key: cord-0667316-lo80zfle
authors: Little, Mark P; Zhang, Wei; Hamada, Nobuyuki
title: Pneumonia after bacterial or viral infection preceded or followed by radiation exposure -- a reanalysis of older radiobiological data for discussion on low dose radiotherapy for COVID-19 pneumonia
date: 2020-08-06
journal: nan
DOI: nan
sha: cd549139bd0b2876a6e4bc9ed1d7f93aff9cd149
doc_id: 667316
cord_uid: lo80zfle

Currently, there are at least 14 ongoing clinical studies on low dose radiotherapy (LDRT) for COVID-19 pneumonia in seven countries. One of the underlying assumptions is that irradiation at the level of about 1 Gy is effective at ameliorating viral pneumonia. Its rationale, however, relies on early human case studies or animal studies mostly obtained in the pre-biotic era, where rigorous statistical analyses were not performed. It therefore remains unclear whether those early data would support such assumptions. With state-of-the-art statistical models, we re-analyzed eleven radiobiological animal datasets (generally dating from the 1920s to early 1970s) in which animals received moderate doses of radiation before or after bacterial or viral inoculation. A number of different model systems (guinea pigs, dogs, cats, mice) and types of challenging infection, both bacterial and viral, are considered. For post-inoculation radiation exposure (which is more relevant to LDRT for COVID-19 pneumonia) the results are heterogeneous, with one study (of six) showing a significant increase in risk associated with radiation exposure, another showing a significant decrease in risk associated with radiation exposure, and all other results being non-significant. For pre-inoculation exposure the results are also heterogeneous, with four (of six) datasets showing significant increase in risk associated with radiation exposure and the other two showing a significant decrease in risk. Collectively, these data do not suggest that there are strong modifying effects of radiation exposure after inoculation. Although there are stronger indications of modifications of risk by radiation exposure before inoculation, the inconsistency of direction of effect makes this body of data difficult to interpret.

Low dose radiotherapy (LDRT) for Coronavirus Disease 2019 (COVID- 19) pneumonia was proposed in early April 2020 (1, 2) . At least 14 clinical studies are currently ongoing in 7 countries (3) . The rationale for clinical benefit (i.e., effectiveness of irradiation at the level of 0.5-1.5 Gy in treating viral pneumonia) largely relies on early human case studies or animal studies mostly obtained in the pre-biotic era, when a number of attempts were made to treat various non-cancer diseases with ionizing radiation, including virally-or bacterially-associated pneumonia. An influential paper underlying a number of proposals made for use of LDRT to treat COVID-19 pneumonia (1,2) was Calabrese and Dhawan in 2013 (4) who reviewed 19 papers, mostly case reports, describing outcomes from low dose radiotherapy with X-rays (LDRT) for pneumonia, among them 3 papers published in 1905-1916 and 16 published between 1925-1943 . Their review identified a total of 863 cases, among which 717 showed good clinical response within three days of treatment (4) . However, the human data reviewed were limited to case series, many based on small numbers of subjects. As the sampling framework in these case reports is unknown, they are subject to ascertainment bias and are effectively uninterpretable. The doses used are also often unknown. Calabrese and Dhawan (4) also identified four radiobiological studies, all from experiments done in the 1940s, namely Fried (5) using a guinea pig model, Lieberman et al. (6) using a canine model, Baylin et al (7) using a cat model, and Dubin et al. (8) using a murine model, the first two of these for bacterially-induced pneumonia and the last two for virally-induced pneumonia. For reasons that are not clear there are at least eight relevant radiobiological studies that were not considered in the review of Calabrese and Dhawan (4). Calabrese and Dhawan (4) did not attempt any statistical reanalysis of this old data. The aim of the present paper is to look at more nearly the totality of radiobiological data relating to radiation exposure before or after inoculation with a viral or bacterial agent likely to result in pneumonia. Because of the age of the data being considered there are shortcomings in the original statistical analysis that was performed -indeed in all but a few cases (9,10) there does not appear to have been any formal statistical analysis in the original reports. It is the purpose of this paper to report reanalysis of this data so far as that is achievable, using state-of-the-art statistical survival models in order to assess modification of pneumonia morbidity or mortality risk by radiation exposure before or after inoculation.

We aimed to capture all radiobiological datasets relating to moderate or low dose radiation therapy whether given before or after viral or bacterial inoculation leading to pneumonia. We searched literature by means of a PubMed search (using terms ((radiation OR radiotherapy) AND pneumonia AND viral AND animal) OR ((radiation OR radiotherapy) AND pneumonia AND bacterial AND animal)) conducted on 2020-8-8, which returned 184 articles. We also searched for the same date. We did not restrict by date or language of the publication. We selected from these searches all relevant articles with information on LDRT and bacterially-or virally-induced pneumonia. The datasets used are listed in Table 1 . It should be noted that the datasets we used include three of the four cited by Calabrese and Dhawan (4), but did not include the paper of Fried (5) which we judged did not contain any quantitatively useful information. We convert the given free-in-air dose in rad or rep in all studies to Gy via the scaling 1 rad/rep =0.00877 Gy (11) .

