The lungs are paired organs, each contained in its own pleural sac within the thoracic cavity. Both lungs are subdivided into anatomically distinct lobes. The right lung normally has three lobes separated by two fissures. The horizontal fissure separates the upper and middle lobes while the oblique fissure separates the lower lobe from the middle and upper lobes. The left lung normally has two lobes (upper and lower) divided by an oblique fissure. The lobes can be further subdivided into segments. The lingula (Latin-little tongue) is part of the upper lobe and is composed of a superior and inferior segments (Fig. 2).

The architecture of the lung can be considered as subunits arranged in both parallel and series. Centrally, the structure is “in series” as the large vessels and airways in the mediastinum and hilum support all of the distal vessels and airways to which they are connected. Fibrosis/stenosis of a central vessel or bronchus will render the downstream alveoli nonfunctional. Conversely, in the periphery, the smaller alveolar/capillary units function relatively independently (i.e., “in parallel”). Resection or injury to a region of lung will not compromise function of adjacent regions. The alveolar/capillary units appear far more sensitive to the effects of RT than the central conducting airways and vessels. Therefore, most RT-induced injury is referable to these “peripheral” structures and the lung is classically considered a “parallel” organ regarding RT-induced effects.

The functional unit of the lung is the pulmonary lobule which consists of a terminal bronchiole and the lung parenchyma distal to it (i.e., alveolar/capillary unit) (Fig. 3a, b). The terminal bronchiole is the final conducting branch of the tracheobronchial tree not involved in gaseous exchange. Terminal bronchioles branch into transitional airways (respiratory bronchioles and alveolar ducts) which contribute to gaseous exchange. The transitional airways terminate in alveolar sacs which open into the alveoli.

The principal function of the lungs is to facilitate gas exchange between alveoli and adjoining capillaries. Gas exchange is most efficient when the thickness of intervening tissues is minimal. Adjacent alveoli share a common wall, termed the interalveolar septum, which contains capillaries and a fine connective tissue framework. Within the interalveolar septum, the adjacent basal lamina of the alveolar epithelium and endothelium are often fused providing a thin barrier between the erythrocytes in the capillaries and the alveolar air space (~0.5 μm). The capillaries wrap around the alveoli creating a prodigious surface area for diffusion (50–100 m²).

Several cell types are present in the pulmonary lobule. Type 1 pneumocytes are flat epithelial cells with minimal cytoplasm and few organelles that form a complete, thin (0.2 μm), lining of the alveoli. Type II pneumocytes are as numerous as type I pneumocytes but only cover 2–5% of the alveolar surface. They are round cells with larger nuclei, and contain prominent lamellar bodies (cytosomes) which contain dipalmitoyl phosphatidylcholine (surfactant), which reduces surface tension within the alveoli. Type II pneumocytes can differentiate into type I pneumocytes in response to damage to the alveolar lining. Alveolar macrophages (dust cells) derive from circulating monocytes and phagocytose microorganisms and other particulate matter that is deposited in the alveoli.

Radiation injury of the lung is categorized as two distinct phases: acute inflammatory (pneumonitis) and late fibroproliferative. The progression of radiation injury is well established in both mice and humans. In humans, the pneumonitis phase peaks between 2–3 months after radiation exposure. This phase is characterized by a dense inflammatory cell infiltrate, exudation of proteinaceous material, and edema. The chronic fibroproliferative phase occurs months to years after exposure and is characterized by diffuse interstitial fibrosis, focal scarring, and impaired ventilation.

The lung is considered to be a late responding tissue, as the time between exposure and symptomatic injury can last months to years, presumably as a result of innate tissue kinetics. During the asymptomatic latency period, radiographic changes can be observed (Skoczylas et al. 2000; Brush et al. 2007; Fleckenstein et al. 2007). Although the period of time between exposure and symptomatic injury may appear to be quiet, there is a firestorm of events occurring at the molecular and cellular level that is just beginning to be better understood.

