Low dose hyper-radiosensitivity in metastatic tumors

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 low dose-rate radiotherapy: radiobiology and dosimetry

Pulsed low dose-rate radiotherapy (PLDR) relies on two radiobiological findings, the hyper-radiosensitivity of tumor cells at small doses and the reduced normal tissue toxicity at low dose rates. This is achieved by delivering the daily radiation dose of 2 Gy in 10 sub-fractions (pulses) with a 3 min time interval, resulting in an effective low dose rate of 0.067 Gy min−1. In vitro cell studies and in vivo animal experiments demonstrated the therapeutic potential of PLDR treatments and provided useful preclinical data. Various treatment optimization strategies and delivery techniques have been developed for PLDR on existing linear accelerators. Preliminary results from early clinical studies have shown favorable outcomes for various treatment sites especially for recurrent cancers. This paper reviews the experimental findings of PLDR and dosimetric requirements for PLDR treatment planning and delivery, and summarizes major clinical studies on PLDR cancer treatments.

[Comparison of 186Re to 662 keV photon radiation concerning biological radiation effect on the human B-cell line BV-173]

ZIEL: Ziel der Untersuchung ist es, die Strahlenwirkung des β--Emitters 186Re und von 662keV-Photonenstrahlung zu ermitteln, um die biologische Wirkung von Strahlung niedriger Dosisleistung (186Re) mit der hoher Dosisleistung zu vergleichen.

MATERIAL UND METHODEN

Zellen der humanen Leukämie-Zelllinie BV-173 wurden mit 662keV-Photonenstrahlung respektive 186Re bestrahlt. In einem Inkubationszeitraum von 7 Tagen wurden Zahl und Vitalität der Zellen täglich bestimmt und als Dosiseffektkurven basierend auf der Vitalität dargestellt. Hierfür wurde der Zeitpunkt mit minimalem Überleben verwendet (72h 186Re und 24h Photonenstrahlung).

ERGEBNISSE

Beide Strahlenarten zeigen am Auswertezeitpunkt (72h nach Versuchsbeginn für 186Re und 24h nach Versuchsbeginn für Photonenstrahlung) eine Überlebenskurve mit biexponentiellem Verlauf. Für Photonenstrahlung ist dies erklärbar durch eine Hypersensitivität im niedrigen Dosisbereich bis 1Gy, für die sich eine D0 von 3,3Gy ergibt, für Dosen über 1,0Gy liegt die D0 bei 10Gy. Für die 186Re-Inkubation ergibt sich eine D0 von 11,1Gy bei niedrigen Dosen verursacht durch die Reparatur subletaler Schäden, durch welche die biologische Wirkung abgeschwächt wird. Ab einer akkumulierten Dosis von etwa 1,6Gy zeichnet sich für 186Re ein wesentlich steilerer Kurvenverlauf mit einer D0 von 4,0Gy ab, der eine in diesem Bereich 2,5-fach stärkere biologische Wirkung als akute Photonenstrahlung wiedergibt (D0 4Gy für 186Re bzw. 10Gy für Photonen).

SCHLUSSFOLGERUNG

Strahlung niedriger Dosisleistung zeigt eine geringere biologische Wirkung als eine akute Bestrahlung. Es existiert aber ein Grenzwert der akkumulierten Dosis, ab dem die biologische Wirkung von β-Strahlung die der Photonenstrahlung sogar übertrifft.

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.

Practical Clinical Implementation of the Special Physics Consultation Process in the Re-irradiation Environment

Purpose

The purpose of this work is to present a practical, structured process allowing for consistent, safe radiation therapy delivery in the re-treatment environment.

Methods and materials

A process for reirradiation is described with documentation in the form of a special physics consultation. Data acquisition associated with previous treatment is described from highest to lowest quality. Methods are presented for conversion to equieffective dose, as well as our departmental assumptions for tissue repair. The generation of organ-at-risk available physical dose for use in treatment planning is discussed. Results using our methods are compared with published values after conversion to biologically effective dose. Utilization of pulsed-low-dose-rate delivery is described, and data for reirradiation using these methods over the previous 5 years are presented.

Results

Between 2015 and 2019, the number of patients in our department requiring equieffective dose calculation has doubled. We have developed guidelines for estimation of sublethal damage repair as a function of time between treatment courses ranging from 0% for <6 months to 50% for >1 year. These guidelines were developed based on available spinal cord data because we found that 84% of organs at risk involved nerve-like tissues. The average percent repair used increased from 32% to 37% over this time period. When comparing the results obtained using our methods with published values, 99% of patients had a cumulative biologically effective dose below the limits established for acceptable myelopathy rates. Pulsed-low-dose-rate use over this period tripled with an average prescription dose of 49 Gy.

Conclusions

The methods described result in safe, effective treatment in the reirradiation setting. Further correlation with patient outcomes and side effects is warranted.

Pulsed Reduced Dose Rate Radiotherapy in Conjunction With Bevacizumab or Bevacizumab Alone in Recurrent High-grade Glioma: Survival Outcomes

Purpose:

Dismal prognosis and limited treatment options for recurrent high-grade glioma have provoked interest in various forms of reirradiation. Pulsed reduced dose rate radiation therapy (pRDR) is a promising technique that exploits low-dose hyper-radiosensitivity of proliferating tumor cells while sparing adjacent nonproliferating normal brain tissue. Large radiation treatment volumes can thus be used to target both contrast-enhancing and FLAIR abnormalities thought to harbor recurrent gross and microscopic disease, respectively. The aim of this retrospective study was to determine whether the addition of pRDR to bevacizumab improves survival over bevacizumab alone for recurrent high-grade glioma.

Methods and Materials:

Eighty patients with recurrent high-grade glioma were included in this study; 47 patients received bevacizumab monotherapy (BEV), and 33 patients received pRDR with bevacizumab (BEV/pRDR). Progression-free survival (PFS) and overall survival were compared between the BEV and BEV/pRDR groups. Regression analysis was performed to identify and control for confounding influences on survival analyses.

Results:

Significant (P <.05) advantages in PFS (12 vs 4 months; hazard ratio = 2.37) and OS (16 vs. 9 months; hazard ratio = 1.68) were observed with BEV/pRDR compared with BEV alone.

Conclusions:

This retrospective analysis suggests that treatment with pRDR in addition to bevacizumab could significantly prolong PFS and overall survival compared with bevacizumab alone for recurrent high-grade glioma.

