Does Unintentional Splenic Radiation Predict Outcomes After Pancreatic Cancer Radiation Therapy?: Beyond the Abstract

Pancreatic cancer is the second most common gastrointestinal cancer in the United States with an estimated incidence of 53,070 cases in the year 2016.1 Survival outcomes are dismal with an estimated 5-year overall survival of less than 5%.2  The lack of validated prevention strategies and screening tools for early detection contribute to diagnosis of disease at more advanced stages.

Even when it is diagnosed before the onset of overt metastatic disease, pancreatic cancer is inherently resistant to treatment and follows an aggressive clinical course. Intense desmoplasia, a hallmark of these tumors, and the high proportion of immunosuppressive cells in the pancreatic tumor microenvironment also contribute to the suboptimal outcomes.2 Emerging immunotherapy strategies in other disease sites are also being considered in pancreatic cancer to overcome this immune-privileged tumor microenvironment. Radiation therapy (RT) has a dual effect on the immune system. It can induce an in-situ vaccination effect via release of autoantigens and radiation-induced neoantigens, increase expression of co-stimulatory immune molecules on the surface of cancer cells, and express hallmarks of immunogenic cytotoxicity leading to enhanced cell death.3 But RT can also be immunosuppressive by inducing tumor overexpression of MHC-1, death receptors, and checkpoint proteins that drive co-inhibitory pathways to evade immune eradication;4 depleting lymphocytes in circulation and/or those sequestered in secondary lymphoid organs;5 inducing lymphocyte apoptosis via secretion of galectin-1 by tumors;6 and inducing TGF-β secretion that impedes the ability to generate an effective cytotoxic T cell response to tumor antigens.7 

Researchers from John Hopkins have shown that post-treatment lymphopenia is correlated with increased morality in resectable and locally advanced pancreatic cancer. RT toxicity to circulating lymphocytes was postulated as the likely cause of lymphopenia that was independent of chemotherapy usage. Mortality was reported to be due to tumor progression rather than lymphopenia-related opportunistic infections.8,9 The spleen is not routinely considered a dose limiting organ and is commonly included in the RT portal in the treatment of pancreatic cancer. The spleen is a rich reservoir of T and B lymphocytes with a very slow circulation time due to the sinusoidal architecture of flow channels. Splenic dose beyond 10 Gy causes eventual stromal fibrosis leading to a dysfunctional spleen.10 The net effect is reduced levels of CD8+ cytotoxic T lymphocytes and CD4 + helper T cells available for tumor infiltration. Surgical resection specimens have shown that tumor infiltration of CD8+ T cells is associated with improved survival outcomes.11,12 Our publication showed that dose to the spleen (mean dose exceeding 9Gy and V15 above 20%) is an independent predictor of post-chemoradiation lymphopenia in locally advanced pancreatic cancer patients and that post-chemoradiation lymphopenia was strongly associated with poorer survival outcomes.13 Initial analyses noted that post-chemoradiation lymphopenia was not related to tumor size and therefore, presumably field size since patients were treated with local fields primarily. Subsequent analyses noted that both the irradiated volume and the treated volume (encompassed by the 50% and 95% isodose lines, respectively) were also not predictive of lymphopenia suggesting that depletion of circulating lymphocytes may not be a dominant mechanism of developing lymphopenia. If the splenic dose is independently validated in other studies to be a determinant of lymphopenia, a known driver of poor outcomes following chemoradiation, then there is a compelling rationale for triaging treatment plans based on their ability to adequately spare the spleen especially in patients with baseline lymphopenia prior to initiating chemoradiation, patients who are candidates for immunotherapy, and patients being considered for dose-escalated RT protocols.14 In addition to choosing spleen-sparing beam angles for 3D conformal RT, effective spleen sparing can also be accomplished by employing intensity modulated radiation dose painting, charged particle therapy, stereotactic body radiation therapy and high dose rate RT.15,16 More refined normal tissue complication probability (NTCP) modeling may also serve as a predictive tool to correlate fractional radiated volumes of the spleen (dose volume histograms (DVH)) with lymphopenia. 