At the time of the initial irradiation, exposed cells rapidly spread the “danger signal” to neighboring cells as far away as 1 mm (Tsoutsou and Koukourakis 2006; Pusey 1905). This results in an orchestrated response of tissue-specific cells, the vascular endothelium, and inflammatory mediators in order to resolve the injury (Skoczylas et al. 2000; Fleckenstein et al. 2007; Tsoutsou and Koukourakis 2006; Bernier et al. 2004).

The intercellular conversation is initiated at the moment of irradiation when injury to the cell membrane, DNA, cytoplasmic bodies, and other cell components occurs, resulting in an immediate release of cytokine mRNA. The identification of a “perpetual cytokine cascade” (Fleckenstein et al. 2007) caused a major paradigm shift from the classical “target cell theory” (α/β) to one that posits a dynamic molecular crosstalk among numerous cells in normal tissue (the lung has more than 40 cell types (Evans and Leucutia 1925)) that propagates the injurious process (Fig. 4).

The cytokine and chemokine reaction observed by Rubin and colleagues clearly demonstrated the absence of a molecular and cellular latent period prior to observed histologic changes. These findings suggest that radiation initiates a downstream sequence of spatial and temporal events that eventually express themselves as clinical manifestations months later.

Coincident with the release of inflammatory cytokines is the development of oxidative stress. Free radical species can participate in a host of cellular signaling events. Indeed, many of the signaling pathways implicated in radiation injury, such as the TGF-β pathway, are sensitive to changes in cellular redox status (Tyler and Blackman 1922; Groover et al. 1922). The transient nature of ionizing radiation suggests that a mechanism of amplification of ROS/RNS occurs after radiation. Proposed mechanisms include activation of oxidant-generating enzymes, such as NADPH oxidases, mitochondrial leakage, and the respiratory burst from activated inflammatory mediators recruited to the site of injury. The role of NADPH oxidases is supported by the fact that TGF-β is activated by radiation and that in turn, TGF-β can activate the NADPH oxidase isoform, Nox4. Newer data coming to light recently have shown a critical role for Nox4 in TGF-β mediated apoptosis (Hines 1922) and downstream Smad signaling (Davis 1924).

In the lung, chronic oxidative stress has been observed using markers for DNA oxidation (Skoczylas et al. 2000) and lipid peroxidation (Groover et al. 1922; Fleckenstein et al. 2007). Oxidative stress can contribute to a number of changes within injured tissue including endothelial cell activation, increased vascular permeability and edema, lipid peroxidation, inflammation, and fibrosis (Skoczylas et al. 2000; Bernier et al. 2004; Johnston et al. 1996; Rubin et al. 1995). The end result is a cyclic progression of oxidative stress and activation of growth factors, transcription factors, and cytokines, all of which can contribute to lung injury.

It is hypothesized that the imbalance between oxidant-generating enzymes and antioxidant capacity is a primary cause of hypoperfusion following radiation (Skoczylas et al. 2000; Vujaskovic et al. 2001). Fleckenstein et al. demonstrated that tissue perfusion begins to decrease 3 days after radiation exposure, resolves for approximately 2 weeks, and then progressively decreases throughout the time of disease progression (Skoczylas et al. 2000). In parallel, the lung tissue becomes increasingly hypoxic (Skoczylas et al. 2000). The late decrease in tissue oxygenation is likely a result of fibrous obliteration of capillaries and decreased gas exchange due to increased alveolar wall thickness and edema/congestion within the alveolar space. The early drop in perfusion cannot be explained by these events and is more likely a result of vasoconstriction, although this is not confirmed. The activation of NADPH oxidase after radiation increases superoxide production, which may suppress vasodilation by oxidizing key enzymes within the NO signaling pathway or through rapid reaction with NO itself to form peroxynitrite. Both of these reactions can interfere with NO signaling leading to vasoconstriction, a transient loss in tissue perfusion, and tissue hypoxia. Tissue hypoxia, a fundamental feature underlying the development of lung injury, develops several weeks after lung irradiation and progresses with time, presumably as a result of injury to the pulmonary vascular and decreased perfusion (Skoczylas et al. 2000; Bernier et al. 2004). Changes in pulmonary perfusion have been observed within months following high doses of radiation in humans (Bentzen 2006; Mikkelsen and Wardman 2003) and within weeks in rodent models (Skoczylas et al. 2000; Rubin et al. 1995; Jackson et al. 2006). In studies by Fleckenstein et al. (Skoczylas et al. 2000), the transient decrease in perfusion was associated with increased edema and elevated expression of hypoxia-inducible factor-1 alpha (HIF-1α) and TGF-β. In that study, the vascular perfusion changes were noted before the development of histologically apparent and functional disease.

