What Are DoR, ORR, CR, and pCR? How Is Response Rate Measured in Cancer Clinical Trials?
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Traditional oncology endpoints— overall survival (OS), disease-free survival (DFS) and event-free survival (EFS) —require particularly long follow-up. Today, however, responses to more biologically targeted drugs (e.g., PARP inhibitors, CDK4/6 inhibitors, PD-1/PD-L1 blockers), immunotherapies, and antibody–drug conjugates can sometimes occur very rapidly. Early assessment of these responses has become crucial for clinicians and regulatory agencies (FDA, EMA).
Modern clinical trials therefore frequently use four main parameters to measure how much treatment shrinks a tumor, how long this shrinkage lasts, and whether the tumor truly ‘disappears’ in some patients: Duration of Response (DoR), Objective Response Rate (ORR), Complete Response (CR), Pathologic Complete Response (pCR).
These indicators are more than numbers; they also seek to answer: ‘Does this treatment truly change the patient’s tumor biology, or does it merely produce temporary shrinkage?’
1. Duration of Response (DoR)
Definition: Duration of Response (DoR)describes, after a patient achieves a complete response (CR) or partial response (PR) , how long that response lasts until progression (renewed disease growth/advancement) or death occurs.
DoR = Time from the response start date → to the date of first progression or death.
DoR answers the question, ‘This treatment produced a response, but is it durable?’ With immunotherapies in particular (such as anti-PD-1/PD-L1 and anti-CTLA-4), some patients’ tumors have been observed to remain suppressed for very long periods, almost becoming a chronic disease. Because DoR can demonstrate ‘durable clinical benefit’ from immunotherapy, it is now interpreted as a quality indicator rather than simply a value as well.
Example: If measurable tumor volume shrinks dramatically in a metastatic melanoma patient receiving a PD-1 inhibitor and remains stable for 14 months, the patient’s DoR is 14 months. This is highly valuable compared with responses lasting a few months with conventional chemotherapy.
2. Objective Response Rate (ORR)
Definition: Objective Response Rate (ORR)is the proportion of patients achieving a complete response (CR) or partial response (PR) to treatment, relative to the entire treated population.
ORR = (Number of patients with CR + PR) / (Total number of treated patients)
ORR is generally determined using standardized criteria such as RECIST 1.1 (by measuring percentage reductions in tumor diameters). It is objective rather than subjective, hence ‘objective.’ A high ORR in an early-phase (phase II) trial is interpreted as evidence that the drug ‘truly shrinks tumors’ and can justify larger phase III studies.
In rapidly proliferating tumors such as breast cancer, small cell lung cancer, and head and neck cancer, a high ORR means ‘urgent volume control’: a mass causing breathing difficulty, bleeding, or compression can be shrunk rapidly. This can also be critical for palliative relief.
3. Complete Response (CR)
Definition: Complete Response (CR)means that imaging, physical examination, and laboratory findings show no remaining detectable evidence of a tumor . In other words, cancer sites appear to have disappeared completely on imaging.
microscopic residual disease (minimal residual disease, MRD) may remain, especially in hematological malignancies.
CR is an important goal associated with long-term prognosis in blood cancers (e.g., multiple myeloma, acute leukemia) and some solid tumors. In multiple myeloma, a ‘deep response’ (e.g., CR + MRD negativity) indicates stronger biological disease control and may therefore be associated with longer progression-free survival (PFS) and potentially longer overall survival (OS).
Its clinical importance is this: Numerous published studies show significant separation of both EFS and DFS curves between patients achieving CR and those who do not. CR is therefore a prognostic signal as well as a radiological finding.
4. Pathologic Complete Response (pCR)
Definition: Pathologic Complete Response (pCR)is the situation after neoadjuvant treatment (preoperative chemotherapy / immunotherapy / targeted therapy) in which the removed surgical specimen and regional lymph nodes show no invasive tumor cells . In other words, no viable invasive cancer remains under the microscope.
Particularly in neoadjuvant breast cancer trials, pCR is a crucial endpoint. Patients achieving pCR have repeatedly been shown to generally have higher DFS (time lived without recurrence) and better OS (overall survival) than those not achieving it. pCR has therefore been used as a primary biological marker in FDA accelerated approvals for certain neoadjuvant treatments.
Example: Marked increases in pCR rates after adding immunotherapy to chemotherapy in triple-negative breast cancer (TNBC) paved the way for subsequent use of the combination in the adjuvant phase. This represents a major turning point in biologically aggressive TNBC.