Similar to pancreatic cancer, post-RT lymphopenia has been correlated with inferior survival outcomes in lung cancer and high grade gliomas. In lung cancer, the greatest correlation between treatment parameters and post-chemoradiation lymphocyte nadirs was gross tumor volume (GTV) with larger GTVs causing lower lymphocyte nadirs.17 In high glioma patients, treatment-induced lymphopenia was again a poor prognostic factor for overall survival.18 In addition to the lymphotoxic effects of temozolomide and corticosteroids, lymphopenia was attributed to lymphotoxic radiation doses to circulating blood which in turn correlated with planning target volume (PTV) size but not radiation delivery technique.19 Furthermore, the lymphopenia failed to trigger a compensatory increase in interleukin-7 (IL-7) and interleukin-15 (IL-15), the key homeostatic cytokines that mediate the recovery of CD8+ cytotoxic T cells and CD4+ helper T cells.20 

Accumulating evidence of a synergy between immunotherapy and radiotherapy has led to a flurry of clinical trials being planned for the treatment of pancreatic cancer. Currently there are over 30 clinical trials evaluating immunotherapy in pancreatic cancer. Optimal levels of CD8+ T cells and CD4+ helper cells a prerequisite for immunotherapeutic agents to act.21 Therefore, in addition to strategies that create an immune-permissive milieu within tumors via blockade of TGF-β and checkpoint signaling, treatments that directly increase lymphocyte counts could amplify the synergy and thus prevent or overcome the immunosuppressive effects of radiation. This immune-mediated tumoricidal effect could be effective locally within the tumor and/or distantly at sites of metastatic disease via long-term immunological memory of tumor associated antigens. Approaches being explored in this realm include IL-7 or IL-15 supplementation during RT, restitution of lymphocytes with autologous transfusion, and inhibition of galectin 1-mediated lymphocyte apoptosis possibly with thiodigalactoside. Preclinical models have demonstrated that administration of IL-7 to irradiated mice results in a preferential expansion of CD8+ cytotoxic T cells rather than T regulatory cells.22 Similarly, IL-15 generates a durable antitumor response when combined with RT, long-term immune memory against tumor rechallenge and improved survival.23,24 Currently there is one phase 1 clinical trial exploring IL-7 supplementation in high grade gliomas treated with RT.25 IL-15 is being evaluated as monotherapy in metastatic melanoma and renal cell carcinoma.26 Lymphocyte restitution is frequently utilized as a rescue strategy following myeloablative chemoradiation therapy in transplant protocols and adoptive T cell therapy.27 In one study, autologous lymphocytes harvested before temozolomide-radiation for high grade gliomas were reinfused after completion of treatment. The return of lymphocyte counts following radiation-induced depletion was, however, no different between reinfused patients and matched controls.28 Preclinical studies suggest that intratumoral thiodigalactoside administration prior to radiation rescued circulating CD8+ T cell and CD4+ T cell counts that were depleted by radiation.7 

A preponderance of evidence suggests that lymphopenia confers a poor prognosis in the treatment of cancer.5,8,9,13,17,29-31 Coupled with this evidence, the recent illustration of a direct correlation between dose to critical normal tissues in radiation treatment plans and post-treatment lymphopenia in multiple tumor types is a harbinger of concerted efforts to prevent or overcome this detrimental effect of radiation. This can be achieved by selectively modifying treatment plans and delivery techniques (beam arrangements, intensity modulation, stereotactic radiation, proton therapy, and high dose rate), in scenarios where secondary lymphoid organs receive large doses of radiation that could contribute to lymphopenia, or replenishing and/or protecting lymphocytes from damage mediated by radiation therapy where the dose of radiation to the circulating blood pool (within the tumor or surrounding normal tissue) is the predominant cause of lymphopenia. 