The same study showed that tissue hypoxia and perfusion changes coincide with the accumulation and activation of macrophages within the residual alveolar space and interstitium, as well as enhanced cytokine and growth factor production. This could, in part, contribute to the presence of cytokines in the irradiated tissue weeks to months after exposure (Skoczylas et al. 2000). Jackson et al. (Johnston et al. 1996) found that in vitro TGF-β production by macrophages could be induced by hypoxia (0.5% O₂), prior to the onset of VEGF production. In that study, incubation with a superoxide dismutase mimetic led to decreased TGF-β and VEGF production (Johnston et al. 1996). Thus, it appears that ROS signaling plays a key role in macrophage-associated expression of the above cytokines under hypoxic conditions.

The pro-inflammatory/pro-fibrogenic cytokine transforming growth factor-beta (TGF-β) has been most extensively studied in regard to its role in the development of radiation-induced lung injury (Fleckenstein et al. 2007; Novakova-Jiresova et al. 2007; Kang et al. 2003; Feletou et al. 1995). TGF-β is secreted as an inactive polypeptide by a number of cells including macrophages, fibroblasts, and epithelial cells of the lung. The inactive precursor is sequestered in the extracellular matrix with its latency-associated peptide (LAP), which has been suggested to be the “sensory” portion of the protein (Marks et al. 1997; Theuws et al. 2000; Peterson et al. 1992; Dor et al. 2001; Li et al. 2001). The LAP is sensitive to free radicals, changes in cellular pH, and integrins, all of which can induce activation of TGF-β which readily binds the TGF-β type II receptor to stimulate the Smad second messenger signaling pathway (Bartholdi et al. 1997). Activation of TGF-β and subsequent translocation of Smad2/3 protein into the nucleus of the responding cell leads to transcription of genes involved in fibroproliferation and tissue repair (Li et al. 2007; van Hinsbergh et al. 2001). An increase in TGF-β protein can be observed within 24 hours following radiation exposure (Skoczylas et al. 2000; Dor et al. 2001; Anscher et al. 1998, 2006). Epperly et al. (Rodemann et al. 1996) found a biphasic TGF-β surge associated with the early and delayed radiation response in mice after 20 Gy whole lung irradiation. The establishment of TGF-β as a mediator of radiation-induced lung injury has led to a number of studies evaluating circulating serum TGF-β levels as a potential biomarker for increased risk of lung injury as well as pre-clinical testing of inhibitors of TGF-β and/or its signaling pathway. These studies have used either neutralizing antibodies or adenoviral vectors to demonstrate significantly reduced histological and functional damage in rodent models of lung injury following radiation exposure. Reduced lung damage was also associated with decreased cytokine activity, fewer macrophages, and less oxidative stress (Kang et al. 2003; Barcellos-Hoff 1996; Rahimi and Leof 2007).