Pulsed low dose-rate irradiation response in isogenic HNSCC cell lines with different radiosensitivity

Background

Management of locoregionally recurrent head and neck squamous cell carcinomas (HNSCC) is challenging due to potential radioresistance. Pulsed low-dose rate (PLDR) irradiation exploits phenomena of increased radiosensitivity, low-dose hyperradiosensitivity (LDHRS), and inverse dose-rate effect. The purpose of this study was to evaluate LDHRS and the effect of PLDR irradiation in isogenic HNSCC cells with different radiosensitivity.

Materials and methods

Cell survival after different irradiation regimens in isogenic parental FaDu and radioresistant FaDu-RR cells was determined by clonogenic assay; post irradiation cell cycle distribution was studied by flow cytometry; the expression of DNA damage signalling genes was assesed by reverse transcription-quantitative PCR.

Results

Radioresistant Fadu-RR cells displayed LDHRS and were more sensitive to PLDR irradiation than parental FaDu cells. In both cell lines, cell cycle was arrested in G₂/M phase 5 hours after irradiation. It was restored 24 hours after irradiation in parental, but not in the radioresistant cells, which were arrested in G₁-phase. DNA damage signalling genes were under-expressed in radioresistant compared to parental cells. Irradiation increased DNA damage signalling gene expression in radioresistant cells, while in parental cells only few genes were under-expressed.

Conclusions

We demonstrated LDHRS in isogenic radioresistant cells, but not in the parental cells. Survival of LDHRS-positive radioresistant cells after PLDR was significantly reduced. This reduction in cell survival is associated with variations in DNA damage signalling gene expression observed in response to PLDR most likely through different regulation of cell cycle checkpoints.

Resensitization of cisplatin resistance ovarian cancer cells to cisplatin through pretreatment with low-dose fraction radiation

Objective

Cisplatin is the first‐line chemotherapy for ovarian cancer. However, cisplatin resistance is severely affecting the treatment efficacy. FOXO3a has been reported to be involved in reversing chemotherapy resistance. However, whether low‐dose fraction radiation therapy (LDFRT) can reverse cisplatin resistance remains unclear. This study aimed to explore the effect of LDFRT on cisplatin resistance and its relation with FOXO3a expression in vitro.

Methods

The toxicity of cisplatin on SKOV3/DDP cells was evaluated by CCK8 assay and cell apoptosis was measured by Annexin V‐FITC staining as well as Hoechst33342 staining. The expression of FOXO3a and other relative proteins was measured by western blot.

Results

Our study found that LDFRT enhanced cisplatin‐induced apoptosis of SKOV3/DDP cells and promoted the expression of FOXO3a and pro‐apoptotic protein PUMA. In addition, overexpression of FOXO3a promoted PUMA activity and toxicity of cisplatin on SKOV3/DDP cells.

Conclusion

LDFRT reverses cisplatin resistance of SKOV3/DDP cells possibly by upregulating the expression of FOXO3a and its downstream target PUMA, suggesting that LDFRT might be a potent chemosensitizer for the treatment of ovarian cancer.

Large volume re-irradiation for recurrent meningioma with pulsed reduced dose rate radiotherapy

PURPOSE

Meningiomas comprise up to 30% of primary brain tumors. The majority of meningioma patients enjoy high rates of control after conventional therapies. However, patients with recurrent disease previously treated with radiotherapy have few options for salvage treatment, and systemic interventions have proven largely ineffective. The aim of this study was to determine whether pulsed reduced dose rate radiotherapy (PRDR) was well tolerated in a small cohort of patients with recurrent meningioma.

METHODS

We retrospectively identified eight patients with recurrent intracranial meningioma treated with PRDR from April 2013 to August of 2017 at a single institution. All patients had radiographic and/or pathologic evidence of progression prior to treatment and had previously completed conventional radiotherapy. Acute and late toxicities were graded based on CTCAE 4.0.

RESULTS

Of eight patients, six had histologically confirmed atypical meningiomas upon recurrence. All patients were re-treated with IMRT at an apparent dose rate of 0.0667 Gy/min. Median time between radiation courses was 7.7 years. Median PRDR dose was 54 Gy in 27 fractions to a median volume of 261.6 cm³. Two patients (25%) had in field failure with a median follow up of 23.3 months. PFS at 6 months was 100%. All but one (87.5%) patient was still alive at last follow up. No patient experienced grade ≥ 2 acute or late toxicities.

CONCLUSIONS

PRDR re-irradiation was well tolerated and appeared effective for a small cohort of patients with recurrent meningioma previously treated with radiotherapy. A phase II trial to assess this prospectively is in development.

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.

Chemopotentiation by Ultrafractionated Radiotherapy in Glioblastoma Resistant to Conventional Therapy

Introduction

Induced radiation resistance (IRR) and hyper-radiosensitivity (HRS) are well-described phenomena in basic literature, yet few reports have been published in which such phenomena are exploited clinically for the benefit of patients. Glioblastoma is a prime example.

Case and methods

The case of an 82-year-old woman is described whose resected frontoparietal glioblastoma progressed through treatment administered according to standard methods. With review board and patient approval, we continued her treatment using radiotherapy and temozolomide, but drastically modified the radiotherapy fractionation, administering 50 cGy twice daily on each of the first 5 days of a 14-day cycle. Temozolomide was administered on the first 4 days of each cycle. We use the term “ultrafractionated radiotherapy” to refer to the extremely low doses of radiation used in this case.

Results

This modified regimen resulted in regression of the contrast-enhancing areas of disease recurrence identified on MRI, and the patient survived approximately 6 months following recurrence of her disease, having received 5 cycles of additional therapy after prior full-dose treatment.

Conclusions

Ultrafractionated radiotherapy and concurrent temozolomide were efficacious and tolerable in this patient whose glioblastoma previously progressed through conventional treatment. Additional studies of this approach are warranted. Free full text available at www.tumorionline.it

Multi-institutional phase I study of low-dose ultra-fractionated radiotherapy as a chemosensitizer for gemcitabine and erlotinib in patients with locally advanced or limited metastatic pancreatic cancer

PURPOSE

Gemcitabine (G) has been shown to sensitize pancreatic cancer to radiotherapy but requires lower doses of G and thus delays aggressive systemic treatment, potentially leading to distant failure. We initiated a phase I trial combining ultra-fractionated low-dose radiotherapy with full dose G and erlotinib in the treatment of patients with advanced pancreatic cancer.

METHODS

Patients with locally advanced or metastatic pancreatic cancer confined to the abdomen and an ECOG performance status (PS) of 0-1 who had received 0-1 prior regimens (without G or E) and no prior radiotherapy were eligible. Patients were treated in 21 day cycles with G IV days 1 & 8, E once PO QD, and twice daily RT fractions separated by at least 4h on days 1, 2, 8, and 9. Whole abdominal RT fields were used. Primary endpoint was to define dose limiting toxicity (DLT) and the maximum tolerated dose (MTD).

