this phenomenon is that it reflects an induced radioresistance so that low doses of X-rays in vitro are more effective per Gy than higher doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms. It follows that variation in the amount of inducible radioresistance might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosensitive because they have a diminished inducible response.

Micronucleus induction in 10 human tumour cells after high- and low-dose radiation

A number of data measuring survival of animal or human cells to low LET ionizing radiation have demonstrated that these cells may be hypersensitive to doses below 1 Gy, possibly due to the absence of an inducible repair mechanism, which is observed at higher doses. The production of micronuclei (MN) in cells exposed to ionizing radiation reflects genotoxic damage. Moreover, the micronucleus assay is sensitive to low radiation doses. We have exposed 10 human tumour cell lines to doses ranging between 0.12 and 4 Gy. Using cytochalasin B to block the cells in a binucleate phase, we have scored the fraction of binucleate cells (BNC) expressing MN, as well as the number of MN per BNC, as a function of gamma-ray dose. Experimental points were fitted with a binomial equation. Doses from 1 to 4 Gy were fitted separately from those below 1 Gy, and the initial slopes after both fits were compared. Taken together, the initial slopes of MN induction after low-dose (LD) irradiation were not different from those after high-dose (HD) irradiation. Only in one cell line was a significant increase in MN production detected after LD irradiation. This cell line had the shallowest linear term after HD irradiation. It appeared that the likeliness of expressing hypersensitivity at LD was correlated with the quadratic term of MN induction at HD, which does not contradict an inducible repair hypothesis. However, the failure of observation of a significant hypersensitivity at LD for nine cell lines, and the high variability of response at LD suggests that this occasional effect may be influenced by other factors as well.