In summary, our knowledge of the mechanisms of radiation injury has vastly improved over the past 30 years. It is now established that chronic oxidative stress and continuous production of cytokines, particularly TGF-β, play a significant role in sustaining inflammation and fibroproliferation in irradiated lung during the progression of disease. Symptomatic and histologically relevant lung injury is preceded by cytokine production and communication, inflammatory cell infiltration, changes in blood flow and perfusion, and increased vascular permeability (Bernier et al. 2004; Roberts 1999; Barcellos-Hoff 1993; Barcellos-Hoff et al. 1994; Stone et al. 2003, Vujaskovic et al. 2000). Although great progress has been made in identifying the mechanisms underlying radiation-induced lung injury, more comprehensive studies are needed to link the events occurring at the molecular level to cellular and tissue pathology.

Despite these laboratory findings, the results in the clinic are less encouraging. To date, the observed associations between changes in a variety of biological markers and clinical pneumonitis have been inconsistent (Table 1).

Radiation pneumonitis, typically manifested as shortness of breath, dry cough, and occasionally fever, occurring 1–6 months after treatment (Tsoutsou and Koukourakis 2006), appears to result from an acute inflammatory process within the alveolar spaces. While the radiologic changes associated with this inflammatory reaction are generally limited to the irradiated regions of the lung (Mah et al. 1987; Marks et al. 2000), radiologic changes have been occasionally observed to extend beyond the irradiated field (Gibson et al. 1988; Roberts et al. 1993). Furthermore, bronchoalveolar lavage (BAL) following unilateral irradiation reveals increased lymphocytes from both the irradiated and unirradiated lung (Gibson et al. 1988; Roberts et al. 1993; Martin et al. 1999). These observations suggest that radiation pneumonitis may be type of hypersensitivity pneumonitis (bilateral lymphocytic alveolitis) (Abratt and Morgan 2002). Further, the lymphocytes retrieved with BAL are activated T-lymphocytes (Martin et al. 1999), suggesting that immunologic processes are participating in the radiation reaction. The observation that pneumonitis typically responds well to oral prednisone (Salinas and Winterbauer 1995) also supports the inflammatory/immunologic nature of radiation pneumonitis.

In patients with symptomatic pneumonitis, physical exam is often normal, although rales or a friction rub can sometimes be appreciated. Radiological studies are often, but not always, abnormal. Thus, pneumonitis is a clinical diagnosis. Radiographic abnormalities without symptoms usually do not warrant intervention. Indeed, asymptomatic patients frequency has radiologic findings similar to those seen in symptomatic patients. Thus symptoms and radiologic findings do not necessarily parallel each other. Further, since abnormalities on CT and CXR are very common following RT, reported studies that consider asymptomatic radiologic findings as ‘an event’ likely over-estimate the rate of ‘clinical toxicity’.

Differentiating radiation pneumonitis from other etiologies (e.g., tumor progression, infection, pulmonary emboli, heart disease) can be challenging (Kocak et al. 2005). Ideally, one should be reasonably sure that symptoms suggestive of pneumonitis are not due to these other conditions before therapy is initiated. A short course of antibiotics is occasionally useful if there is concern for infection, with a rapid institution of steroids if symptoms do not respond.

The culminating event in RT-induced lung injury is replacement of normal lung parenchyma with fibrosis, which is often appreciated on radiographic studies but is usually asymptomatic (Marks et al. 2000). However, if the baseline pulmonary reserve of a patient is limited and/or the region of fibrosis is extensive, symptoms can develop. In this circumstance, dyspnea can be progressive and difficult to manage, often requiring long-term steroids, oxygen, and/or rehabilitation. The severity of radiographic abnormalities is not always well-correlated with the presence or severity of pulmonary symptoms (Marks et al. 2000). The time course for the development of RT-associated lung symptoms is shown in Fig. 6a, b.

In patients treated with high doses of radiation (e.g., ≥70 Gy), unusual pulmonary complications, such as bronchial stenosis, bronchopleural fistula, and fatal hemoptysis have been reported (Miller et al. 2005; Socinski et al. 2004).

