Repair in the mouse lung during low dose-rate irradiation

EANM expert opinion: How can lessons from radiobiology be applied to the design of clinical trials? Part I: back to the basics of absorbed dose-response and threshold absorbed doses

PURPOSE

This study by the EANM radiobiology working group aims to analyze the efficacy and toxicity of targeted radionuclide therapy (TRT) using radiopharmaceuticals approved by the EMA and FDA for neuroendocrine tumors and prostate cancer. It seeks to understand the correlation between physical parameters such as absorbed dose and TRT outcomes, alongside other biological factors.

METHODS

We reviewed clinical studies on TRT, focusing on the relationship between physical parameters and treatment outcomes, and applying basic radiobiological principles to radiopharmaceutical therapy to identify key factors affecting therapeutic success.

RESULTS

The analysis revealed that mean absorbed dose alone is insufficient to predict treatment response or toxicity. For absorbed doses below a certain threshold, outcomes are unpredictable, while doses above this threshold improve the likelihood of biological responses. However, even at higher absorbed doses, response plateaus indicate the need for additional parameters to explain outcome variability, including heterogeneity in target expression, anatomical disease location, (epi)genetics, DNA repair capacity, and the tumor microenvironment, aspects that will be discussed in Part II of this analysis.

CONCLUSION

Understanding radiobiology is crucial for optimizing TRT. More dosimetric data is needed to refine treatment protocols. While absorbed dose is critical, it alone does not determine TRT outcomes. Future research should integrate biological parameters with physical dosimetry to enhance efficacy and minimize toxicity.

Selecting the Most Relevant Mouse Strains for Evaluating Radiation-Induced Multiple Tissue Injury after Leg-Shielded Partial-Body Gamma Irradiation

Animal studies are needed that best simulate a large-scale, inhomogeneous body exposure after a radiological or nuclear incident and that provides a platform for future development of medical countermeasures. A partial-body irradiation (PBI) model using 137Cs gamma rays with hind limb (tibia) shielding was developed and assessed for the sequalae of radiation injuries to gastrointestinal tract, bone marrow (BM) and lung and among different genetic mouse strains (C57BL/6J, C57L/J, CBA/J and FVB/NJ). In this case, a marginal level of BM shielding (∼2%) provided adequate protection against lethality from infection and hemorrhage and enabled escalation of radiation doses with evaluation of both acute and delayed radiation syndromes. A steep radiation dose-dependent body weight loss was observed over the first 5 days attributed to enteritis with C57BL/6J mice appearing to be the most sensitive strain. Peripheral blood cell analysis revealed significant depression and recovery of leukocytes and platelets over the first month after PBI and were comparable among the four different mouse strains. Latent pulmonary injury was observed on micro-CT imaging at 4 months in C57L/J mice and confirmed histologically as severe pneumonitis that was lethal at 12 Gy. The lethality and radiological densitometry (HUs) dose responses were comparable to previous studies on C57L/J mice after total-body irradiation (TBI) and BM transplant rescue as well as after localized whole-thorax irradiation (WTI). Indeed, the lethal radiation doses and latency appeared similar for pneumonitis appearing in rhesus macaques after WTI or PBI as well as predicted for patients given systemic radiotherapy. In contrast, PBI treatment of C57BL/6 mice at a higher dose of 14 Gy had far longer survival times and developed extreme and debilitating pIeural effusions; an anomaly as similarly reported in previous thorax irradiation studies on this mouse strain. In summary, a radiation exposure model that delivers PBI to unanesthetized mice in a device that provides consistent shielding of the hind limb BM was developed for 137Cs gamma rays with physical characteristics and relevance to relatively high photon energies expected from the detonation of a nuclear device or accidental release of ionizing radiation. Standard strains such as C57BL/6J mice may be used reliably for early GI or hematological radiation syndromes while the C57L/J mouse strain stands out as the most appropriate for evaluating the delayed pulmonary effects of acute radiation exposure and recapitulating this disease in humans.

Incidence, Risk Factors, and Outcomes of Idiopathic Pneumonia Syndrome after Allogeneic Hematopoietic Cell Transplantation

Our current knowledge of idiopathic pneumonia syndrome (IPS) predates improved specificity in the diagnosis of IPS and advances in hematopoietic cell transplantation (HCT) and critical care practices. In this study, we describe and update the incidence, risk factors, and outcomes of IPS. We performed a retrospective cohort study of all adults who underwent allogeneic HCT at the Fred Hutchinson Cancer Research Center between 2006 and 2013 (n = 1829). IPS was defined using the National Heart, Lung, and Blood Institute consensus definition: multilobar airspace opacities on chest imaging, absence of lower respiratory tract infection, and hypoxemia. We described IPS incidence and mortality within 120 and 365 days after HCT. We examined conditioning intensity (nonmyeloablative versus myeloablative with high-dose total body irradiation [TBI] versus myeloablative with low-dose TBI) as an IPS risk factor in a time-to-event analysis using Cox models, controlled for age at transplant, HLA matching, stem cell source, and pretransplant Lung function Score (a combined measure of impairment in Forced Expiratory Volume in the first second (FEV₁) and Diffusion capacity for carbon monoxide (DLCO)). Among 1829 HCT recipients, 67 fulfilled IPS criteria within 120 days (3.7%). Individuals who developed IPS were more likely to be black/non-Hispanic versus other racial groups and have severe pulmonary impairment but were otherwise similar to participants without IPS. In adjusted models, myeloablative conditioning with high-dose TBI was associated with increased risk of IPS (hazard ratio, 2.5; 95% confidence interval, 1.2 to 5.2). Thirty-one patients (46.3%) with IPS died within the first 120 days of HCT and 47 patients (70.1%) died within 365 days of HCT. In contrast, among the 1762 patients who did not acquire IPS in the first 120 days, 204 (11.6%) died within 120 days of HCT and 510 (29.9%) died within 365 days of HCT. Our findings suggest that although the incidence of IPS may be declining, it remains associated with post-transplant mortality. Future study should focus on early detection and identifying pathologic mediators of IPS to facilitate timely, targeted therapies for those most susceptible to lung injury post-HCT.

Radiation-induced lung toxicity in mice irradiated in a strong magnetic field

Strong magnetic fields affect radiation dose deposition in MRI-guided radiation therapy systems, particularly at interfaces between tissues of differing densities such as those in the thorax. In this study, we evaluated the impact of a 1.5 T magnetic field on radiation-induced lung damage in C57L/J mice. We irradiated 140 mice to the whole thorax with parallel-opposed Co-60 beams to doses of 0, 9.0, 10.0, 10.5, 11.0, 12.0, or 13.0 Gy (20 mice per dose group). Ten mice per dose group were irradiated while a 1.5 T magnetic field was applied transverse to the radiation beam and ten mice were irradiated with the magnetic field set to 0 T. We compared survival and noninvasive assays of radiation-induced lung damage, namely respiratory rate and metrics derived from thoracic cone-beam CTs, between the two sets of mice. We report two main results. First, the presence of a transverse 1.5 T field during irradiation had no impact on survival of C57L/J mice. Second, there was a small but statistically significant effect on noninvasive assays of radiation-induced lung damage. These results provide critical safety data for the clinical introduction of MRI-guided radiation therapy systems.

Influence of Total Body Irradiation Dose Rate on Idiopathic Pneumonia Syndrome in Acute Leukemia Patients Undergoing Allogeneic Hematopoietic Cell Transplantation

PURPOSE

To determine the relationship between dose rate and other factors in the development of idiopathic pneumonia syndrome (IPS) in patients with acute lymphoblastic leukemia or acute myeloid leukemia who are undergoing total body irradiation (TBI)-based myeloablative conditioning for allogeneic hematopoietic cell transplantation (HCT).

