Please assess the completion status of the following clinical trial:

Trial Title: Association of Survival With Maintenance Therapy in Patients With Metastatic Breast Cancer After First-line Chemotherapy

Study Design Characteristics:
- Allocation: None
- Intervention Model: None
- Observational Model: COHORT
- Primary Purpose: None
- Time Perspective: RETROSPECTIVE
- Masking: None
- Phase: None
- Number of Arms: None

Eligibility Criteria:
Inclusion Criteria:~* Patients with pathologically clear invasive breast cancer between 2003 and 2018;~* Female,18-75 years old;~* Measurable metastatic lesion according to RECIST 1.1 evaluation criteria;~* The first-line chemotherapy regimen is a breast cancer combination or single-agent chemotherapy regimen recommended by the NCCN guidelines;~* First-line chemotherapy is effective (according to RECIST1.1 evaluation criteria, the efficacy is evaluated as complete response (CR), partial response (PR), or steady state (SD));~* After the last cycle of first-line chemotherapy, patients should still be in a state of no progress for at least 4 weeks;~* Patients' Karnofsky performance status (KPS) scores were ≥70.~Exclusion Criteria:~* Unmeasurable metastatic lesion according to RECIST 1.1 evaluation criteria.

- Gender: FEMALE
- Minimum Age: 18 Years
- Maximum Age: 75 Years
- Healthy Volunteers: f

Study Arms:
- MCT Group (Type: None): People who received chemotherapy as a maintenance therapy after first-line chemotherapy reaching a state of no progress for at least 4 weeks. One patient only received one of the intervention drugs for maintenance therapy.
- MET Group (Type: None): People who received endocrine therapy as a maintenance therapy after first-line chemotherapy reaching a state of no progress for at least 4 weeks. One patient only received one of the intervention drugs for maintenance therapy.
- Observation Group (Type: None): People who didn't receive any maintenance therapy after first-line chemotherapy reaching a state of no progress for at least 4 weeks.

Based on this trial design, predict the completion outcome and termination type (if applicable).