Mortality and morbidity risks in the radiobiological cohorts of Murphy and Sturm (12), Lieberman et al. (6) and Dubin et al. (8) were assessed using a Cox proportional hazards models (13) , with time after radiation exposure, if that followed the inoculation, or time after bacterial or viral inoculation, if that followed the radiation exposure, as timescale, in which the relative risk (RR) (=hazard ratio) of death for animal i at time a after inoculation was given by a linear model in dose:

or alternatively using a log-linear model in dose:

where i D is the total dose (in Gy), α is the excess relative risk coefficient (ERR) per unit dose (Gy 

Occasionally the more standard log-logistic model is fitted to the data (generally on number of animals died in each group):

For the data of Fried (14) , numbering only 7 animals and using as outcome improvement in pneumonia in relation to unirradiated controls, an exact logistic model was used (15) , as non-exact methods did not converge. It is well known that the excess odds ratio approximates to the excess relative risk (16) . All confidence intervals (CIs) and two-sided p-values are profile-partiallikelihood based (17) . In the murine dataset of Dubin et al. (8) in various subgroups risks were assessed in relation to radiation dose administered after inoculation or dose before inoculation. In the murine dataset of Quilligan et al (18) pre-inoculation dose was given to all animals. There is some uncertainty associated with the number of mice in the first of the control groups in this dataset, so a range is employed, spanning the plausible range of 6-10 mice. The model was stratified by the three experiments reported in the data of Dubin et al. (8) and by the three groups used by Lieberman et al. (6) . Tables 3 and 8 and Figure 1 and 2 show the risks in relation to dose for these two datasets. In the analysis of the data from Hale and Stoner (19) in some cases adjustment was made for the type of challenging infection. In the fits to the pneumonia intensity data of Baylin et al. (7) we used either log-logistic regression (as described above) comparing each pneumonia intensity group and those with greater intensity vs every group with reduced intensity;

we also used ordinal regression with log-linear link (20) fitting to all the ordered intensity groups.

In fits of the days of infection data of Baylin et al. (7) we used a linear regression model, estimating the CIs via the bias-corrected advanced method (21) . All models were fitted via Epicure (22), R (23) or LogXact (15) . Table 2 demonstrates that there are weak indications (0.05 < p <0.1) of decreased risk of pneumonia with post-inoculation dose in the dataset of Fried (14) , whether for all guinea pigs or restricting to the six guinea pigs receiving Staphylococcus aureus inoculation. Table 3 demonstrates that there is a highly significant decreasing trend (p<0.001) of mortality with post-inoculation dose in the dataset of Lieberman et al. (6) with EOR per Gy = -0.23 (95% CI -0.24, -0.16), as also shown by Figure 1 . However, Table 3 shows that this is largely driven by a single group, group 3, as also shown by Figure 2 . There is a non-significant positive trend with dose (p>0.4) in the murine data of Tanner and McConchie (24) ( Table 4 and Figure 4 ).

There are few indications of trend of degree of pneumonia infection with dose in the feline data of Baylin et al. (7), whether using logistic or ordinal models (Table 5 ). However, Table 6 indicates that there is a significant decreasing trend of days of acute infection with dose in this dataset, with days of infection / Gy changing by -2.56 (95% CI -4.59, -0.33) (p=0.015), i.e., duration of infection decreasing with dose, as also shown by Figure 3 . Table 7 demonstrates that there is a highly significant (p<0.001) increased risk of death associated with X-ray exposure after inoculation with Pneumococcus in the murine data of Murphy and Sturm (12), with RR = 3.67

(95% CI 1.84, 7.61). (9), so that for pneumonitis morbidity the EOR/Gy = -0.24 (95% CI -0.28, -0.17) and for pneumonitis mortality the EOR/Gy = -0.21 (95% CI -0.26, -0.14), as shown in Table 11 . In contrast Table 12 and Figure 5 show reanalysis of slightly later data of Berlin and

Cochran (10), which exhibits slightly heterogeneous results, with one set of experiments (given in (10)) indicating a significant increase (p<0.05) in influenza mortality, whether or not adjusted for mode of administration of virus, but a different experimental set (reported in Table II of the paper) showing no significant effect (p>0.1) of radiation exposure on influenza morbidity or mortality. These experiments use a similar murine system, also given 3.5 Gy whole body air-dose exposure, as in the earlier paper of Berlin (9). Lundgren et al (25) used a novel type of radiation exposure, aerosolized 144 CeO2, which delivers localized β dose to the lungs of C57BL/6J mice. As can be seen from Table 13 there was a small but highly significant increase in mortality risk associated with radiation exposure, with EOR/Gy = 0.008 (95% CI 0.002, 0.019, p=0.002).

We have re-analyzed eleven radiobiological animal datasets, dating from the 1920s to the early 1970s, in which bacterial or viral agents were administered to induce pneumonia in animals that were also exposed to moderate doses of radiation before or after inoculation. The statistical analysis in the original papers was limited, indeed in all but two cases (9,10) there does not appear to have been any formal statistical analysis. We therefore judged it necessary to statistically reanalyze the original datasets with something like state-of-the-art statistical models. Table 1 ).

Altogether, early radiobiology data do not suggest that there are strong modifying effects of radiation exposure after inoculation. In particular, the heterogeneity in the results of our statistical analysis suggest that these early datasets do not serve as supportive evidence that LDRT of infected 

Observed Odds ratio = 1

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