METHODS AND MATERIALS

From 2006 to 2016, 202 patients with acute leukemia (111 acute lymphoblastic leukemia, 91 acute myeloid leukemia) ranging in age from 1 to 57 years (median, 25 years) underwent allogeneic HCT at University of Minnesota. Pretransplantation conditioning included cyclophosphamide (120 mg/kg) with (68%) or without fludarabine (75 mg/m²) followed by 13.2 Gy TBI given in 8 twice-daily fractions of 1.65 Gy over 4 days. Dose rate varied based on linear accelerator availability and ranged from 8.7 to 19.2 cGy/min. Patients were stratified by receipt of high-dose-rate (HDR; >15 cGy/min; 56%) or low-dose-rate (LDR; ≤15 cGy/min; 44%) TBI for all 8 fractions. IPS was defined as pulmonary injury based on clinical symptoms, radiographic evidence, or pulmonary function testing within 100 days of HCT in the absence of concurrent infection.

RESULTS

IPS developed in 42 patients (21%) between 4 and 73 days (median, 16 days) after transplantation. HDR TBI was associated with a higher rate of IPS compared with LDR TBI (29% vs 10%; P < .01). On multiple regression analysis, HDR remained a significant predictor of IPS (hazard ratio, 2.6; 95% confidence interval, 1.2-5.3; P = .01), and this led to inferior 1-year overall survival (60% vs 76%; P = .01) and increased 1-year nonrelapse mortality (28% vs 15%; P = .02).

CONCLUSIONS

TBI dose rates ≤15 cGy/min reduce the risk of posttransplantation IPS and improve overall survival. LDR TBI should be strongly considered as an easily implemented parameter to improve the safety of pretransplantation TBI-based conditioning.

Longitudinal microcomputed tomography-derived biomarkers for lung metastasis detection in a syngeneic mouse model: added value to bioluminescence imaging

With more patients dying from metastasis than from primary cancers, metastasis is a very important area in cancer research. Investigators thereby heavily rely on animal models of metastasis to common organs such as the lung to improve our insight into the pathogenesis and to research novel therapeutic approaches to combat metastasis. In this experimental context, novel tools that allow longitudinal monitoring of lung metastasis in individual animals are highly needed. We have therefore evaluated for the first time microcomputed tomography (μCT) as a very efficient and crossvalidated means to noninvasively and repeatedly monitor metastasis to the lung in individual, free-breathing syngeneic mice. Two individual clones of KLN205 cancer cells were intravenously injected in syngeneic DBA/2 mice and lung metastasis was monitored weekly during 3 weeks using μCT, and was compared with the current gold standard histology and bioluminescence imaging (BLI). μCT enabled us to visualize diffuse tumor morphology and also to extract four different biomarkers that quantify not only tumor load but also aerated space in the lung as a marker of vital lung capacity and potential compensatory mechanisms. Complementary to BLI, applying this novel μCT-based approach enabled us to unravel sensitively and efficiently differences in metastatic potential between two cellular clones. In conclusion, μCT and BLI offer biomarkers that describe different and complementary aspects of lung metastasis, underlining the importance of multimodality follow-up. The added value of μCT findings is important to better assess lung metastasis and host/lung response in preclinical studies, which will be valuable for translational applications.

A survey of changing trends in modelling radiation lung injury in mice: bringing out the good, the bad, and the uncertain

Within this millennium there has been resurgence in funding and research dealing with animal models of radiation-induced lung injury to identify and establish predictive biomarkers and effective mitigating agents that are applicable to humans. Most have been performed on mice but there needs to be assurance that the emphasis on such models is not misplaced. We therefore considered it timely to perform a comprehensive appraisal of the literature dealing with radiation lung injury of mice and to critically evaluate the validity and clinical relevance of the research. A total of 357 research papers covering the period of 1970-2015 were extensively reviewed. Whole thorax irradiation (WTI) has become the most common treatment for studying lung injury in mice and distinct trends were seen with regard to the murine strain, radiation dose, intended pathology investigated, length of study, and assays. Recently, the C57BL/6 strain has been increasingly used in the majority of these studies with the notion that they are susceptible to pulmonary fibrosis. Nonetheless, many of these investigations depend on animal survival as the primary end point and neglect the importance of radiation pneumonitis and the anomaly of lethal pleural effusions. A relatively large variation in survival times of C5BL/6 mice is also seen among different institutions pointing to the need for standardization of radiation treatments and environmental conditions. An analysis of mitigating drug treatments is complicated by the fact that the majority of studies are limited to the C57BL/6 strain with a premature termination of the experiments and do not establish whether the treatment actually prevents or simply delays the progression of radiation injury. This survey of the literature has pointed to several improvements that need to be considered in establishing a reliable preclinical murine model of radiation lung injury. The lethality end point should also be used cautiously and with greater emphasis on other assays such as non-invasive lung functional and imaging monitoring in order to quantify specific pulmonary injury that can be better extrapolated to radiation toxicity encountered in our own species.

Clinical characteristics and dose-volume histogram parameters associated with the development of pleural effusions in non-small cell lung cancer patients treated with chemoradiation therapy

BACKGROUND

To investigate descriptive characteristics and dose metric (DM) parameters associated with development of pleural effusions (PlEf) in non-small cell lung cancer (NSCLC) treated with definitive chemoradiation therapy (CRT).

MATERIALS AND METHODS

We retrospectively assessed treatment records and follow-up imaging of 66 NSCLC patients to identify PlEf formation after CRT. PlEf association between mean heart dose (MHD), mean lung dose (MLD), heart V5-V60 (HV), and lung V5-V60 (LV) were evaluated using Cox Proportional Hazard Models.

RESULTS

A total of 52% (34 of 66 patients) of our population developed PlEf and the actuarial rates at 6 months, 12 months, and 18 months were 7%, 30%, and 42%, respectively. Median time to diagnosis was five months (range 0.06-27 months). The majority of PlEfs were grade one (67%) and developed at a median of four (0.06-13) months, followed by grade two (15%) at a median 11 (5-12) months, and grade three (18%) at a median of 11 (3-27) months. On multivariate analysis, increasing HV5-HV50, LV5-LV50, MHD, and MLD were associated with greater risk of PlEf. Higher grade PlEf was also associated with higher doses of radiation to the heart, while lung DM parameters were not significantly associated with higher PlEf grades. At five-months post-CRT, MHD of 25 Gy was associated with a 100% chance of grade one PlEf, an 82% risk of grade two PlEf, and a 19% risk of grade three PlEf.

CONCLUSIONS

Post-CRT PlEf is common in NSCLC with the majority being grade one. Increasing heart and lung irradiation was associated with increased risk of PlEf. Increasing heart irradiation also correlated with development of increasing grades of PlEf. The impact of potential cardiopulmonary toxicity and resultant PlEfs after CRT requires additional study.