Written by: Bhanu Venkatesulu, Lakshmi Shree K Mahadevan, Maureen L. Aliru, Monica H. Bodd, Awalpreet S. Chadha, Steven H. Lin, and Sunil Krishnan.

References:
1. Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA: a cancer journal for clinicians. 2016;66(1):7-30.

2. Hidalgo M. Pancreatic cancer. The New England journal of medicine. 2010;362(17):1605-1617.

3. Reynders K, De Ruysscher D. Radiotherapy and Immunotherapy: Improving Cancer Treatment through Synergy. Prog Tumor Res. 2015;42:67-78.

4. Vatner RE, Cooper BT, Vanpouille-Box C, Demaria S, Formenti SC. Combinations of immunotherapy and radiation in cancer therapy. Front Oncol. 2014;4:325.

5. Grossman SA, Ellsworth S, Campian J, et al. Survival in Patients With Severe Lymphopenia Following Treatment With Radiation and Chemotherapy for Newly Diagnosed Solid Tumors. Journal of the National Comprehensive Cancer Network : JNCCN. 2015;13(10):1225-1231.

6. Kuo P, Bratman SV, Shultz DB, et al. Galectin-1 mediates radiation-related lymphopenia and attenuates NSCLC radiation response. Clinical cancer research : an official journal of the American Association for Cancer Research. 2014;20(21):5558-5569.

7. Vanpouille-Box C, Diamond JM, Pilones KA, et al. TGFbeta Is a Master Regulator of Radiation Therapy-Induced Antitumor Immunity. Cancer research. 2015;75(11):2232-2242.

8. Wild AT, Ye X, Ellsworth SG, et al. The Association Between Chemoradiation-related Lymphopenia and Clinical Outcomes in Patients With Locally Advanced Pancreatic Adenocarcinoma. Am J Clin Oncol. 2015;38(3):259-265.

9. Balmanoukian A, Ye X, Herman J, Laheru D, Grossman SA. The association between treatment-related lymphopenia and survival in newly diagnosed patients with resected adenocarcinoma of the pancreas. Cancer Invest. 2012;30(8):571-576.

10. Weinmann M, Becker G, Einsele H, Bamberg M. Clinical indications and biological mechanisms of splenic irradiation in chronic leukaemias and myeloproliferative disorders. Radiother Oncol. 2001;58(3):235-246.

11. Dahlin AM, Henriksson ML, Van Guelpen B, et al. Colorectal cancer prognosis depends on T-cell infiltration and molecular characteristics of the tumor. Mod Pathol. 2011;24(5):671-682.

12. Yasuda K, Nirei T, Sunami E, Nagawa H, Kitayama J. Density of CD4(+) and CD8(+) T lymphocytes in biopsy samples can be a predictor of pathological response to chemoradiotherapy (CRT) for rectal cancer. Radiat Oncol. 2011;6:49.

13. Chadha AS, Liu G, Chen HC, et al. Does Unintentional Splenic Radiation Predict Outcomes After Pancreatic Cancer Radiation Therapy? International journal of radiation oncology, biology, physics. 2017;97(2):323-332.

14. Krishnan S, Chadha AS, Suh Y, et al. Focal Radiation Therapy Dose Escalation Improves Overall Survival in Locally Advanced Pancreatic Cancer Patients Receiving Induction Chemotherapy and Consolidative Chemoradiation. International journal of radiation oncology, biology, physics. 2016;94(4):755-765.

15. Crocenzi T, Cottam B, Newell P, et al. A hypofractionated radiation regimen avoids the lymphopenia associated with neoadjuvant chemoradiation therapy of borderline resectable and locally advanced pancreatic adenocarcinoma. J Immunother Cancer. 2016;4:45.

16. Wild AT, Herman JM, Dholakia AS, et al. Lymphocyte-Sparing Effect of Stereotactic Body Radiation Therapy in Patients With Unresectable Pancreatic Cancer. International journal of radiation oncology, biology, physics. 2016;94(3):571-579.

