Line of therapy
In one sentence
A line of therapy is a distinct treatment strategy in a defined disease setting, counted using the clinical record or a study’s stated rules.
The intuition
A line is like a chapter in a treatment story. Several medicines, visits and planned stages can belong to one chapter. Turning a page because the strategy changes is different from counting every medicine as a new chapter.
The analogy has a limit: clinical records and trial protocols may draw chapter boundaries differently. The number becomes useful only when its setting and counting rule are clear.
How it works
First-line therapy is the initial treatment strategy for a specified disease setting. Second-line therapy generally follows when the first strategy does not work or stops working. Intolerable toxicity can also lead to a new strategy. The National Cancer Institute's first-line and second-line definitions establish the basic sequence.
A regimen is a planned combination or sequence of treatments. A cycle is a repeating treatment-and-recovery interval. A planned block can use different medicines from the preceding block without automatically becoming another line. Neoadjuvant and adjuvant describe timing around the main treatment; they do not mean first and second line.
In advanced breast-cancer studies, “first line” often refers specifically to the first systemic strategy for advanced disease. Earlier treatment given with curative intent is recorded separately and can still affect eligibility. It is not safe to translate “first-line metastatic” into “never received an anticancer medicine.”
For example, the KEYNOTE-355 primary report studied previously untreated locally recurrent inoperable or metastatic triple-negative breast cancer. Its randomization also accounted for previous chemotherapy in the neoadjuvant or adjuvant setting. Those are compatible statements: prior early-disease exposure and prior advanced-disease treatment are different fields. This example explains a trial's terminology, not current access or a prescribing choice. Primary report.
Why it matters in cancer
A study in a first-line population may not transfer to people whose cancer progressed through several strategies. Previous exposure can affect drug resistance, tolerability and the comparison being tested. The same drug name in two studies therefore does not make their populations equivalent.
A “later line” is not a forecast of remaining life or a guarantee that options are exhausted. It is a history label. Disease biology, available evidence and the person's circumstances remain separate questions.
How it is recorded
| Record field | What it clarifies |
|---|---|
| Disease setting | Early, recurrent operable, unresectable or metastatic disease |
| Strategy and dates | Medicines, planned sequence, start and stop dates |
| Reason for change | Progression, toxicity, planned completion or another documented reason |
| Counting convention | How the clinical record, label or protocol handles switches and maintenance |
When a study labels continued treatment as maintenance, check whether its counting convention includes that phase with the initial line or counts it separately. A dose reduction, pause or removal of one component also does not automatically create a new line. Preserve what happened before assigning a number.
Common confusions
- Three drugs can form one regimen; three infusions are not three lines.
- A switch built into the original plan differs from changing strategy after progression.
- Early/adjuvant exposure may matter to trial eligibility even when it is not counted as a prior metastatic line.
- A database's algorithmic line count may differ from the clinician's account. Check its rules before comparing groups.
Try it
A fictional plan gives combination A before surgery and medicine B afterward as its planned adjuvant phase. Years later, metastatic disease receives its first systemic regimen. Does the earlier plan automatically make this third-line metastatic treatment?
Answer: No. The earlier treatments must remain in the history, but they were given in another setting. Count advanced-disease lines using the relevant clinical or protocol rule; do not count calendar phases as metastatic lines.
Explain it back
“Before interpreting ‘second line,’ I need ___.”
One possible answer: “The disease setting, the earlier strategy and the counting rule.”
Takeaway
A line describes a strategy in context, not a drug count or an infusion count.
Related concepts
Sources and scope
Source-checked October 10, 2026. General education; the treatment story is fictional. Expert and learner review remain pending. No universal algorithm for counting every switch is asserted.
- NCI: First-line therapy — initial treatment and a possible set of treatments.
- NCI: Second-line therapy — treatment after the initial treatment does not work or stops working.
- NCI: Chemotherapy — cycles, differing roles and schedule changes.
- Cortes and colleagues, 2020: KEYNOTE-355 — primary abstract; an advanced-disease population with previous neoadjuvant/adjuvant exposure recorded separately.