Longitudinal in vivo microcomputed tomography of mouse lungs: No evidence for radiotoxicity

Before microcomputed tomography (micro-CT) can be exploited to its full potential for longitudinal monitoring of transgenic and experimental mouse models of lung diseases, radiotoxic side effects such as inflammation or fibrosis must be considered. We evaluated dose and potential radiotoxicity to the lungs for long-term respiratory-gated high-resolution micro-CT protocols. Free-breathing C57Bl/6 mice underwent four different retrospectively respiratory gated micro-CT imaging schedules of repeated scans during 5 or 12 wk, followed by ex vivo micro-CT and detailed histological and biochemical assessment of lung damage. Radiation exposure, dose, and absorbed dose were determined by ionization chamber, thermoluminescent dosimeter measurements and Monte Carlo calculations. Despite the relatively large radiation dose delivered per micro-CT acquisition, mice did not show any signs of radiation-induced lung damage or fibrosis when scanned weekly during 5 and up to 12 wk. Doubling the scanning frequency and once tripling the radiation dose as to mimic the instant repetition of a failed scan also stayed without detectable toxicity after 5 wk of scanning. Histological analyses confirmed the absence of radiotoxic damage to the lungs, thereby demonstrating that long-term monitoring of mouse lungs using high-resolution micro-CT is safe. This opens perspectives for longitudinal monitoring of (transgenic) mouse models of lung diseases and therapeutic response on an individual basis with high spatial and temporal resolution, without concerns for radiation toxicity that could potentially influence the readout of micro-CT-derived lung biomarkers. This work further supports the introduction of micro-CT for routine use in the preclinical pulmonary research field where postmortem histological approaches are still the gold standard.

Radiation-induced lung injury is mitigated by blockade of gastrin-releasing peptide

Gastrin-releasing peptide (GRP), secreted by pulmonary neuroendocrine cells, mediates oxidant-induced lung injury in animal models. Considering that GRP blockade abrogates pulmonary inflammation and fibrosis in hyperoxic baboons, we hypothesized that ionizing radiation triggers GRP secretion, contributing to inflammatory and fibrotic phases of radiation-induced lung injury (RiLI). Using C57BL/6 mouse model of pulmonary fibrosis developing ≥20 weeks after high-dose thoracic radiation (15 Gy), we injected small molecule 77427 i.p. approximately 1 hour after radiation then twice weekly for up to 20 weeks. Sham controls were anesthetized and placed in the irradiator without radiation. Lung paraffin sections were immunostained and quantitative image analyses performed. Mice exposed to radiation plus PBS had increased interstitial CD68(+) macrophages 4 weeks after radiation and pulmonary neuroendocrine cells hyperplasia 6 weeks after radiation. Ten weeks later radiation plus PBS controls had significantly increased pSmad2/3(+) nuclei/cm(2). GRP blockade with 77427 treatment diminished CD68(+), GRP(+), and pSmad2/3(+) cells. Finally, interstitial fibrosis was evident 20 weeks after radiation by immunostaining for α-smooth muscle actin and collagen deposition. Treatment with 77427 abrogated interstitial α-smooth muscle actin and collagen. Sham mice given 77427 did not differ significantly from PBS controls. Our data are the first to show that GRP blockade decreases inflammatory and fibrotic responses to radiation in mice. GRP blockade is a novel radiation fibrosis mitigating agent that could be clinically useful in humans exposed to radiation therapeutically or unintentionally.

A further comparison of pathologies after thoracic irradiation among different mouse strains: finding the best preclinical model for evaluating therapies directed against radiation-induced lung damage

The human lung is among the most sensitive and critical tissues of concern in localized and systemic radiation exposures, and it is a subject of active preclinical research for evaluating mitigating therapies within the radiation countermeasures program. Our previous study comparing C57BL/6, CBA and C57L mice after whole-thorax irradiation pointed to the problems of late pleural effusions that prevented the full development of lung injury in C57BL/6 mice and suggested that the CBA and C57L strains are more favorable for modeling lung injury in humans (Jackson et al., Radiat. Res. 173, 10-20, 2010). We extended these comparisons to include three other mouse strains (BALB/c, C57BR/J and A/J mice) irradiated with 10, 12.5 or 15 Gy. Most of these mice were unable to survive the first 6 months and presented with a mixture of lung injury and pleural effusions as determined from gross pathology, histology and micro-CT. The independent and varying development of compressive pleural effusions of ill-defined etiology represents a concern for these strains in that they may not satisfy the preclinical requirements for approval of medical countermeasures (e.g. radiation mitigators) for human use. Thus, among the various different mouse strains studied so far for these pathologies, only three (CBA, C3H and C57L) appear to be desirable in exhibiting an early wave of pulmonary dysfunction attributed exclusively to radiation pneumonitis and for further assessment of radioprotective and mitigating therapies. C57L mice are particularly relevant in that they show significant lung damage at lower radiation doses that are closer to what is predicted for humans.

Phantom and cadaver measurements of dose and dose distribution in micro-CT of the chest in mice

BACKGROUND

Micro-computed tomography (CT) allows high-resolution imaging of the chest in mice for small animal research with a significant radiation dose applied.

PURPOSE

To report on measurement of the applied radiation dose using different scan protocols in micro-CT of the chest in mice.

MATERIAL AND METHODS

Repetitive dose measurements were performed for four different micro-CT protocols (with/without respiratory gating) and for micro-CT fluoroscopy used for chest imaging. Measurements were carried out using thermoluminescence dosimeters (TLD) in mouse cadavers and in a PMMA phantom allowing measurement of the radiation dose in the direct path of rays and assessment of scattered radiation.

RESULTS

The dose measured inside and outside the chests of the cadavers varied between 190 und 210 mGy, respectively. The expected mean doses in mice in the direct path of rays for the four examined micro-CT protocols varied between 170 and 280 mGy. The mean values for 1 and 5 minutes of fluoroscopy were 17 mGy and 105 mGy, respectively.

CONCLUSION

The measured dose values are similar to the dose values for micro-CT of the chest reported so far. A relevant dose can be delivered by micro-CT of the chest, which could possibly interact with small animal studies. Therefore, the applied dose for a specific protocol should be known and adverse radiation effects be considered.

Revisiting strain-related differences in radiation sensitivity of the mouse lung: recognizing and avoiding the confounding effects of pleural effusions

The mouse has been used extensively to model radiation injury to the lung, a major dose-limiting organ for radiotherapy. Substantial differences in the timing and sensitivity of this tissue between mouse strains have been reported, with some strains, including C57BL/6, being designated as "fibrosis-prone". Pleural effusions have also been reported to be a prominent problem in many mouse strains, but it remains unclear how this affects the lung function and survival of the standard C57BL/6 mouse. The purpose of this investigation was to re-evaluate this strain in comparison with C57L and CBA mice after whole-thorax irradiation at doses ranging from 10 to 15 Gy. Breathing rate measurements, micro-computerized tomography, lung tissue weight, pleural fluid weight and histopathology showed that the most prominent features were an early phase of pneumonitis (C57L and CBA) followed by a late incidence of massive pleural effusions (CBA and C57BL/6). A remarkable difference was seen between the C57 strains: The C57L mice were exquisitely sensitive to early pneumonitis at 3 to 4 months while C57BL/6 mice showed a delayed response, with most mice presenting with large accumulations of pleural fluid at 6 to 9 months. These results therefore caution against the routine use of C57BL/6 mice in radiation lung experiments because pleural effusions are rarely observed in patients as a consequence of radiotherapy. Future experiments designed to investigate genetic determinants of radiation lung damage should focus on the high sensitivity of the C57L strain (in comparison with CBA or C3H mice) and the possibility that they are more susceptible to pulmonary fibrosis as well as pneumonitis.