Why Are These Measures Clinically Important?
They help clinicians answer two crucial questions:
- Is there a response? (ORR, CR, pCR)
This can be vital when rapid tumor burden control is needed (e.g., thoracic masses threatening breathing, obstructing mediastinal lymph nodes, or liver metastases posing a risk of liver failure). - Is the response durable? (DoR)
Shrinking a tumor is not enough; if it grows again after 2 months, the clinical gain is limited. DoR makes this distinction visible.
These parameters also indicate whether a treatment simply reduces size or truly suppresses tumor biology. For example, immunotherapy studies may not always appear as ‘high’ in ORR as conventional chemotherapy, but DoR can be strikingly long among responders. This pattern of ‘fewer but deep and durable responses’ underpins immunotherapies’ ability to open the door to lasting disease control in many tumor types.
In multiple myeloma, deep response (CR or MRD-negative CR) significantly improves the progression-free survival (PFS) curve. Similarly, long-term recurrence risk falls dramatically in triple-negative breast cancer patients achieving pCR after neoadjuvant treatment, making pCR a prognostic marker.
Their Role in Clinical Trials and Drug Approval
These metrics are more than statistics in scientific papers; they also play a direct role in approving new drugs.
- ORR (Objective Response Rate) being high in a phase II trial can support FDA ‘accelerated approval,’ especially in aggressive or relapsed disease with limited treatment options. This can provide access without waiting for conventional long-term survival data.
- pCR (Pathologic Complete Response)is used by regulators as an indicator of ‘early biological benefit’ in neoadjuvant + adjuvant combination strategies, particularly in breast cancer. Patients achieving pCR have repeatedly shown better DFS and OS curves, making pCR a bridging endpoint that guides clinical practice.
- DoR (Duration of Response)is now reported as a critical subsection in many immunotherapy labels (licensing documents) because it supports the argument that immunotherapies and certain targeted treatments ‘not only shrink the tumor but also suppress it for a long time.’
- CR (Complete Response)has raised the target standard in treatment algorithms by demonstrating that treatment can ‘eliminate the disease burden,’ particularly in hematological malignancies.
These early endpoints (ORR, DoR, pCR) are often regarded as surrogate endpoints predicting later DFS / EFS / OS advantages . Regulators may therefore grant early approval on the expectation that an ‘early biological signal’ will translate into actual survival benefits. This means faster access to oncology treatments.
How Do They Connect with DFS / EFS?
DFS (disease-free survival) and EFS (event-free survival) are time-based follow-up metrics addressing whether disease returns or treatment is disrupted. ORR, CR, pCR, and DoR measure treatment’s early, direct effect on the tumor itself.
The following patterns are common in research:
- High pCR in the neoadjuvant phase → DFS tends to be better in the postoperative adjuvant phase.
- High ORR and long DoR in metastatic disease → The regimen may significantly extend EFS or PFS.
- CR achieved in hematological malignancies → EFS and even OS curves are more favorable than in patients without CR.
Duration of Response (DoR), Objective Response Rate (ORR), Complete Response (CR) and Pathologic Complete Response (pCR) are early efficacy indicators forming the backbone of modern oncology studies.
These measures:
- Show which treatments have rapid, powerful effects on tumors (ORR, CR).
- Show whether this effect is temporary or biologically durable (DoR).
- Show whether preoperative treatment truly clears all cancer tissue (pCR).
- Help model the likelihood of these early signals translating into long-term outcomes (DFS, EFS, OS).
- Provide regulators with a rationale for bringing new drugs to patients faster .
In brief, these metrics are more than numbers: they are strategic clinical tools directly affecting individual prognosis, clinical decisions, guideline development, and the licensing of certain drugs.
References
- RECIST 1.1 Criteria for Response Evaluation in Solid Tumors. European Journal of Cancer. 2009.
- US FDA Oncology Center of Excellence. Guidance for Industry: Clinical Trial Endpoints for the Approval of Cancer Drugs and Biologics. 2023.
- Masuda N et al. Pathologic complete response and prognosis after neoadjuvant chemotherapy in breast cancer. J Clin Oncol. 2020.
- Schmid P et al. Pembrolizumab in early triple-negative breast cancer. N Engl J Med. (relationship between neoadjuvant immunotherapy and pCR)
- Wolchok JD et al. PD-1 blockade in metastatic melanoma: durability of response and survival impact. N Engl J Med.
- ASCO Educational Book. “Depth and durability of response as predictors of long-term control in solid tumors and hematologic malignancies.” 2024.
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