Remission of pulmonary alveolar proteinosis after radiotherapy in a patient with comorbid small cell lung cancer.

To the Editor: Pulmonary alveolar proteinosis (PAP) is a rare disease characterized by abnormal and excessive accumulation of pulmonary surfactant in the alveolar cavity. Cases of PAP are usually categorized into three etiological forms: congenital, secondary, and idiopathic PAP. Whole-lung lavage (WLL) has been widely used to treat PAP.[1] According to the literature, case reports of PAP associated with lung cancer are rare. Furthermore, to our knowledge, few cases of PAP with lung cancer receiving definitive thoracic radiation therapy (TRT) has been reported. Herein, we reported a case of PAP associated with limited stage-small cell lung cancer (LS-SCLC), which was treated using a combination of chemotherapy and sequential TRT. Unexpectedly, the patient's clinical and radiographical abnormalities of PAP were completely relieved after TRT. This study was approved by the Institutional Review Board of the National Cancer Center, Chinese Academy of Medical Sciences and Peking Union Medical College (approval No. 23/139-3885). All procedures performed in our study were in accordance with the Declaration of Helsinki or comparable ethical standards. All patients provided written informed consent. In 2019, a 55-year-old man who had both SCLC and PAP was admitted to our hospital with intermittent shortness of breath and chest tightness. Chest computed tomography (CT) revealed that reticulations on both lungs were superimposed on ground-glass opacities, forming a "crazy-paving appearance" [Figure 1 and Supplementary Figure 1A, https://links.lww.com/CM9/B594], while a hyperdense lesion was observed in the lingual lobe of his left lung. Bronchoscopy biopsy confirmed SCLC [Supplementary Figure 1B, https://links.lww.com/CM9/B594]. He was diagnosed with LS-SCLC (T1cN1M0, stage IIB). PAP was also diagnosed based on typical CT imaging findings and pathological findings from bronchoalveolar lavage fluid (BALF) in 2013. His blood test for anti-granulocyte-macrophage colony stimulating factor (GM-CSF) antibodies was positive. No medical intervention was given between 2013 and 2019 because his symptoms were mild. Routine follow-up was carried out every year. There was a concern that concurrent chemoradiation would aggravate his PAP; therefore, he started six courses of chemotherapy alone with etoposide and cisplatin (EP). A chest CT scan in September 2019 (after the sixth course of chemotherapy) revealed that the SCLC had achieved complete remission (CR), while the PAP remained stable [Figure 1]. The patient then received definitive TRT with a total dose of 60 Gy (6MV-X/VMAT, Varian EDGE linear accelerator, Palo Alto, CA, USA) in our department in October 2019. Surprisingly, a CT scan after 20 fractions of radiotherapy showed that the reticulations and ground-glass opacities induced by PAP were partly diminished in both lungs. Prophylactic cranial irradiation (PCI) was started one month after definitive TRT. A CT scan before PCI revealed little sign of PAP. Surprisingly, we found that during follow-up, radiotherapy for SCLC also led to the CR of his PAP after February 2020 [Figure 1]. Unfortunately, the tumor recurred with metastasis of the mediastinal lymph node nine months after chest radiotherapy. He received two courses of chemotherapy with EP regimen. The patient refused any further medical interventions thereafter. He was diagnosed with multiple metastases of the brain, skull, ribs, and right ilium in January 2021. Overall, the patient was followed up >24 months after TRT. He had systemic metastasis of SCLC but was still alive during follow-up. Meanwhile, no sign of PAP was found on CT scans after February 2020. No treatment for PAP other than radiotherapy has been administered to this patient, and there have been no significant changes in his pulmonary function tests since 2013.Figure 1: Chest CT presented that patient's radiographical abnormalities of PAP were relieved after thoracic radiotherapy.CT: Computed tomography; PAP: Pulmonary alveolar proteinosis; SCLC: Small cell lung cancer.Therefore, we hypothesized that radiotherapy could promote remission of PAP and might be a potential treatment of this rare disease. Further laboratory experiments were conducted to explore the possible mechanisms of this phenomenon. Healthy mice (strain name: C57BL/6J) were used to evaluate the influence of radiation on myeloid cells. The mice used for the radiation treatment were first anesthetized by intraperitoneal injection. They were then divided into three groups to receive radiation treatment, each with 9 mice. Different doses and regimens were used as indicated in the text. Since the mean dose delivered to the PAP-involved areas of the patient was <1 Gy, the radiotherapy groups received 0.2 Gy and 2 Gy of X-ray radiation, respectively, based on the calculation of the radiotherapy planning system, while the negative control group did not receive radiation. After completing all planned irradiations, the mice were euthanized at 3, 6 and 9 weeks, and their lung tissues were collected for further analysis. Flow cytometry of alveolar macrophages (AMs) and monocytes (Mos) was performed following Misharin's method.