Hematopoietic Progenitor Stem Cell Homing in Mice Lethally Irradiated with Ionizing Radiation at Differing Dose Rates

It has recently been shown that specific lineage-depleted murine hematopoietic stem cells that home to the bone marrow 2 days after transplantation of ablated primary recipients are capable of long-term engraftment and repopulation of secondary recipients. We were interested in determining whether the rate at which the ablating radiation dose was delivered to the mice affected the homing of lineage-depleted stem cells to the bone marrow and/or sites of tissue damage. Fractionated, lineage-depleted donor marrow cells were isolated and labeled with the membrane dye PKH26. Recipient mice were lethally irradiated with 11 Gy ionizing radiation using varying dose rates and were immediately injected with PKH26-labeled progenitor stem cells. With the exception of the lowest dose-rate group, all irradiated mice had an approximately fivefold (P = 0.014 to 0.025) reduction in stem cell homing to the bone marrow compared to unirradiated control animals. A fivefold reduction of stem cell homing to the spleen compared to unirradiated animals was also observed, though this was not statistically significant for any dose-rate group (P = 0.072 to 0.233). This difference in homing could not be explained by increased stem cell apoptosis/necrosis or non-marrow tissue homing to the intestine, lung or liver. We show that the dose rate at which a lethal dose of total-body radiation is delivered does not augment hematopoietic progenitor stem cell homing to the bone marrow, spleen or sites of early radiation-mediated tissue damage at either 2 or 5 days postirradiation/transplantation. The observation that greater homing was seen in unirradiated control mice calls into question the concept that adequate bone marrow stem cell homing requires radiation-induced "space" to be made in the marrow, certainly for the enriched early progenitor hematopoietic stem cells used for this set of experiments. Further experiments will be needed to determine whether these homed cells are as capable of giving rise to long-term engraftment/repopulation of the marrow of secondary recipients as they are in irradiated recipients.

Tolerance of rat spinal cord to continuous interstitial irradiation

PURPOSE

To study the kinetics of repair in rat spinal cord during continuous interstitial irradiation at different dose rates and to investigate the impact of a rapid dose fall off over the spinal cord thickness.

MATERIAL AND METHODS

Two parallel catheters were inserted on each side of the vertebral bodies from the level of T10 to L4. These catheters were afterloaded with two 192Ir- wires of 4 cm length each (activity 1-10 mCi/cm) or connected to the HDR- microSelectron. Experiments have been carried out to obtain complete dose response curves at 7 different dose rates: 0.53, 0.90, 1.64, 2.56, 4.4, 9.9 and 120 Gy/h. Paralysis of the hindlegs after 5 - 6 months and histopathological examination of the spinal cord of each animal were used as experimental endpoints.

RESULTS

The distribution of the histological damage was a good reflection of the rapid dose fall - off over the spinal cord, with white matter necrosis or demyelination predominantly seen in the dorsal tracts of the spinal cord or dorsal roots. With each reduction of the dose rate, spinal cord tolerance was significantly increased, with a maximum dose rate factor of 4.3 if the dose rate was reduced from 120 Gy/h to 0.53 Gy/h (ED50 of 17.3 Gy and 75.0 Gy, respectively). Estimates of the repair parameters using different types of analysis are presented. For the direct analysis the best fit of the data was obtained if a biexponential function for repair was used. For the 100% dose prescribed at the ventral side of the spinal cord the alpha/beta ratio is 1.8 Gy (0.8 - 2.8) and two components of repair are observed: a slow component of repair of 2.44 h (1.18 - infinity) and a fast component of 0.15 h (0.02 - infinity). The proportion of the damage repaired with the slow component is 0.59 (0.18 - 1). For the maximum of 150% of the prescribed dose at the dorsal side of the spinal cord the alpha/beta ratio is 2.7 Gy (1.5 - 4.4); the two components for the kinetics of repair remain the same.

CONCLUSIONS

Spinal cord radiation tolerance is significantly increased by a reduction in dose rate. Depending on the dose prescription, the alpha/beta ratio is 1.8 or 2.7 Gy for the 100% and 150% of the reference dose (rate), respectively; for the kinetics of repair a biphasic pattern is observed, with a slow component of 2.44 hours and a fast component of 0.15 hours, which is independent of the dose prescription.

Radiation biology of lung cancer

The enormous problem that is lung cancer still defies satisfactory therapeutic strategy. This article summarizes some of the more important laboratory efforts directed at understanding the biology of this complex disease. The radiation sensitivities of established lung cancer cell lines are outlined. The effect of radiation dose rate and chemotherapy is explored. The emerging biology of oncogenetic alterations is explored as it relates to radiation sensitivity in general, and lung cancer in particular. Finally, novel therapeutic approaches including photodynamic therapy are introduced.

Radiation-induced lung damage in rats: the influence of fraction spacing on effect per fraction

PURPOSE

When the linear-quadratic model is used to predict fractionated treatments which are isoeffective, it is usually assumed that each (equal size) treatment fraction has an equal effect, independent of the time at which it was delivered during a course of treatment. Previous work by our group has indicated that this assumption may not be valid in the context of radiation-induced lung damage in rats. Consequently we tested directly the validity of the assumption that each fraction has an equal effect, independent of the time it is delivered.

METHODS AND MATERIALS

An experiment was completed in which fractionated irradiation was given to whole thoraces of Sprague-Dawley rats. All treatment schedules consisted of eleven equal dose fractions in 36 days given as a split course, with some groups receiving the bulk of the doses early in the treatment schedule, before a 27-day gap, and others receiving most of the dose toward the end of the treatment schedule, after the time gap. To monitor the incidence of radiation-induced damage, breathing rate and lethality assays were used.

RESULTS

The maximum differences in the LD50s and breathing rate ED50s for the different fractionation schedules were 4.0% and 7.7% respectively. The lethality data and breathing rate data were consistent with results expected from modelling using the linear-quadratic model with the inclusion of an overall time factor, but not the generalized linear-quadratic model which accounted for fraction spacing.

CONCLUSION

For conventional daily fractionation, and within the range of experimental uncertainties, the results indicate that the effect of a treatment fraction does not depend on the time at which it is given (its position) in the treatment. The results indicate no need to extend isoeffect formulae to consider the effect of each fraction separately for radiation-induced lung damage.

Compromising effect of low dose-rate total body irradiation on allogeneic bone marrow engraftment

The protraction of total body irradiation (TBI) to a continuous low dose-rate has been investigated for its effect on donor marrow engraftment in murine bone marrow transplant (BMT) models of varying histocompatibility. Three different BMT combinations were used: syngeneic [B6-Gpi-1a-->B6-Gpi-1b], H-2 compatible allogeneic [BALB.B (H-2b)-->B6 (H-2b)] and H-2 mismatched allogeneic [BALB/c (H-2d)-->B6 (H-2b)]. TBI was delivered over a range of doses at either a high (HDR, 40 cGy/min) or low (LDR, 2 cGy/min) dose rate followed by infusion of 10(7) bone marrow cells from syngeneic or allogeneic donors. The level of donor (Gpi-1a) engraftment was determined from blood Gpi-typing at different times after TBI and BMT. Radiation dose-response relationships corresponding to long-term haemopoietic engraftment at 20 weeks showed a dose-sparing effect of LDR that became more prominent with increasing genetic disparity between donor and host. For fully allogeneic (H-2 incompatible) BMT, a dose as high as 16Gy LDR was still not sufficient for achieving chimerism in all recipients. In many cases allogeneic BMT gave transient blood chimerism enabling the recipient to survive the acute effects of high dose TBI with full long-term repopulation from surviving stem cells of the host. Radiation cell survival curves were obtained for the frequency of alloreactive precursors of proliferating T-lymphocytes (pPTL) remaining in the spleen at 1 day after TBI. A radiation dose-sparing effect of LDR was also found for pPTL depletion. These data suggest that radiation damage repair during LDR irradiation in an immunocyte target cell population is mainly responsible for enhanced graft rejection thus rendering protracted TBI less effective for application in clinical allogeneic BMT.