RESULTS

27 patients (median age 64 years and 15 male) were enrolled between 11/24/08 and 4/12/12. 1 patient withdrew consent prior to receiving any protocol therapy. 17 patients had a PS of 1. The majority of patients were stage IV. One DLT was noted out of 7 patients at dose level (DL) 1. Subsequently no DLTs were noted in 3 patients each enrolled at DL2-4 or 11 patients in the expansion cohort. The majority of grade 3 toxicities were hematologic with 1 grade 5 bowel perforation in dose level 1 in cycle 4. Best response in 24 evaluable patients: PR (8), stable (15), PD 1. Median survival for the entire group was 9.1 months.

CONCLUSION

This phase I study combining low-dose ultra-fractionated RT as a sensitizer to full dose G plus E was well tolerated with encouraging efficacy. This represents a novel strategy worthy of further investigation in advanced pancreatic cancer patients.

Intensity-modulated radiation therapy for pancreatic and prostate cancer using pulsed low-dose rate delivery techniques

Reirradiation of patients who were previously treated with radiotherapy is vastly challenging. Pulsed low-dose rate (PLDR) external beam radiotherapy has the potential to reduce normal tissue toxicities while providing significant tumor control for recurrent cancers. This work investigates treatment planning techniques for intensity-modulated radiation therapy (IMRT)-based PLDR treatment of various sites, including cases with pancreatic and prostate cancer. A total of 20 patients with clinical recurrence were selected for this study, including 10 cases with pancreatic cancer and 10 with prostate cancer. Large variations in the target volume were included to test the ability of IMRT using the existing treatment planning system and optimization algorithm to deliver uniform doses in individual gantry angles/fields for PLDR treatments. Treatment plans were generated with 10 gantry angles using the step-and-shoot IMRT delivery technique, which can be delivered in 3-minute intervals to achieve an effective low dose rate of 6.7cGy/min. Instead of dose constraints on critical structures, ring structures were mainly used in PLDR-IMRT optimization. In this study, the PLDR-IMRT plans were compared with the PLDR-3-dimensional conformal radiation therapy (3DCRT) plans and the PLDR-RapidArc plans. For the 10 cases with pancreatic cancer that were investigated, the mean planning target volume (PTV) dose for each gantry angle in the PLDR-IMRT plans ranged from 17.6 to 22.4cGy. The maximum doses ranged between 22.9 and 34.8cGy. The minimum doses ranged from 8.2 to 17.5cGy. For the 10 cases with prostate cancer that were investigated, the mean PTV doses for individual gantry angles ranged from 18.8 to 22.6cGy. The maximum doses per gantry angle were between 24.0 and 34.7cGy. The minimum doses per gantry angle ranged from 4.4 to 17.4cGy. A significant reduction in the organ at risk (OAR) dose was observed with the PLDR-IMRT plan when compared with that using the PLDR-3DCRT plan. The volume receiving an 18-Gy (V18) dose for the left and right kidneys was reduced by 10.6% and 12.5%, respectively, for the pancreatic plans. The volume receiving a 45-Gy (V45) dose for the small bowel decreased from 65.3% to 45.5%. For the cases with prostate cancer, the volume receiving a 40-Gy (V40) dose for the bladder and the rectum was reduced significantly by 25.1% and 51.2%, respectively. When compared with the RapidArc technique, the volume receiving a 30-Gy (V30) dose for the left and the right kidneys was lower in the IMRT plans. For most OARs, no significant differences were observed between the PLDR-IMRT and the PLDR-RapidArc plans. These results clearly demonstrated that the PLDR-IMRT plan was suitable for PLDR pancreatic and prostate cancer treatments in terms of the overall plan quality. A significant reduction in the OAR dose was achieved with the PLDR-IMRT plan when compared with that using the PLDR-3DCRT plan. For most OARs, no significant differences were observed between the PLDR-IMRT and the PLDR-RapidArc plans. When compared with the PLDR-3DCRT plan, the PLDR-IMRT plan could provide superior target coverage and normal tissue sparing for PLDR reirradiation of recurrent pancreatic and prostate cancers. The PLDR-IMRT plan is an effective treatment choice for recurrent cancers in most cancer centers.

Large volume reirradiation as salvage therapy for glioblastoma after progression on bevacizumab

Outcomes after bevacizumab failure for recurrent glioblastoma (GBM) are poor. Our analysis of 16 phase II trials (n = 995) revealed a median overall survival (OS) of 3.8 months (±1.0 month SD) after bevacizumab failure with no discernible activity of salvage chemotherapy. Thus, the optimal treatment for disease progression after bevacizumab has yet to be elucidated. This study evaluated the efficacy of reirradiation for patients with GBM after progression on bevacizumab. An IRB approved retrospective (2/2008-5/2013) analysis was performed of 23 patients with recurrent GBM (after standard radiotherapy/temozolomide) treated with bevacizumab (10 mg/kg) every 2 weeks until progression (median age 53 years; median KPS 80; median progression free survival on bevacizumab 3.7 months). Within 7-14 days of progression on bevacizumab, patients initiated reirradiation to a dose of 54 Gy in 27 fractions using pulsed-reduced dose rate (PRDR) radiotherapy. The median planning target volume was 424 cm(3). At the start of reirradiation, bevacizumab (10 mg/kg) was given every 4 weeks for two additional cycles. The median OS and 6 month OS after bevacizumab failure was 6.9 months and 65 %, respectively. Reirradiation was well tolerated with no symptomatic grade 3-4 toxicities. Favorable outcomes of reirradiation after bevacizumab failure in patients with recurrent GBM suggest its role as a treatment option for large volume recurrences not amenable to stereotactic radiosurgery. As PRDR is easily accomplished from a technological standpoint, we are in the process of expanding this approach to a multi-institutional cooperative group trial.