[2] AMs were identified as CD45+, Siglec F+, and CD11b ± cells. Similarly, Mos were identified as CD45+, CD11b+, and CD64 ± cells. In the negative control group of mice, the average relative numbers of AMs and Mos before irradiation were 2.3% and 15.6%, respectively. After irradiation of mouse lungs, the relative number of AMs decreased and then returned to the baseline or even higher in both groups (0.2 Gy group: 1.64% at three weeks, 2.08% at six weeks, and 2.36% at nine weeks; 2 Gy group: 1.02% at three weeks, 1.18% at six weeks, and 4.70% at nine weeks) [Supplementary Figure 1C, https://links.lww.com/CM9/B594]. Conversely, Mos, as precursors of AMs, increased during the first 6 weeks and then decreased (0.2 Gy group: 17.0% at three weeks, 20.9% at six weeks, and 18.1% at nine weeks; in the 2 Gy group: 26.3% at three weeks, 27.3% at six weeks, and 20.4% at nine weeks) [Supplementary Figure 1D, https://links.lww.com/CM9/B594]. The above flow cytometry results are summarized as Supplementary Figure 1E, https://links.lww.com/CM9/B594. To evaluate the clinical value of GM-CSF in patients with or without PAP, we collected the patient's blood samples in vacutainers with heparin and centrifuged them at 300 g for 10 min. The supernatant was transferred to a new sterile tube for further analysis. The enzyme-linked immunosorbent assay (ELISA) method was used to detect GM-CSF, and the concentration of anti-GM-CSF antibodies in the patient's blood sample was measured based on the method established by Uchida et al[3]. Anti-GM-CSF antibody concentration >5 µg/mL was identified as a positive threshold value. The other three non-small cell lung cancer (NSCLC) patients without PAP were recruited as the control group, and they were all treated by the same doctor with similar regimens. The GM-CSF concentration was measured in this PAP patient. Compared with the concentration before irradiation, the GM-CSF level increased significantly during and one month after radiotherapy [Supplementary Figure 1F, https://links.lww.com/CM9/B594]. Similarly, we could also observe an increase in GM-CSF levels during and after radiotherapy in three other patients who had NSCLC without PAP [Supplementary Figure 1G, https://links.lww.com/CM9/B594]. Based on the above findings, we hypothesized that radiation could lead to the self-renewal of AMs, which might help to restore normal pulmonary surfactant clearance. A previous study showed that radiation could result in alteration to AMs and Mos in the lungs.[4] Furthermore, Fedorocko et al[5] found that radiation could increase the production of hemopoietic and proinflammatory cytokines, such as GM-CSF. The GM-CSF signal could lead to transformation of Mos into AMs. In addition, Suzuki et al[6] used Mo transplantation to cure PAP in mice. Combined with these previous findings, our study indicated that radiotherapy might overcome the blockage of anti-GM-CSF antibodies and facilitate the transformation of peripheral blood macrophages to AMs. Excessive deposition of pulmonary surfactant in a patient's lungs could therefore be removed by the increased number of AMs. Moreover, the removal of pulmonary surfactant in patients with PAP would produce a rapid improvement in clinical symptoms and imaging features. In this case, the AM self-renewal theory could be a reasonable explanation for the observed phenomenon. Radiotherapy is recognized as an important treatment for cancer. However, radiotherapy could also be used for the management of benign diseases. Low-dose radiotherapy (LDRT) has been adopted to treat inflammatory diseases, such as scapulohumeral periarthritis, humeral epicondylitis, enthesopathies, and osteoarthritis.[7,8] Our study revealed a role for LDRT in the treatment of PAP. Since 1988, studies have demonstrated that LDRT could result in a shift of immune cells from an immune suppressive state to a responsive state. Klug et al[9] demonstrated that LDRT can program macrophages into the M1 phase. Previous studies suggested that LDRT decreased the inflammatory process by modulating M1 and M2 macrophages to decrease their production of cytokines, such as tumor necrosis factor 1 alpha (TNF-1α), interleukin (IL)-1β, and IL-6. This could be beneficial for coronavirus disease 2019 (COVID-19) induced pneumonitis.[10] There are several limitations to our study. Further exploration is needed to answer the following questions: (1) Which molecules/cytokines are involved in radiation-activated GM-CSF signaling pathways in PAP? (2) Why does short-term radiation exposure lead to such long-term remission? (3) We did not measure the concentration of GM-CSF and anti-GM-CSF antibodies in the mice. However, we measured the concentration of GM-CSF in both PAP patients and non-PAP patients receiving TRT. In the future, we plan to establish an animal model of PAP and explore the GM-CSF concentration variation after radiotherapy. Detection of cytokines, C-reactive protein (CRP), and immune cell count in both human and animal models may be beneficial for revealing the related mechanism. In conclusion, PAP is a rare disease and there has been rarely previous report that confirmed the feasibility of definitive TRT in patients with PAPs. Herein, we reported a patient with LS-SCLC treated successfully with combined chemotherapy and sequential TRT. Unexpectedly, his PAP was significantly relieved after radiotherapy. Our work implied that a chemotherapy sequential radiation regimen might be safe and effective for patients with both SCLC and PAP. As a potential therapeutic strategy for PAP, radiation therapy warrants further investigation in the future. Funding This study was supported by the National Natural Sciences Foundation Key Program (No. 82173348) and CAMS Innovation Fund for Medical Sciences (No. CIFMS 2021-I2M-1-012). Conflicts of interest None.