Radioimmunotherapy: clinical results and dosimetric considerations

Radiolabeled antibodies for cancer therapy are being investigated in clinical trials in more than 30 centers. 131Iodine-labeled antibody (Ab) therapy of solid tumors has produced few responses when given alone. When given in conjunction with chemotherapy and external beam therapy in hepatoma patients, objective responses have occurred. Because of the short range of 131I, 90Y and 186Re are being studied and objective responses have occurred in patients without the addition of other therapies. 131I-labeled Ab therapy of lymphoma, a radioresponsive tumor, has produced a much higher objective response rate than in other solid tumors. Regional RIT has not been shown to offer a definite advantage over the intravenous route. Tumor doses have generally been less than 2000 cGy per treatment with some tumors receiving higher doses. The bone marrow is the dose-limiting organ for RIT and marrow cryopreservation with subsequent reinfusion may prove useful.

Recovery kinetics of X-ray damage in mouse skin: the influence of dose per fraction

The rate of recovery from radiation damage, as a function of dose per fraction, was investigated in mouse skin. Two different experimental designs were used, both incorporating the neutron top-up technique which enables the X-ray dose per fraction to be kept constant whilst changing the interfraction interval. Either equally spaced X-ray fractions (concertina design) or single or multiple pairs of X-ray doses (single and multiple split-dose designs) were given at varying intervals, followed by graded doses of neutrons. A wide range of X-ray doses per fraction were investigated (from 1 to 10.5 Gy) and the data were analysed using the Thames Incomplete Repair (IR) model modified for use with neutron top-up doses. Analyses of the data, obtained from five different experiments, indicate that the rate of recovery from radiation damage is significantly faster at doses per fraction between 1 and 4.4 Gy than at 10.5 Gy. These data appear not to support the assumption, made by most recovery models, that the rate of recovery is independent of dose.

Isoeffect relationships for fractionated biologically targeted radiotherapy

This paper describes a method of analysis of the biological effects on normal tissues of fractionated administrations of biologically targeted radiotherapy (BTR). The linear-quadratic (LQ) model as extended by Dale [2] is used to consider the case in which administrations may be separated by time gaps down to the order of a single day. It is assumed that the pharmacokinetics of clearance are linear and that dose-rate profiles in organs are simple exponential decays. The method adopted is to calculate the extrapolated response doses (ERDs) for individual time periods of the treatment between one administration and the next (assuming complete recovery between periods) and additional components which are corrections for incomplete recovery between these time periods. The overall ERD for the course of administrations is given by the sum of these factors. No account is taken of cellular repopulation. As it is likely that fractionated biologically targeted radiotherapy (BTR) will be used in practice, this subject is of clinical relevance. The method is illustrated by a numerical example.

Inhaled thiol and phosphorothiol radioprotectors fail to protect the mouse lung

Pretreatment with an inhaled aerosol of WR 2721, cysteamine or N-acetylcysteine failed to influence the development of radiation-induced lung damage at 14-16 weeks as assessed by a rise in breathing rate in the mouse. In contrast, intraperitoneal WR 2721 400 mg/kg and cysteamine 230 mg/kg were effective indicating that, unlike N-acetylcysteine, they are effective radioprotectors if correctly delivered.

Radiobiological aspects of continuous low dose-rate irradiation and fractionated high dose-rate irradiation

The biological effects of continuous low dose-rate irradiation and fractionated high dose-rate irradiation in interstitial and intracavitary radiotherapy and total body irradiation are discussed in terms of dose-rate fractionation sensitivity for various tissues. A scaling between dose rate and fraction size was established for acute and late normal-tissue effects which can serve as a guideline for local treatment in the range of dose rates between 0.02 and 0.005 Gy/min and fraction sizes between 8.5 and 2.5 Gy. This is valid provided cell-cycle progression and proliferation can be ignored. Assuming that the acute and late tissue responses are characterised by alpha/beta values of about 10 and 3 Gy and a mono-exponential repair half-time of about 3 h, the same total doses given with either of the two methods are approximately equivalent. The equivalence for acute and late non-hemopoietic normal tissue damage is 0.02 Gy/min and 8.5 Gy per fraction; 0.01 Gy/min and 5.5 Gy per fraction; and 0.005 Gy/min and 2.5 Gy per fraction. A very low dose rate, below 0.005 Gy/min, is thus necessary to simulate high dose-rate radiotherapy with fraction sizes of about 2 Gy. The scaling factor is, however, dependent on the repair half-time of the tissue. A review of published data on dose-rate effects for normal-tissue response showed a significantly stronger dose-rate dependence for late than for acute effects below 0.02 Gy/min. There was no significant difference in dose-rate dependence between various acute non-hemopoietic effects or between various late effects. The consistent dose-rate dependence, which justifies the use of a general scaling factor between fraction size and dose rate, contrasts with the wide range of values for repair half-time calculated for various normal-tissue effects. This indicates that the model currently used for repair kinetics is not satisfactory. There are also few experimental data in the clinical dose-rate range, below 0.02 Gy/min. It is therefore necessary to verify further the presented scaling between fraction size and dose rate.

Tumor and normal tissue tolerance for fractionated low-dose-rate radiotherapy

Radiobiological evidence suggests that an improved therapeutic ratio might be achieved through the use of smaller than conventional dose fractions. The ultimate in small dose fractions for external beam radiotherapy would be fractionated low-dose-rate (LDR) irradiation, and clinical trials of fractionated external beam LDR therapy are already in progress. Using the BA1112 rat sarcoma, we have determined the 50% tumor control dose for LDR and for conventional-dose-rate (CDR) fractionated radiotherapy. These tumor control doses were compared to normal tissue tolerance doses for hemi-body irradiation in similar CDR and LDR schedules. Animals were treated 3 times per week without anesthesia using 10-19 fractions. LDR radiotherapy was done with 60Co at dose rates of 0.028-0.033 Gy/min; CDR radiotherapy was done with 250 kVp X rays at dose rates of 0.54-2.1 Gy/min. The tumor control dose modification factor (DMF) for LDR compared to CDR irradiation was 1.3 (1.0-1.5). For LDR and CDR hemi-body irradiation, the dose modification factor for 7 day lethality (gastrointestinal damage) was 1.7 (1.5-1.9), for 100 day morbidity was 1.8 (1.6-2.2), and for radiation nephritis at 90 days was 1.9 (1.7-2.3). The therapeutic gain factor for fractionated low-dose-rate irradiation compared to conventional-dose-rate fractionated radiotherapy was therefore 1.8/1.3 = 1.4 (1.2-1.8). The study shows that there is an experimental as well as a theoretical basis for anticipating a therapeutic benefit from the use of external beam fractionated LDR radiotherapy, and implies that the recognized therapeutic efficacy of brachytherapy is not due solely to the high localized dose.

Kinetics of recovery from sublethal radiation damage in four murine tumors

The kinetics of repair of sublethal radiation damage (SLD) was studied in four transplantable C3H mouse tumors, i.e. mammary carcinoma AT17, fibrosarcoma SSK2, and squamous cell carcinomas AT51 and AT478. Tumors were irradiated with 4 fractions of 300 kV X-rays given under local hypoxia at intervals ranging from 0 to 6 h. Radiation response was measured by growth delay, which was directly analyzed using a general curve description based on the extended linear-quadratic model (exponential repair kinetics). In contrast to existing methods all growth delay values were utilized to estimate the alpha/beta ratios and the half-times as well as their confidence limits in a non-linear least squares analysis. The half-times were 42, 44, 54 and 31 min, respectively. It is concluded that repair of SLD is virtually complete after 5 h in these tumors. This is also due to the relatively small proportion of repairable damage in these tumors reflected in their alpha/beta values, which were 38, 30, 54 and 42 Gy, respectively.