The role of nitric oxide radicals in removal of hyper-radiosensitivity by priming irradiation

In this study, a mechanism in which low-dose hyper-radiosensitivity (HRS) is permanently removed, induced by low-dose-rate (LDR) (0.2-0.3 Gy/h for 1 h) but not by high-dose-rate priming (0.3 Gy at 40 Gy/h) was investigated. One HRS-negative cell line (NHIK 3025) and two HRS-positive cell lines (T-47D, T98G) were used. The effects of different pretreatments on HRS were investigated using the colony assay. Cell-based ELISA was used to measure nitric oxide synthase (NOS) levels, and microarray analysis to compare gene expression in primed and unprimed cells. The data show how permanent removal of HRS, previously found to be induced by LDR priming irradiation, can also be induced by addition of nitric oxide (NO)-donor DEANO combined with either high-dose-rate priming or exposure to prolonged cycling hypoxia followed by reoxygenation, a treatment not involving radiation. The removal of HRS appears not to involve DNA damage induced during priming irradiation as it was also induced by LDR irradiation of cell-conditioned medium without cells present. The permanent removal of HRS in LDR-primed cells was reversed by treatment with inducible nitric oxide synthase (iNOS) inhibitor 1400W. Furthermore, 1400W could also induce HRS in an HRS-negative cell line. The data suggest that LDR irradiation for 1 h, but not 15 min, activates iNOS, and also that sustained iNOS activation is necessary for the permanent removal of HRS by LDR priming. The data indicate that nitric oxide production is involved in the regulatory processes determining cellular responses to low-dose-rate irradiation.

Three-times daily ultrafractionated radiation therapy, a novel and promising regimen for glioblastoma patients

Glioblastomas are considered to be one of the most radio resistant tumors. Despite new therapies, the prognosis of this disease remains dismal. Also, the mechanisms of radiation resistance in mammalian cells are more complex than once believed. Experimental studies have indicated that some human cell lines are sensitive to low radiation doses of <1 Gy. This phenomenon has been termed low-dose hyper-radio-sensitivity (HRS), and is more apparent in radio resistant cell lines, such as glioblastoma cells. Sensitivity may result from the inability of low dose radiation to efficiently induce repair mechanisms, whereas higher doses cause enough damage to trigger repair responses for radio resistance. In vitro studies have demonstrated this phenomenon using various human malignant glioma cell lines: (1) daily repeated irradiation of cells with low doses compared to irradiation using a single biologically equivalent dose resulted in significantly higher cell killing; (2) experiments conducted on glioma xenografts demonstrated that repeated irradiation with low doses was more effective for inhibiting tumor growth than a single dose. In order to confirm and validate these promising studies on HRS, a few phase II trials were developed. For translating the experimental observations into the clinic, ultra fractionation protocols (with three daily doses) were tested in glioblastoma patients. Tolerance and toxicity were the primary endpoints, with overall survival as a secondary endpoint. These protocols were initiated before concomitant radio chemotherapy became the standard of care. For these trials, patients with an unfavorable clinical prognostic factor of newly unresectable GBM were included. When comparing the results of these trials with international literature using multivariate analysis for both progression free survival and overall survival, ultra fractionated irradiation showed superiority over radiotherapy alone. In addition, it was found to be equivalent to treatment using radiotherapy and temozolomide. Therefore, ultra fractionated protocols may prolong survival of glioblastoma patients. In this review, we describe the main experimental data regarding low-dose hypersensitivity as well as the findings of clinical trials that have investigated this new radiotherapy regimen.

Determining if low dose hyper-radiosensitivity (HRS) can be exploited to provide a therapeutic advantage: a cell line study in four glioblastoma multiforme (GBM) cell lines

PURPOSE

To determine if ultra-fractionation using repeated pulses of radiation (10 × 0.2 Gray [Gy]) would be more cytotoxic than continuously-delivered radiation to the same total dose (2 Gy) in four glioma cell lines.

MATERIALS AND METHODS

Human T98G, U373, U87MG and U138MG cells were conventionally X-irradiated with 0.1-8 Gy and clonogenic survival assessed. Next, cells were treated with either a single dose of 2 Gy or 10 pulses of 0.2 Gy using a 3-min inter-pulse interval and DNA (Deoxyribonucleic acid) repair (pHistone H2A.X), G2-phase cell cycle checkpoint arrest (pHistone H3) and apoptosis (caspase-3) compared between the two regimens. A dose of 0.2 Gy was selected as this reflects the hyper- radiosensitivity (HRS)/increased radioresistance (IRR) transition point of the low-dose cell survival curve.

RESULTS

T98G, U87MG and U138MG exhibited distinct HRS responses and survival curves were well-described by the Induced Repair model. Despite the prolonged delivery time, ultra-fractionation (10 × 0.2 Gy) was equally effective as a single continuously-delivered 2 Gy dose. However, ultra-fractionation was more effective when given for five consecutive days to a total dose of 10 Gy. The increased effectiveness of ultra-fractionation could not be attributed directly to differences in DNA damage, repair processes or radiation-induced apoptosis.

CONCLUSIONS

Ultra-fractionation (10 × 0.2 Gy) is an effective modality for killing glioma cell lines compared with standard 2 Gy dosing when multiple days of treatment are given.

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.

Low-dose radiation hyper-radiosensitivity in multicellular tumour spheroids

OBJECTIVE

We propose and study a new model aimed at describing the low-dose hyper-radiosensitivity phenomenon appearing in the survival curves of different cell lines.

METHODS

The model uses the induced repair assumption, considering that the critical dose at which this mechanism begins to act varies from cell to cell in a given population. The model proposed is compared with the linear-quadratic model and the modified linear-quadratic model, which is commonly used in literature and in which the induced repair is taken into account in a heuristic way. The survival curve for the MCF-7 line of human breast cancer is measured at low absorbed doses and the uncertainties in these doses are estimated using thermoluminiscent dosemeters.

RESULTS

It is shown that these multicellular spheroids present low-dose hyper-radiosensitivity. The new model permits an accurate description of the data of two human cell lines (previously published) and of the multicellular spheroids of the MCF-7 line here measured.

CONCLUSION

The model shows enough flexibility to account for data with very different characteristics and considers in a faithful way the hypothesis of the repair induction.

