To evaluate the efficacy and safety of Pembrolizumab + Pemetrexed in elderly patients with non-squamous non-small cell lung cancer with Programmed cell death (PD) -ligand1 (L1) tumor proportion score (TPS) of less than 50%.

To evaluate the efficacy and safety of Pembrolizumab + Pemetrexed in elderly patients with non-squamous non-small cell lung cancer with Programmed cell death (PD) -ligand1 (L1) tumor proportion score (TPS) of less than 50%.Primary endpoint is overall response rate and secondary endpoints are progression free survival (PFS), 1 year PFS rate, overall survival and adverse event.

Inclusion Criteria:

1. Non-squamous non-small cell lung cancer (NSCLC) confirmed by histology or cytology.
2. Not received prior systemic treatment with stage IV, or recurrent NSCLC. Subjects who received adjuvant therapy are eligible if the adjuvant therapy was completed at least 12 months prior to the development of metastatic disease.
3. PD-L1 TPS of less than 50% with 22C3 antibody.
4. With at least one measurable lesion based on RECIST 1.1.
5. Age of 75 years or older on the day of informed consent.
6. ECOG Performance Status 0-1.
7. Without activating mutation in EGFR or ALK chromosomal translocation.
8. Absence of severe impairments of major organs.
9. Life expectancy of 12 weeks or more from the treatment start date.
10. Prior to the study registration, able to provide written informed consent after a thorough explanation of the study content.

Exclusion Criteria:

1. Before the first dose of trial treatment:

   Had major surgery (<3 weeks prior to the first dose)

2. Received radiation therapy to the lung that exceeds 30 Gy within 6 months of the first dose of the study treatment.

3. Completed palliative radiotherapy within 7 days of the first dose of the treatment.

4. Has received a live-virus vaccination within 30 days of planned treatment initiation.

   Seasonal flu vaccines that do not contain live virus are permitted.

5. Has clinically active diverticulitis, intra-abdominal abscess, GI obstruction, peritoneal carcinomatosis.

6. Has a history of malignancy except if the subject has undergone curative therapy without recurrence for 5 years since initiation of that therapy.

7. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis.

8. Previously had a severe hypersensitivity reaction to treatment with another mAb.

9. Has a known sensitivity to any component of pemetrexed

10. Has active autoimmune disease that has required systemic treatment in past 2 years

11. Is on chronic systemic steroids. Subjects with asthma that require intermittent use of bronchodilators, inhaled steroids, or local steroid injections would not be excluded from the study.

12. Is unable to interrupt aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs), other than an aspirin.

13. Is unable or unwilling to take folic acid or vitamin B12 supplementation.

14. Had prior treatment with any other anti-PD-1, or PD-L1 or PD-L2 agent or an antibody targeting other immuno-regulatory receptors or mechanisms.

15. Has an active infection requiring therapy.

16. Has a history of Human Immunodeficiency Virus (HIV)

17. Has known active Hepatitis B or C. Active Hepatitis B is defined as a known positive HBsAg result.

18. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the Principal Investigator.

19. Has symptomatic ascites or pleural effusion.

20. Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis.

21. Patients wishing their partner to become pregnant during the study.