Early and late effects of fractionated irradiation and the kinetics of repair in rat lung

The thorax of WAG/Rij rats was irradiated with fractionated doses of X rays. Irradiation schedules were designed either to allow virtually complete repair of sublethal damage between subsequent fractions by fractionating at 6-h intervals, or to result in incomplete repair by allowing only 1-h intervals between subsequent fractions. Combination of the data from both experimental series permitted the calculation of alpha/beta ratios and values for the repair halftime T1/2. The animals were monitored by assessment of the breathing frequency and by recording deaths. At the end of the experiments, 18 months after treatment, the hydroxyproline content of the lung tissue was determined as a biochemical indicator of radiation-induced fibrosis, and an histopathological analysis was performed. Early endpoints, indicative of radiation-induced pneumonitis, resulted in an alpha/beta ratio of 3.5 Gy and a T1/2 value of 0.95 h. Late endpoints were presumed to be indicative of radiation-induced fibrosis. Based on the combined analysis of data from three different late endpoints, the mean alpha/beta ratio was 2.3 Gy, and the T1/2 value was 1.13 h. The difference in alpha/beta ratio and T1/2 value between early and late endpoints was not significant, since the 95% confidence limits were overlapping. For each individual early or late endpoint as well as for the two early or the three late endpoints combined, there was a trend for lower alpha/beta ratios and higher T1/2 values associated with low doses per fraction. However, widely overlapping confidence limits indicated that again the differences were not significant.

Effect of X-irradiation on the stomach of the rat

A model for localized 300 kV X-irradiation of the rat stomach was developed. After irradiation with single doses, three distinct gastric disorders were observed which occurred at different latency times. Acute death 2-3 weeks after irradiation was caused by an erosive and ulcerative gastritis and occurred in all animals given 28.5 Gy without diet, in 17% of the animals given 28.5 Gy plus diet, and in 13% of the animals given 23 Gy. Subacute to chronic fatal disorders 4 weeks to 7 months after irradiation were seen as stomach dilatation and gastroparesis, associated with the replacement of the normal gastric mucosa by a hyperkeratinized multilayered squamous epithelium. These disorders occurred in 40-100% of the animals after doses between 16 Gy and 28.5 Gy (+diet). An ED 50 value of 19.2 Gy (16.5-21.2 Gy, 95% confidence interval) was calculated for this gastroparesis. Late gastric obstruction exceeding 7 months after irradiation was seen in the rats because of profound changes in the gastric wall in 13-18% of the animals after doses between 23 Gy and 14 Gy. In animals surviving these three periods, an atrophic mucosa and intestinal metaplasia developed. From functional and morphohistological studies, it can be concluded that there are differences in the pathogenesis of the fatal radiation damage for each of these periods after irradiation.

Total body irradiation in acute myeloid leukemia and chronic myelogenous leukemia: influence of dose and dose-rate on leukemia relapse

From June 1981 to March 1987, 106 patients--59 with acute myeloid leukemia (AML) and 47 with chronic myelogenous leukemia (CML)--were treated with Cyclophosphamide 60 mg/kg X 2 d and total body irradiation (TBI-990 cGy/3fr/3d described dose) before allogeneic bone marrow transplantation. Seventy-nine patients are evaluable for risk of relapse: 32 with chronic myelogenous leukemia (23 in first chronic phase, 9 in accelerated phase) and 47 with acute myeloid leukemia (38 in first complete remission, 9 in subsequent phases). Actual TBI doses delivered to these patients varied between 839 and 1250 cGy (mean 956 +/- 101)/3 fr/3d, with dose rates between 2.7 and 7.25 cGy/min (mean 4.2 +/- 1.8). Patients receiving high (greater than 990 cGy) and low (less than or equal to 990) dose and/or dose rate (greater than 4 cGy/min and less than or equal to 4, respectively) have been evaluated overall and stratified by type of leukemia and phase of disease. When the patients are considered altogether, high total dose is significantly correlated with decreased risk of relapse (p = 0.0005) as well as high dose rate (p = 0.03). When considering specific subgroups, the influence of total dose on relapse rate is evident both for "early" and "advanced" leukemias, while an impact of dose rate appears only for chronic myelogenous leukemia in 1st chronic phase. Pertinent radiobiological and clinical literature is reviewed, and a possible role of dose fractionation and dose rate in leukemic control rate is evidenced; in this TBI setting, total dose not less than 990 cGy/3fr/3d and dose rate not less than 4 cGy/min have to be guaranteed.

Late tissue-specific toxicity of total body irradiation and busulfan in a murine bone marrow transplant model

Total body irradiation (TBI) and busulfan were compared for late effects in a murine model of bone marrow transplantation (BMT). Male C57BL/6 mice were given fractionated TBI or busulfan given in 4 equal daily doses followed by infusion of 10(7) syngeneic bone marrow cells. Total doses of 16.4 Gy TBI and 3.4 mg busulfan were chosen for their equivalence in inducing near complete engraftment of allogeneic marrow from donor mice of the LP strain. The two treatment groups had a late wave of mortality starting at about 80 weeks after transplantation. Specific tissue damage was manifested in bone marrow stem cells, splenic T-cell precursors, hair greying and cataract formation for both TBI and busulfan but to varying degrees. Severe nephrotoxicity and anemia were observed only after TBI. Although both busulfan and TBI kill early marrow stem cells and are effective preparative agents in bone marrow transplantation, their effects on other stem cell and organ systems are not similar. In addition, many of the injuries seen are late to occur. The delayed expression of injury deserves careful long-term evaluation of BMT recipients before the therapeutic potential of effective preparative regimens can be fully appreciated.

Repair kinetics of mouse lung

Pathological manifestations of lung damage after irradiation support the use of death between 11 weeks (80 days) and 23 weeks (160 days) as an assay for pneumonitis in the C3Hf strain of mice. The proportion of deaths and the time to reach a specific percent of lethality were found to be dose and dose per fraction-related. The median survival time and the latency period for the time of occurrence of pneumonitis, were dependent on total biological dose over a narrow range, equivalent to "normalized" doses of 36-41 Gy in 2 Gy fractions. All these criteria entered into the analysis of data. The linear-quadratic isoeffect model gives a good fit for the results over a range of doses from 1.6 to 8 Gy per fraction, that is, with the exclusion of the single dose data. The value of alpha/beta was always approximately 4.0 Gy from three kinds of analyses. The half-time for repair from sublethal injury was approximately 0.75 h. Both of these values are in agreement with the results from other investigators.

Expression of the dose rate effect in clinical curietherapy

The role of the dose rate on the biological effect can be assessed by a simple formula involving only two parameters related to the repair ability (alpha/beta) and to the repair kinetics (repair time constant t or repair half time Tr). The result of the computation provides a value in which the dose rate is taken into account along with the physical dose. It can be expressed by: the ERD (extrapolated response dose), the equivalent dose at a constant dose rate used as a reference, or by the equivalent dose for a conventionally fractionated dose (5 x 2 Gy/week). The computation permits a description of the treatment by the ERD distribution or the "equivalent dose" distribution which is more significant than the dose distribution. It expresses the relative decline in the effect in the low dose region, and the relative increase in the high dose region which are related to the difference in dose rate: the quantitative comparison of different treatments with identical dose distribution delivered in different times; a rational timing of a partial removal of the sources when a uniform treatment time would result in an overdosage of a part of the treated volume. The computation has been used for an analysis of the conflicting clinical data on the variation of the isoeffect dose as a function of the treatment time. The basic radiobiological mechanisms implied in the method of computation and the values of the parameters which have been used are discussed.