Cell division cycle 25 homolog c effects on low-dose hyper-radiosensitivity and induced radioresistance at elevated dosage in A549 cells

The underlying mechanisms behind both low-dose hyper-radiosensitivity (HRS) and induced radioresistance (IRR), generally occurring at elevated radiation levels, remain unclear; however, elucidation of the relationship between cell cycle division 25 homolog c (Cdc25c) phosphatase and HRS/IRR may provide important insights into this process. Two cell lines with disparate HRS status, A549 and SiHa cells, were selected as cell models for comparison of dose-dependent Cdc25c phosphatase expression subsequent to low-dose irradiation. Knockdown of Cdc25c in A549 cells was mediated by transfection with a pGCsi-RAN-U6neo vector containing hairpin siRNA sequences. S216-phosphorylated Cdc25c protein [p-Cdc25c (Ser216)], cell survival and mitotic ratio were measured by western blot, colony-forming assay and histone H3 phosphorylation analysis. Variant p-Cdc25c (Ser216) expression was observed in the two cell lines after irradiation. The p-Cdc25c (Ser216) expression noted in SiHa cells after administration of 0-1 Gy radiation was similar to the radioresistance model; however, in A549 cells, the dose response for the phosphorylation of the Cdc25c Ser216 residue overlapped the level required to overcome the HRS response. Furthermore, Cdc25c repression prior to low-dose radiation induced more distinct HRS and prevented the development of IRR. The dose required to overcome the HRS response coincided with the effect of early G2-phase checkpoint arrest in A549 cells (approximately 0.3 Gy), and Cdc25c knockdown in A549 cells (approximately 0.5 Gy) corresponded to the phosphorylation of the Cdc25c Ser216 residue. Resultant data confirmed that dose-dependent Cdc25c phosphatase does effectively act as an early G2-phase checkpoint, thus indicating mechanistic importance in the HRS to IRR transition in A549 cells.

Pulsed reduced dose-rate radiotherapy as re-irradiation for brain metastasis in a patient with lung squamous-celled carcinoma

The recurrence and progression of brain metastases after brain irradiation are a major cause of mortality and morbidity in patients with cancer. The risk of radiation-induced neurotoxicity and efficacy probably leads oncologists to not consider re-irradiation. We report the case of a 48-year-old Asian male diagnosed with squamous cell lung cancer and multiple brain metastases initially treated with 40 Gy whole-brain radiotherapy and 20 Gy partial brain boost. Fourteen gray stereotactic radiosurgery as salvage for brain metastases in the left occipital lobe was performed after initial irradiation. The recurrence of brain metastases in the left occipital lobe was demonstrated on magnetic resonance imaging at 9 months after initial radiotherapy. He received the second course of 28 Gy stereotactic radiosurgery for the recurrent brain metastases in the left occipital lobe. The third relapse of brain metastases was demonstrated by a magnetic resonance imaging scan at 7 months after the second radiotherapy. The third course of irradiation was performed because he refused to undergo surgical resection of the recurrent brain metastases. The third course of irradiation used a pulsed reduced dose-rate radiotherapy technique. It was delivered in a series of 0.2 Gy pulses separated by 3-min intervals. The recurrent brain metastases were treated with a dose of 60 Gy using 30 daily fractions of 2 Gy. Despite the brain metastases receiving 162 Gy irradiation, this patient had no apparent acute or late neurologic toxicities and showed clinical improvement. This is the first report of the pulsed reduced dose-rate radiotherapy technique being used as the third course of radiotherapy for recurrent brain metastases.

Use of cetuximab in combination with pulsed reduced dose-rate radiotherapy in a patient with recurrence of nasopharyngeal carcinoma in the neck

Reirradiation is a major therapeutic modality for patients with locally recurrent head and neck carcinoma. Due to normal tissue tolerances, reirradiation using conventional techniques has a narrow therapeutic ratio in the regional recurrence of nasopharyngeal carcinoma (NPC). Pulsed reduced dose-rate radiotherapy (PRDR), which delivers a series of 0.2 Gy pulses separated by 3 min intervals, is a new reirradiation technique. Head and neck carcinoma cells have high levels of epidermal growth factor receptor expression and cetuximab shows a clear benefit to locally advanced head and neck carcinoma. We report a 56-year-old male with a recurrent lesion of NPC in the neck following initial radical radiochemotherapy. The patient was retreated with PRDR and concurrent cetuximab. The total dose of PRDR was 70 Gy, using 35 daily fractions of 2.0 Gy. The recurrent lesion of this patient had a complete response with no apparent radiation-induced normal tissue complications. This is the first study concerning PRDR combined with cetuximab for the treatment of recurrent head and neck carcinoma following radiotherapy. The outcome of this patient reveals that treatment with PRDR and concurrent cetuximab is a promising therapeutic option for patients with recurrent head and neck carcinoma following radiotherapy.

Human Lung Cancer Risks from Radon - Part III - Evidence of Influence of Combined Bystander and Adaptive Response Effects on Radon Case-Control Studies - A Microdose Analysis

Since the publication of the BEIR VI (1999) report on health risks from radon, a significant amount of new data has been published showing various mechanisms that may affect the ultimate assessment of radon as a carcinogen, in particular the potentially deleterious Bystander Effect (BE) and the potentially beneficial Adaptive Response radio-protection (AR). The case-control radon lung cancer risk data of the pooled 13 European countries radon study (Darby et al 2005, 2006) and the 8 North American pooled study (Krewski et al 2005, 2006) have been evaluated. The large variation in the odds ratios of lung cancer from radon risk is reconciled, based on the large variation in geological and ecological conditions and variation in the degree of adaptive response radio-protection against the bystander effect induced lung damage. The analysis clearly shows Bystander Effect radon lung cancer induction and Adaptive Response reduction in lung cancer in some geographical regions. It is estimated that for radon levels up to about 400 Bq m(-3) there is about a 30% probability that no human lung cancer risk from radon will be experienced and a 20% probability that the risk is below the zero-radon, endogenic spontaneous or perhaps even genetically inheritable lung cancer risk rate. The BEIR VI (1999) and EPA (2003) estimates of human lung cancer deaths from radon are most likely significantly excessive. The assumption of linearity of risk, by the Linear No-Threshold Model, with increasing radon exposure is invalid.

The effects of G2-phase enrichment and checkpoint abrogation on low-dose hyper-radiosensitivity

PURPOSE

An association between low-dose hyper-radiosensitivity (HRS) and the "early" G2/M checkpoint has been established. An improved molecular understanding of the temporal dynamics of this relationship is needed before clinical translation can be considered. This study was conducted to characterize the dose response of the early G2/M checkpoint and then determine whether low-dose radiation sensitivity could be increased by synchronization or chemical inhibition of the cell cycle.

METHODS AND MATERIALS

Two related cell lines with disparate HRS status were used (MR4 and 3.7 cells). A double-thymidine block technique was developed to enrich the G2-phase population. Clonogenic cell survival, radiation-induced G2-phase cell cycle arrest, and deoxyribonucleic acid double-strand break repair were measured in the presence and absence of inhibitors to G2-phase checkpoint proteins.