Multiple radiological studies have been used to assess regional lung injury. The most widely studied include chest X-ray and CT (which evaluate structure and tissue density), and single photon emission computed tomography (SPECT) (which can evaluate the functional endpoints of ventilation and perfusion). The sensitivity of these radiological modalities varies, and thus, the incidence of lung injury depends on the modality utilized. Furthermore, radiological changes are common in patients with lung cancer (who receive relatively large doses to large volumes of lung), and less common in patients with breast cancer or lymphoma (where the doses/volumes are lower). In general, 3D imaging modalities are more sensitive than 2D modalities (CT is more sensitive than CXR), and SPECT-based ventilation/perfusion imaging is more sensitive than either CXR or CT. Comparisons between studies are challenging since the patient populations and RT doses used are variable. Nevertheless, these generalizations appear to be true in studies that have considered multiple imaging modalities (Jackson et al. 2006).

PFTs objectively assess multiple pulmonary parameters including lung volumes, the amount and rate of air flow (spirometry), and the ability of the lung to transfer gas at the alveolar level. Diffusing capacity of the lung for carbon monoxide (DLCO) is likely the best parameter to assess lung function after RT, since the ultimate role of the lungs is to facilitate gas exchange. Loss of alveolar surface area and thickening of the intraalveolar septi are the primary causes for reductions in DLCO. Thus, both acute (edema, extravasation of material into the alveoli) and chronic (fibrosis, loss of alveoli) injury can affect DLCO. The forced expiratory volume in 1 s (FEV1) is another useful parameter and often interpreted in the context of the FEV1/FVC ratio (FVC = forced vital capacity). A decreased FEV1 with a normal FEV1/FVC ratio is most consistent with a restrictive process, such as fibrosis. A decreased FEV1 and FEV1/FVC ratio is more consistent with an obstructive process.

Assessing the impact of RT on PFTs in patients with lung cancer is challenging. Many patients with lung cancer have baseline pulmonary dysfunction from smoking, such as chronic obstructive pulmonary disease, and some patients continue to smoke after RT with continued accelerated diminutive effects on function. Also, pulmonary function may transiently improve as large central tumors regress. Finally, there is limited data on long-term PFT changes since most patients with lung cancer succumb to their disease within 1–2 years.

With these caveats, most studies show a decline in pulmonary function after RT. Reductions in FEV1 are appreciated approximately 3–6 months post-RT (Table 5), sometimes followed by partial/complete recovery at ≈12 months. This transient decline may be coincident with the acute inflammatory reaction associated with pneumonitis. DLCO typically is reduced to a greater degree than FEV1 (Table 5), and many studies appears to show less recovery at ≈12 months than FEV1. Most studies show a decline in both FEV1 and FVC (Borst et al. 2005; Miller et al. 2003), such that the FEV1/FVC ratio remains in the normal range (Myers et al. 2005), consistent with a restrictive process. In the few patients with long-term changes in PFTs systematically studied beyond one year, there appears to be long-term continued diminution in PFTs 2–8 years post-RT (Miller et al. 2003).

Multiple studies, primarily in children, have confirmed that pulmonary function tests are abnormal after whole lung irradiation (Benoist et al. 1982; Ellis et al. 1992; Littman et al. 1976; Miller et al. 1986; Weiner et al. 2006). In general, these studies demonstrate decreased lung volumes (vital capacity, total lung capacity), impaired air flow (forced expiratory volume in 1 second), and perhaps diminished diffusing capacity (DLCO). Multiple factors likely play a role, including impaired chest wall growth, a reduction in the number of functioning alveolar units, and fibrotic changes within the lung. Whether function improves over time (Ellis et al. 1992) or continues to deteriorate (Weiner et al. 2006) is unclear.

Irradiation of both lungs occurs in multiple clinical contexts. This includes total body irradiation (TBI) as part of the preparative regimen for stem cell transplants, hemibody irradiation in the setting of diffuse symptomatic metastatic disease, and whole lung irradiation for pulmonary metastases from various malignancies (not commonly done recently). Clinical experience in these settings has permitted the tolerance of whole lung irradiation to be estimated.