17. Campian JL, Ye X, Brock M, Grossman SA. Treatment-related lymphopenia in patients with stage III non-small-cell lung cancer. Cancer Invest. 2013;31(3):183-188.

18. Yovino S, Grossman SA. Severity, etiology and possible consequences of treatment-related lymphopenia in patients with newly diagnosed high-grade gliomas. CNS Oncol. 2012;1(2):149-154.

19. Yovino S, Kleinberg L, Grossman SA, Narayanan M, Ford E. The etiology of treatment-related lymphopenia in patients with malignant gliomas: modeling radiation dose to circulating lymphocytes explains clinical observations and suggests methods of modifying the impact of radiation on immune cells. Cancer Invest. 2013;31(2):140-144.

20. Ellsworth S, Balmanoukian A, Kos F, et al. Sustained CD4+ T cell-driven lymphopenia without a compensatory IL-7/IL-15 response among high-grade glioma patients treated with radiation and temozolomide. Oncoimmunology. 2014;3(1):e27357.

21. Teixido C, Gonzalez-Cao M, Karachaliou N, Rosell R. Predictive factors for immunotherapy in melanoma. Ann Transl Med. 2015;3(15):208.

22. Faltynek CR, Wang S, Miller D, et al. Administration of human recombinant IL-7 to normal and irradiated mice increases the numbers of lymphocytes and some immature cells of the myeloid lineage. Journal of immunology (Baltimore, Md. : 1950). 1992;149(4):1276-1282.

23. Mathios D, Park CK, Marcus WD, et al. Therapeutic administration of IL-15 superagonist complex ALT-803 leads to long-term survival and durable antitumor immune response in a murine glioblastoma model. International journal of cancer. Journal international du cancer. 2016;138(1):187-194.

24. Pilones K, Aryankalayil J, Formenti S, S. D. Intratumoral IL-15 potentiates radiation-induced anti-tumor immunity. J Immunother Cancer. 2015;3((Suppl 2)):P239.

25. IL-7 in Increasing Low CD4 Counts After Concurrent Radiation and Temozolomide Treatment in Patients With High Grade Gliomas. ClinicalTrials.gov [cited 2017 Apr 5]. Available from: https://clinicaltrials.gov/ct2/show/NCT02659800?term=interleukin+7+And+radiation&rank=1. 2017.

26. Recombinant Interleukin-15 in Treating Patients With Advanced Melanoma, Kidney Cancer, Non-small Cell Lung Cancer, or Squamous Cell Head and Neck Cancer. ClinicalTrials.gov [cited 2017 Apr 5]. Available from: https://clinicaltrials.gov/ct2/show/NCT01727076?term=Interleukin+15&rank=1. 2017.

27. Dudley ME, Yang JC, Sherry R, et al. Adoptive cell therapy for patients with metastatic melanoma: evaluation of intensive myeloablative chemoradiation preparative regimens. Journal of clinical oncology : official journal of the American Society of Clinical Oncology. 2008;26(32):5233-5239.

28. Campian JL, Ye X, Gladstone DE, et al. Pre-radiation lymphocyte harvesting and post-radiation reinfusion in patients with newly diagnosed high grade gliomas. J Neurooncol. 2015;124(2):307-316.

29. Tang C, Liao Z, Gomez D, et al. Lymphopenia association with gross tumor volume and lung V5 and its effects on non-small cell lung cancer patient outcomes. International journal of radiation oncology, biology, physics. 2014;89(5):1084-1091.

30. Wu ES, Oduyebo T, Cobb LP, et al. Lymphopenia and its association with survival in patients with locally advanced cervical cancer. Gynecol Oncol. 2016;140(1):76-82.

31. Campian JL, Sarai G, Ye X, Marur S, Grossman SA. Association between severe treatment-related lymphopenia and progression-free survival in patients with newly diagnosed squamous cell head and neck cancer. Head Neck. 2014;36(12):1747-1753.

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