RESULTS

For MR4 cells, the dose required to overcome the HRS response (approximately 0.2 Gy) corresponded with that needed for the activation of the early G2/M checkpoint. As hypothesized, enriching the number of G2-phase cells in the population resulted in an enhanced HRS response, because a greater proportion of radiation-damaged cells evaded the early G2/M checkpoint and entered mitosis with unrepaired deoxyribonucleic acid double-strand breaks. Likewise, abrogation of the checkpoint by inhibition of Chk1 and Chk2 also increased low-dose radiosensitivity. These effects were not evident in 3.7 cells.

CONCLUSIONS

The data confirm that HRS is linked to the early G2/M checkpoint through the damage response of G2-phase cells. Low-dose radiosensitivity could be increased by manipulating the transition of radiation-damaged G2-phase cells into mitosis. This provides a rationale for combining low-dose radiation therapy with chemical synchronization techniques to improve increased radiosensitivity.

Reirradiation of large-volume recurrent glioma with pulsed reduced-dose-rate radiotherapy

PURPOSE

Pulsed reduced-dose-rate radiotherapy (PRDR) is a reirradiation technique that reduces the effective dose rate and increases the treatment time, allowing sublethal damage repair during irradiation.

PATIENTS AND METHODS

A total of 103 patients with recurrent glioma underwent reirradiation using PRDR (86 considered to have Grade 4 at PRDR). PRDR was delivered using a series of 0.2-Gy pulses at 3-min intervals, creating an apparent dose rate of 0.0667 Gy/min to a median dose of 50 Gy (range, 20-60) delivered in 1.8-2.0-Gy fractions. The mean treatment volume was 403.5±189.4 cm3 according to T2-weighted magnetic resonance imaging and a 2-cm margin.

RESULTS

For the initial or upgraded Grade 4 cohort (n=86), the median interval from the first irradiation to PRDR was 14 months. Patients undergoing PRDR within 14 months of the first irradiation (n=43) had a median survival of 21 weeks. Those treated ≥14 months after radiotherapy had a median survival of 28 weeks (n=43; p=0.004 and HR=1.82 with a 95% CI ranging from 1.25 to 3.10). These data compared favorably to historical data sets, because only 16% of the patients were treated at first relapse (with 46% treated at the second relapse, 32% at the third or fourth relapse, and 4% at the fourth or fifth relapse). The median survival since diagnosis and retreatment was 6.3 years and 11.4 months for low-grade, 4.1 years and 5.6 months for Grade 3, and 1.6 years and 5.1 months for Grade 4 tumors, respectively, according to the initial histologic findings. Multivariate analysis revealed age at the initial diagnosis, initial low-grade disease, and Karnofsky performance score of ≥80 to be significant predictors of survival after initiation of PRDR.

CONCLUSION

PRDR allowed for safe retreatment of larger volumes to high doses with palliative benefit.

Effects of irradiation on tumor cell survival, invasion and angiogenesis

Ionizing irradiation is a widely applied therapeutic method for the majority of solid malignant neoplasms, including brain tumors where, depending on localization, this might often be the only feasible primary intervention.Without doubt, it has been proved to be a fundamental tool available in the battlefield against cancer, offering a clear survival benefit in most cases. However, numerous studies have associated tumor irradiation with enhanced aggressive phenotype of the remaining cancer cells. A cell population manages to survive after the exposure, either because it receives sublethal doses and/or because it successfully utilizes the repair mechanisms. The biology of irradiated cells is altered leading to up-regulation of genes that favor cell survival, invasion and angiogenesis. In addition, hypoxia within the tumor mass limits the cytotoxicity of irradiation, whereas irradiation itself may worsen hypoxic conditions, which also contribute to the generation of resistant cells. Activation of cell surface receptors, such as the epidermal growth factor receptor, utilization of signaling pathways, and over-expression of cytokines, proteases and growth factors, for example the matrix metalloproteinases and vascular endothelial growth factor, protect tumor and non-tumor cells from apoptosis, increase their ability to invade to adjacent or distant areas, and trigger angiogenesis. This review will try to unfold the various molecular events and interactions that control tumor cell survival, invasion and angiogenesis and which are elicited or influenced by irradiation of the tumor mass, and to emphasize the importance of combining irradiation therapy with molecular targeting.

Treatment modelling: the influence of micro-environmental conditions

The interest in theoretical modelling of radiation response has grown steadily from a fast method to estimate the gain of new treatment strategies to an individualisation tool that may be used as part of the treatment planning algorithms. While the advantages of biological optimisation of plans are obvious, accurate theoretical models and realistic information about the micro-environmental conditions in tissues are needed. This paper aimed to investigate the clinical implications of taking into consideration the details of the tumour microenvironmental conditions. The focus was on the availability of oxygen and other nutrients to tumour cells and the relationship between cellular energy reserves and DNA repair ability as this is thought to influence the response of the various hypoxic cells. The choice of the theoretical models for predicting the response (the linear quadratic model or the inducible repair model) was also addressed. The modelling performed in this project has shown that the postulated radiobiological differences between acute and chronic hypoxia have some important clinical implications which may help to understand the mechanism behind the current success rates of radiotherapy. The results also suggested that it is important to distinguish between the two types of hypoxia in predictive assays and other treatment simulations.

Low-dose hyperradiosensitivity of human glioblastoma cell linesin vitrodoes not translate into improved outcome of ultrafractionated radiotherapyin vivo

PURPOSE

Low dose hyperradiosensitivity (HRS) has been observed in HGL21- and T98G human glioblastoma cells in vitro. The present study investigates whether these effects translate into improved outcome of ultrafractionated irradiation (UF) in vivo.

MATERIAL AND METHODS

T98G or HGL21 were transplanted on the hind leg of nude mice. Tumours were irradiated with UF (3 fractions of 0.4 Gy per day, interval 4 h, 7 days per week) or with conventional fractionation (CF; 1 fraction of 1.68 Gy per day, 5 days per week) over 2 or 4 weeks in HGL21 and 2,4 or 6 weeks in T98G. In HGL21, graded top-up doses under clamped hypoxia were applied after 4 weeks of fractionated irradiation. Additional groups of animals were irradiated with single doses under clamp hypoxic conditions with or without whole body irradiation (WBI) before tumour transplantation. Experimental endpoints were growth delay (time to 5-fold starting volume, GD(V5)) and local tumour control.

RESULTS

In T98G tumours median relative GD(V5) was 1.2 [95% C.I. 0.96; 8] in the CF and 0.8 [0.7; 1.02] in the UF arm (p = 0.009) indicating that ultrafractionation is less efficient than conventional fractionation. The TCD50 value of 33.5 Gy [22; 45] after UF was higher than TCD50 of 23.6 Gy [16; 31] after CF (p = 0.15). In HGL21 the median relative GD(V5) was not significantly different between CF and UF. The top-up TCD50 value of 16.1 Gy [95% C.I. 9; 23 Gy] after CF was significantly lower than the corresponding value of 33.2 Gy [23; 44] after UF irradiation (p = 0.007), indicating a higher efficacy of CF compared to UF.