Repair kinetics as a determining factor for late tolerance of central nervous system to low dose rate irradiation

The effect of continuous irradiation, delivered at four different dose rates (107.6, 14.7, 3.9 and 2 Gy.h-1) has been investigated using the rat cervical spinal cord biological system. The endpoint was the induction of foreleg paralysis at 9 months which corresponds, as has been described before, to white matter necrosis. Paralysis occurring in 50% of the animals was taken as the isoeffect, and the ED50 (radiation dose leading to paralysis in 50% of the animals) was calculated by probit analysis. There was a constant increase in the ED50 with the decrease in the dose rate, resulting from the repair of sublethal damage (SLD) occurring during irradiation. A comparison was made with the previously published results of high dose rate (100-120 Gy.h-1) fractionated irradiations (2, 4 and 10 fractions). alpha/beta (1.6 Gy for the pooled fractionation and dose rate data) and the half-time of SLD repair (82 min) were derived.

Repair kinetics in tissues: alternative models

Monoexponential repair kinetics is based on the assumption of a single, dose-independent rate of repair of sublethal injury in the target cells for tissue injury after exposure to ionizing radiation. Descriptions of the available data based on this assumption have proved fairly successful for both acutely responding (skin, lip mucosa, gut) and late-responding (lung, spinal cord) normal tissues. There are indications of biphasic exponential repair in both categories, however. Unfortunately, the data usually lack sufficient resolution to permit unambiguous determination of the repair rates. There are also indications that repair kinetics may depend on the size of the dose. The data are conflicting on this account, however, with suggestions of both faster and slower repair after larger doses. Indeed, experiments that have been explicitly designed to test this hypothesis show either no effect (gut, spinal cord), faster repair after higher doses (lung, kidney), or slower repair after higher doses (skin). Monoexponential repair appears to be a fairly accurate description that provides an approximation to a more complicated picture, the elucidation of whose details will, however, require very careful and extensive experimental study.

The influence of dose per fraction on repair kinetics

The use of multiple fractions per day (MFD) in radiotherapy requires information about the rate of repair of radiation injury. It is important to know the minimum interval between fractions necessary for maximum sparing of normal tissue damage, whether rate of repair is dependent on the size of dose per fraction and if it is different in early and late responding tissues and in tumours. To address these questions, the rate of repair between radiation dose fractions was measured in mouse skin (acute damage), mouse kidney (late damage) and a mouse tumour (carcinoma NT). Skin and kidney measurements were made using multiple split doses of X-rays, followed by a neutron top-up. For skin, faster recovery was obtained with 4.4 Gy fractions (t1/2 = 1.29 +/- 0.35 h, 95% CL) than with 10.5 Gy fractions (t1/2 = 3.46 +/- 0.88 h). In contrast, kidney showed slower recovery at a low dose per fraction of 2 Gy (t1/2 = 1.69 +/- 0.39 h) than at a higher dose of 7 Gy per fraction (t1/2 = 0.92 +/- 0.1 h). These data show that repair rate is dependent on the size of dose per fraction, but not in a simple way. T1/2 values now available for many different tissues generally lie in the range of 1-2 h, and are not correlated with proliferation status or early versus late response to treatment. At the doses used currently in clinical MFD treatments, these data indicate that damage in almost all normal tissues would increase if interfraction intervals less than 6 h were used.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of single doses of gamma-radiation on pig lung

Using a 133Xe wash-out technique changes in the gas exchange capacity of the pig lung have been studied after irradiation with a range of single doses of 60Co gamma-rays. Lung function was assessed for periods up to 104 weeks after irradiation. It was found that the gas exchange capacity of the lung was impaired as early as 4 weeks after irradiation. After a dose of 9 Gy the initial impairment in lung function was resolved within 13 weeks, while after 14.7 Gy damage persisted. The degree of impairment in lung function during the relatively early pneumonitic and late fibrotic phases was indistinguishable and suggested that the late functional impairment was a continuation of the earlier damage. The results of the lung function tests were converted into quantal data and ED50 values of 9.68 +/- 0.3 Gy for early (4-26 weeks) and 9.73 +/- 0.34 Gy for late (39-104 weeks) damage were obtained. The ED50 value for fibrosis and focal scarring based on the histological assessment, 104 weeks after irradiation, was 11.12 +/- 0.9 Gy. The differences in ED50 values were not significant.

Repair kinetics in mouse lung after multiple X-ray fractions per day

The potential advantage for sparing normal tissue damage by hyperfractionation of low-LET radiation may be limited by the repair kinetics of tissues in the irradiated field. Tissues with slow repair kinetics will limit the number of fractions that may be given on the same day. Results are presented for mouse lung treated with a range of doses per fraction using either two or three fractions per day in multiple X-ray fractionation schedules. The results are analysed to determine whether the repair kinetics follow a single exponential function of time. The calculated repair rate (T1/2) was about 1.2 h for two fractions per day of 2 Gy (10F/5d) but slightly less (T1/2 = 0.8 h) for two fractions of 9 Gy (2F/1d). For smaller doses per fraction of 1.1 Gy, given three times per day (39F/13d), the T1/2 was not significantly less (T1/2 = 0.3-0.7 h). For three fractions per day of 1.1 Gy per fraction an unsatisfactory fit is achieved using a single exponential function of time, and a better fit is obtained using two components of repair. The repair kinetics are slow for lung, in comparison to acute reacting tissues (except skin), and may require that 6-8 h (i.e. four or five half-times) should be allowed between fractions on the same day so that more than 95 per cent of the repairable dose is repaired. At present the variation in repair kinetics with doses per fraction between 1.1 and 9 Gy are not significantly different, so no reduction of interfraction interval should be proposed.

Late effects of fractionated pi-mesons compared to X rays on mouse lung

Early and late delayed effects of up to 20 fractions of pions and X rays were investigated in the mouse lung. The whole thorax of female CD-1 mice was irradiated under Ethrane/O2 anesthesia. Respiration rate was measured by whole body plethysmography at biweekly to monthly intervals. With signs of irreversible respiratory distress, animals were sacrificed and their lungs evaluated histologically. In addition to the effect of fractionation, the influence of dose-rate and anesthesia was studied as well. The degree of injury for the most predominant lesions (macrophage accumulation, fibrosis, vascular congestion) was scored, and the correlation with the relative change in respiratory rate and survival was analyzed. This analysis showed the primary lesion to be radiation pneumonitis at a median survival time of approximately 100 days. Focal fibrosis was observed to occur soon thereafter, and no evidence was obtained for an independent second wave of fibrotic injury. Fibrosis seemed primarily the result of pathological organization in areas with heavy concentration of macrophages. It was observed that the mice were unusually sensitive, with a single dose X ray ED50/180 of 8.8 Gy. A similar value was found for unanesthetized mice. This might have been the result of performing these studies at an altitude of 2100 m. The fractionation effect also seemed more pronounced, with alpha/beta values of 0.6 Gy for X rays and 4 Gy for pions, which is significantly lower compared to reported values. At the pion dose-rate of 0.25 Gy.min-1, RBE values for single doses were 0.9 when compared to high dose-rate X rays, and 1.36 at equivalent dose rates. This clearly shows that significant repair occurs during the relatively low dose-rate pion irradiations. With smaller doses per fraction, the dose-rate effect became less dominant, and for 20 fractions of pions the RBE was 1.4 compared to fractionated high dose-rate X rays. These RBE's are similar to values reported for acute effects in skin.