CONCLUSION

The results on human T98G and HGL21 glioblastoma do not support the hypothesis that HRS in vitro translates into improved outcome of ultrafractionated irradiation in vivo.

Pulsed reduced dose-rate radiotherapy: a novel locoregional retreatment strategy for breast cancer recurrence in the previously irradiated chest wall, axilla, or supraclavicular region

PURPOSE

Reirradiation of breast cancer locoregional recurrence (LRR) in the setting of prior post-mastectomy radiation poses a significant clinical challenge due to the high risk for severe toxicity. In an attempt to reduce these toxicities, we have developed pulsed reduced dose-rate radiotherapy (PRDR), a reirradiation technique in which a series of 0.2 Gy pulses separated by 3-min time intervals is delivered, creating an apparent dose rate of 0.0667 Gy/min. Here we describe our early experience with PRDR.

PATIENTS AND METHODS

We reirradiated 17 patients with LRR breast cancer to the chest wall, axilla, or supraclavicular region using PRDR. The median prior radiation dose was 60 Gy. We delivered a median PRDR dose of 54 Gy (range 40-66 Gy) in 1.8-2.0 Gy per fraction. Eight patients received concomitant low dose capecitabine for radiosensitization. The median treatment volume was 2,084 cm(3) (range 843-7,881 cm(3)).

RESULTS

At a median follow-up of 18 months (range 4-75 months) only 2 patients have had tumor failure in the treatment region. Estimated 2-year local control rate is 92%. Treatment was well tolerated with 4 patients experiencing grade 3 acute skin toxicity. Despite a median cumulative dose of 110 Gy (range 80-236 Gy), there has been only one grade 3 and one grade 4 late toxicity.

CONCLUSIONS

With a median follow-up of 18 months, PRDR appears to be an effective method to reirradiate large volumes of previously irradiated tissue in selected patients with locoregional chest wall, axilla, and supraclavicular recurrences.

Transition in survival from low-dose hyper-radiosensitivity to increased radioresistance is independent of activation of ATM Ser1981 activity

PURPOSE

The molecular basis of low-dose hyper-radiosensitivity (HRS) is only partially understood. The aim of this study was to define the roles of ataxia telangiectasia mutated (ATM) activity and the downstream ATM-dependent G(2)-phase cell cycle checkpoint in overcoming HRS and triggering radiation resistance.

METHODS AND MATERIALS

Survival was measured using a high-resolution clonogenic assay. ATM Ser1981 activation was measured by Western blotting. The role of ATM was determined in survival experiments after molecular (siRNA) and chemical (0.4 mM caffeine) inhibition and chemical (20 microg/mL chloroquine, 15 microM genistein) activation 4-6 h before irradiation. Checkpoint responsiveness was assessed in eight cell lines of differing HRS status using flow cytometry to quantify the progression of irradiated (0-2 Gy) G(2)-phase cells entering mitosis, using histone H3 phosphorylation analysis.

RESULTS

The dose-response pattern of ATM activation was concordant with the transition from HRS to radioresistance. However, ATM activation did not play a primary role in initiating increased radioresistance. Rather, a relationship was discovered between the function of the downstream ATM-dependent early G(2)-phase checkpoint and the prevalence and overcoming of HRS. Four cell lines that exhibited HRS failed to show low-dose (<0.3-Gy) checkpoint function. In contrast, four HRS-negative cell lines exhibited immediate cell cycle arrest for the entire 0-2-Gy dose range.

CONCLUSION

Overcoming HRS is reliant on the function of the early G(2)-phase checkpoint. These data suggest that clinical exploitation of HRS could be achieved by combining radiotherapy with chemotherapeutic agents that modulate this cell cycle checkpoint.

Hyperradiosensibilité aux très faibles doses: impact en radiothérapie des micrométastases

Radiobiologists have pointed out a novel radiobiological phenomenon observed in many tumor and normal cell lines: hyper-radiosensitivity to very low-dose (HRS) followed by induced radioresistance (IRR) after a threshold dose of 0.1-0.3 Gy that depends on the cell line. Radioresistance at high dose (i.e. higher than 0.5 Gy) and metastatic potential of tumor cells are likely major factors of failure in radiotherapy. A careful review of literature suggests that: 1) radiotherapy does not increase the metastatic potential of tumor cells; 2) radioresistance at high dose and metastatic potential are not related. However, inside a given tumor cell line, highly metastatic clones may elicit more cells showing HRS or are more radiosensitive at high dose than poorly metastatic ones. Recent data obtained from molecular techniques (comet and immunofluorescence assays) applied to single cells irradiated at very low radiation doses (1-100 mGy) suggest that DNA single-strand breaks (SSB) and double-strand breaks (DSB) may be the key-lesions responsible for the HRS phenomenon. These data suggest that the HRS phenomenon may find application in radiotherapy for micrometastasis. These early disseminated and probably unvascularised cells may escape the influence of high-dose chemotherapy after excision of the primary tumor. Considering the link between metastatic potential and HRS, we have previously proposed to apply very low-dose total body irradiation (TBI) at M(0) stage that may prevent the development of micrometastases. Literature data suggest that the smallest radiation dose that can produce HRS without increasing the risk of cancer may be in the milliGrays range.

Role of apoptosis in low-dose hyper-radiosensitivity

Little is known about the mode of cell killing associated with low-dose hyper-radiosensitivity, the radiation response that describes the enhanced sensitivity of cells to small doses of ionizing radiation. Using a technique that measures the activation of caspase 3, we have established a relationship between apoptosis detected 24 h after low-dose radiation exposure and low-dose hyper-radiosensitivity in four mammalian cell lines (T98G, U373, MR4 and 3.7 cells) and two normal human lymphoblastoid cell lines. The existence of low-dose hyper-radiosensitivity in clonogenic survival experiments was found to be associated with an elevated level of apoptosis after low-dose exposures, corroborating earlier observations (Enns et al., Mol. Cancer Res. 2, 557-566, 2004). We also show that enriching populations of MR4 and V79 cells with G(1)-phase cells, to minimize the numbers of G(2)-phase cells, abolished the enhanced low-dose apoptosis. These cell-cycle enrichment experiments strengthen the reported association between low-dose hyper-sensitivity and the radioresponse of G(2)-phase cells. These data are consistent with our current hypothesis to explain low-dose hyper-radiosensitivity, namely that the enhanced sensitivity of cells to low doses of ionizing radiation reflects the failure of ATM-dependent repair processes to fully arrest the progression of damaged G(2)-phase cells harboring unrepaired DNA breaks entering mitosis.