Dose-limiting complications from upper half body irradiation in C3H mice

This investigation evaluates the usefulness of an experimental technique in mice that has been used to study lung tolerance as a major dose-limiting tissue in clinical radiotherapy. The pathological sequalae of upper half body irradiation using a range of single and fractionated (2 Gy per fraction once or twice daily) doses was characterized in C3H/HeJ mice. Four phases of potentially lethal syndromes were revealed starting with the very acute effects of oral inflammation within 1 month. Incisor damage occurred between 1 and 3 months when the supplying of powdered food appeared to prevent lethality from starvation. Single radiation doses then produced a predictable incidence of pneumonitis (3 to 6 months) followed by pleural effusions (6 to 12 months). These later two syndromes were absent in mice that survived the acute effects of fractionated UHBI. In accordance with other rapidly proliferating tissues, the estimated alpha/beta ratios for oral epithelial and incisor damage were notably larger than that previously reported for lung. This denotes the smaller capacity of the acute responding target cells to repair sublethal damage. The consequent predominance of acute reactions in the fractionated courses therefore confined the maximal tolerated dose to that which produced an unmeasurable level of pulmonary injury. Our discussion of these results warn against the simple extrapolation of fractionated or low dose rate UHBI lethality data from mouse to man without due consideration of extrapulmonary radiation effects.

Possible dose rate dependence of recovery kinetics as deduced from a preliminary analysis of the effects of fractionated irradiations at varying dose rates

Data relating to jejunal crypt survival following fractionated total body irradiation of C3H mice have been subjected to a preliminary analysis using the linear-quadratic formalism. Because the regimes considered involved fairly closely-spaced fractions, delivered at dose rates in the range 1.2-72 Gy h-1, it was necessary to further develop previously established forms of the model in order to carry out the evaluation. The magnitude of the recovery half-life has been deduced for each of the regimes and found to be in the approximate range 0.1-0.6 h. For any given fraction number, the half-life appears to be markedly dependent on dose rate, increasing with dose rate up to approximately 25 Gy h-1, falling off thereafter. Tentative reasons for this behaviour are given, together with an assessment of the possible shortcomings of the analytical method.

The kinetics of repair in mouse lung after fractionated irradiation

The kinetics of repair of sublethal damage in mouse lung was studied after fractionated doses of 137Cs gamma-rays. A wide range of doses per fraction (1.7-12 Gy) was given with interfraction intervals ranging from 0.5 to 24 h. The data were analysed by a direct method of analysis using the incomplete repair model. The half-time of repair (T1/2) was 0.76 h for the pneumonitis phase of damage (up to 8 months) and 0.65 h for the later phase of damage up to 12 months. The rate of repair was dependent on fraction size for both phases of lung damage and was faster after large dose fractions than after small fractions. The T1/2 was 0.6 h (95 per cent c.1. 0.53, 0.69) for doses per fraction greater than 5 Gy and 0.83 h (95 per cent c.1 0.76, 0.92) for doses per fraction of 2 Gy. Repair was nearly complete by 6 h, at least for the pneumonitis phase of damage. To the extent that extrapolation of these data to humans may be valid, these results imply that treatments with multiple fractions per day that involve the lung will not be limited by the necessity for interfraction intervals much longer than 6 h.

Effects of fractionated schedules of irradiation combined with cis-diamminedichloroplatinum II on the SCCVII/St tumor and normal tissues of the C3H/KM mouse

The interaction of cis-diamminedichloroplatinum II (c-DDP) and a course of 5 daily irradiations was investigated in the SCCVII/St tumor and normal tissues (duodenal crypt cells and lung) of the C3H mouse. Two schedules with daily doses of 2.4 mg/kg c-DDP given immediately before 4 Gy X ray on 5 consecutive days and a single 12 mg/kg c-DDP dose followed 24 hr later by the first of 5 daily 4 Gy X ray doses produced the most consistent and significant supra-additive effects on the SCCVII tumor. This supra-additive effect was also achieved with lower and much less toxic drug doses. These schedules produced high enhancement ratios (dose effect factors DEF) for mouse duodenal crypt cells, but the degree of enhancement was less than that for the SCCVII tumor. Schedules with a 72-hr interval between drug and radiation treatments, which produced low enhancement ratios for the SCCVII tumor and duodenal crypt cells, gave high enhancement ratios for the lung. It is concluded that c-DDP has the potential of enhancing the radiation effect on normal tissues, and the degree of enhancement depends upon the interval between X ray and c-DDP. The enhancement ratios for the SCCVII tumors are greater than for normal tissues, and this results in high therapeutic gain factors (TGF). Comparing the effects on tumor with those on normal tissues, it may be seen that there is clinical usefulness in simultaneous combination treatments and perhaps moreso in the administration of a single drug dose 24 hr before the first of 5 daily X ray fractions.

The dose-rate effect in human tumour cells

The radiation response of 12 cell lines derived from a variety of human tumours has been investigated over the dose-rate range from 150 to 1.6 cGy/min. As the dose rate was lowered, the amount of sparing varied widely; in 2 cell lines it was zero, in the other cell lines the dose required for 10(-2) survival ranged up to twice the value at high dose rate. Low dose-rate irradiation discriminates better than high dose rate between tumour cell lines of differing radiosensitivity. The data are equally well fitted by two mathematical models of the dose-rate effect: the LPL model of Curtis and the Incomplete Repair model of Thames. Analysis by the LPL model leads to the conclusion that the theoretical radiosensitivity in the total absence of repair was rather similar among the 7 cell lines on which this analysis was possible. What differs among these cell lines is the extent of repair and/or the probability of direct infliction of a non-repairable lesion. Recovery from radiation damage was also examined by split-dose experiments in a total of 17 human tumour cell lines. Half-time values ranged from 0.36 to 2.3 h and there was a systematic tendency for split-dose halving times to be longer than those derived from analysis of the dose-rate effect. This could imply that cellular recovery is a two-component process, low dose-rate sparing being dominated by the faster component. The extent of low dose-rate sparing shows some tendency to correlate with the magnitude of split-dose recovery; in our view the former is the more reliable measure of cellular recovery. The clinical implication of these studies is that some human tumour types may be well treated by hyperfractionation or low dose-rate irradiation, while for others these may be poor therapeutic strategies.

Fractionation and dose rate effects in mice: a model for bone marrow transplantation in man

This study was designed to compare several fractionation and dose rate schedules to optimize the therapeutic ratio for total body irradiation (TBI). C3H/HeJ mice were given TBI and the bone marrow survival fraction was calculated using the CFUS assay. Irradiation was given at two dose rates: low dose rate (LDR) at 5 cGy/min or high dose rate (HDR) at 80 cGy/min in single fraction and fractionated regimens. The fractionated regimens were given as either 120 cGy three times daily, 200 cGy twice daily, or 200 cGy daily. The Do was 80 cGy for the single fraction, HDR group and 85 for the LDR group. For the fractionated regimens, the apparent Do's ranged from 55-65 indicating no sparing effect of fractionation for the normal bone marrow stem cells. Indeed, the Do's were smaller suggesting an increased sensitivity to irradiation with fractionation. Low dose rate (LDR) and fractionation were also studied for their influence on normal tissue toxicity following upper half body irradiation (UHBI). All the fractionated regimens had higher LD50/30 and LD50/30-180 values than those achieved by single fraction LDR alone. There was no significant dose rate effect for LD50/30 when 120 or 200 cGy fractions were used. However, dose rate was important for LD50/30-180 with 200 cGy but not with 120 cGy fractions. These results demonstrate protection of non-hematopoietic tissues with fractionation and low dose rate without protecting hematopoietic stem cells and may have implications for human bone marrow transplantation.