Low-Dose Radiation Response of Primary Keratinocytes and Fibroblasts from Patients with Cervix Cancer

The aim of the present study was to examine, using the micronucleus (MN) assay, the low-dose radiation response of normal skin cells from cancer patients and to determine whether the hyper-radiosensitivity (HRS)-like phenomenon occurs in cells of these patients. Primary skin fibroblasts and keratinocytes derived from 40 patients with cervix cancer were studied. After in vitro gamma irradiation with single doses ranging from 0.05 to 4 Gy, MN induction was assessed. For each patient, the linear-quadratic (LQ) model and the induced repair (IR) model were fitted over the whole data set. In fits of the IR model, an HRS-like response after low doses (seen as the deviation over the LQ curve) was demonstrated for the fibroblasts of two patients and for the keratinocytes of four other patients. The alpha(s)/alpha(r) ratio for the six patients ranged from 2.7 to 15.4, whereas the values of the parameter d(c) ranged from 0.13 to 0.36 Gy. No relationship was observed between chromosomal radiosensitivity of fibroblasts and keratinocytes derived from the same donor in the low-dose (0.1-0.25 Gy) region. In conclusion, the fact that low-dose chromosomal hypersensitivity was observed for cells of only six of the patients studied suggests that it is not a common finding in human normal cells and can represent an individual characteristic.

Decomposition analysis of differential dose volume histograms

Dose volume histograms are a common tool to assess the value of a treatment plan for various forms of radiation therapy treatment. The purpose of this work is to introduce, validate, and apply a set of tools to analyze differential dose volume histograms by decomposing them into physically and clinically meaningful normal distributions. A weighted sum of the decomposed normal distributions (e.g., weighted dose) is proposed as a new measure of target dose, rather than the more unstable point dose. The method and its theory are presented and validated using simulated distributions. Additional validation is performed by analyzing simple four field box techniques encompassing a predefined target, using different treatment energies inside a water phantom. Furthermore, two clinical situations are analyzed using this methodology to illustrate practical usefulness. A comparison of a treatment plan for a breast patient using a tangential field setup with wedges is compared to a comparable geometry using dose compensators. Finally, a normal tissue complication probability (NTCP) calculation is refined using this decomposition. The NTCP calculation is performed on a liver as organ at risk in a treatment of a mesothelioma patient with involvement of the right lung. The comparison of the wedged breast treatment versus the compensator technique yields comparable classical dose parameters (e.g., conformity index approximately = 1 and equal dose at the ICRU dose point). The methodology proposed here shows a 4% difference in weighted dose outlining the difference in treatment using a single parameter instead of at least two in a classical analysis (e.g., mean dose, and maximal dose, or total dose variance). NTCP-calculations for the mesothelioma case are generated automatically and show a 3% decrease with respect to the classical calculation. The decrease is slightly dependant on the fractionation and on the alpha/beta-value utilized. In conclusion, this method is able to distinguish clinically important differences between treatment plans using a single parameter. This methodology shows promise as an objective tool for analyzing NTCP and doses in larger studies, as the only information needed is the dose volume histogram.

A modelling study of the potential influence of low dose hypersensitivity on radiation treatment planning

BACKGROUND AND PURPOSE

Low dose hyper-radiosensitivity (HRS) has been observed in both normal tissues and tumours. This modelling study explores the possible impact of HRS on radiation treatment planning.

PATIENTS AND METHODS

The interplay between volume-effect and HRS was studied in an idealized comparison of partial versus whole organ irradiation. In the further studies, CT scans of three previously scanned patients were used to estimate normal tissue complication probability (NTCP) for the kidneys after a conformal and a conventional treatment plan with and without consideration of HRS.

RESULTS

Idealized treatment plans were compared as pairs of a conventional and a conformal plan both treating the same target volume to the same dose per fraction. Contour maps of the difference in NTCP between paired plans showed a strong dependence on the magnitude of both the volume effect and the HRS effect. For more clinically realistic treatment plans with NTCP calculated for the kidney, the balance between the sparing due to the LQ effect and the increased sensitivity due to the HRS effect was dependent on both the dose distribution and the fractionation.

CONCLUSIONS

HRS may potentially affect radiotherapy treatment planning and the relative importance of HRS is larger in a tissue or organ with a pronounced volume effect. If HRS is expressed in some normal tissues or organs, this could offset much of the sparing predicted by the LQ formalism. However, in some clinical situations the NTCP calculated with correction for HRS may still be lower than the NTCP calculated from the uncorrected physical doses.

Not all 2 gray radiation prescriptions are equivalent: Cytotoxic effect depends on delivery sequences of partial fractionated doses

PURPOSE

To test whether or not the commonly prescribed daily dose of 2 Gy (whole fraction), when delivered as various partial fraction (PF) dose sequences simulating clinical treatment fields, produces equal biologic effects.

METHODS AND MATERIALS

Eleven actively proliferating cell lines derived from human and animal tissues were used in this study. 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) and clonogenic assays were used to determine the radiation effects on cell proliferation and survival, respectively. The 2 Gy dose was divided into 2 or more PFs for delivery to simulate the delivery of clinical treatment fields. Most irradiation sequences contained two parts consisting of at least 1 small PF, denoted by S which was 0.5 Gy or less, and a large PF, denoted by L which was 1 Gy or more. Irradiation schemes were designed to include the following conditions: (a) the 2 Gy dose divided into combinations of an L-dose and one or more S-doses; (b) the L-dose given either before or after the S-doses; and (c) delivery of all partial fractions within a fixed total time.

RESULTS

Significant differences in biologic effect were observed between sequences in which the L-dose was given before or after the S-doses in both the MTT and clonogenic assays. Nearly all the latter schemes, that is S-L, produced greater cytotoxic effects than the L-S schemes.

CONCLUSIONS

These data demonstrate that the biologic effects of 2 Gy may differ in different clinical settings depending on the size and sequence of the partial fractions. The variation between cytotoxic effects is likely a result of the combination of low-dose hyper-radiosensitivity (HRS) and higher-dose increased radioresistance (IRR) effects established recently. We suggest that to ensure the optimal biologic effect of a prescribed dose of 2 Gy clinically, it is critical to consider the sequence in which the treatment fields are delivered when partial fractions of different sizes are used.