Testing the Effects of Palbociclib and Fulvestrant or Letrozole for Patients 70 Years of Age of Older With Metastatic Breast Cancer: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Testing the Effects of Palbociclib and Fulvestrant or Letrozole for Patients 70 Years of Age of Older With Metastatic Breast Cancer (NCT03633331), a na phase 2 study. The primary outcome is Incidence of Adverse Events.

2. Clinical Question

The central statistical question is about tolerability in an older patient population, not comparative efficacy. Secondary outcomes include treatment failure, adherence, response, progression-free survival, overall survival, and overall treatment utility. The registry lists these outcomes, but their results are not reported in the registry record. The posted safety summary includes serious adverse-event information, which is relevant to safety but is not a substitute for the specified primary endpoint.

3. Trial Design

This is a phase II, single-group trial sponsored by the Alliance for Clinical Trials in Oncology. The registry reports enrollment of 93 and a completed status. The recorded start date is 2018-08-15, and primary completion is recorded as 2020-09-08. Masking is listed as none, and allocation is not applicable. These features mean the treatment group was not formed through random assignment to competing study arms.

Eligible patients were aged 70 and older and had estrogen receptor-positive, HER2-negative metastatic breast cancer. The treatment group received palbociclib with letrozole or fulvestrant. Palbociclib was taken orally once daily on days 1–21; letrozole was taken orally once daily, or fulvestrant was administered intramuscularly on days 1 and 15 of the first course and day 1 of later courses. Courses repeated every 28 days in the absence of disease progression or unacceptable toxicity. The endocrine therapy choice was at physician discretion, so those receiving letrozole and fulvestrant should not be treated as randomized subgroups.

The primary endpoint concerns severe toxicity over 6 months. The registry defines the primary endpoint population as eligible patients who had not withdrawn before treatment and had adverse-event data available. The posted safety summary identifies 92 participants at risk for serious and other adverse events, while enrollment was 93. The reason for the difference between these counts is not reported in the registry record. Further details of the planned enrollment rationale, including a formal sample-size justification, are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: patients meeting the eligibility criteria who have not withdrawn before treatment and have adverse event data available are considered evaluable for the primary endpoint.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Incidence of Adverse Events, measured over 6 months.

The primary endpoint is the incidence of adverse events, expressed as the proportion of evaluable patients with documentation of grade 3–5 toxicity during the 6 months timeframe. Toxicity is counted regardless of whether it is attributed to the study treatment and is classified using NCI CTCAE version 5.0 criteria. The endpoint population excludes patients who withdrew before treatment and those without adverse-event data.

A proportion is a direct way to describe how frequently a defined event occurred in the study group. A confidence interval communicates the uncertainty around that estimated proportion: it gives a range of values compatible with the data and the statistical procedure, rather than a guarantee that the true value lies within the range. The registry describes constructing a 95% binomial confidence interval for the severe-toxicity rate.

This endpoint is used to characterize tolerability in the trial’s target population. It is not the same as the count of participants with serious adverse events in the posted safety summary. Seriousness and toxicity grade are distinct classifications: an event may be serious under adverse-event reporting criteria without being grade 3–5, and a grade 3–5 event is not necessarily classified as serious. The primary endpoint result itself is not reported in the registry record, so its proportion and confidence interval cannot be described from the posted results.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a proportion describe?

A proportion divides the number of participants meeting a defined event criterion by the number assessed for that criterion. In this trial, the prespecified primary event is documented grade 3–5 toxicity during 6 months, regardless of attribution. The denominator is restricted to eligible patients who had not withdrawn before treatment and had adverse-event data available. This definition matters: enrollment and the evaluable primary endpoint population are not automatically the same group.

A proportion describes frequency, not treatment effect relative to another therapy. In a single-group study, it summarizes the experience of participants in that group. Without a randomized comparator, it cannot by itself distinguish treatment-related effects from events related to the disease, background risk, or other factors.

How should the confidence interval be understood?

The registry specifies a 95% binomial confidence interval for the primary severe-toxicity proportion. In general, a confidence interval expresses the precision of an estimate under the assumptions of the method. A narrower interval indicates greater precision than a wider interval, but neither the interval nor its confidence level means that an individual participant’s outcome is predictable. The primary proportion and interval are not reported in the registry record.

The registry also describes cross-tabulation and Fisher’s exact testing for univariate relationships between severe toxicity and pretreatment characteristics. Cross-tabulation organizes event counts by categories of a characteristic. Fisher’s exact test is a method for assessing evidence of association in categorical data, especially when expected cell counts may be small. A test result, if available, would not establish that a characteristic caused toxicity or that the association will generalize beyond the study population.

What does exploratory logistic regression add?

The planned exploratory logistic regression would assess the relative contributions of selected pretreatment variables to the likelihood of severe toxicity. Logistic regression models the relationship between a binary outcome and explanatory variables; its association measure can be expressed as an odds ratio. The registry specifies reporting an odds ratio with an associated 95% confidence interval. An odds ratio describes an association in odds, not a risk ratio or a causal effect. The registry itself warns that generalizability is limited by the modest sample size. The fitted model, estimates, intervals, and results are not reported in the registry record.

General statistical caution is warranted when multiple characteristics are considered in a modest dataset: estimates can be unstable, and associations may reflect chance or confounding. The record does not report the model specification, variable-selection procedure, diagnostics, or any adjustment for multiple testing. Those details should not be inferred from the outline of the planned analyses.

How do planning illustrations differ from study results?

The registry record’s planning illustrations are described as calculations based on an observed effect, with two-sided significance level 0.05, equal allocation between two hypothetical arms, and a normal approximation. They use a stated fraction of the observed effect as the effect to detect. These are planning assumptions, not features of this trial’s single-group design. The illustrations do not represent observed power, and they should not be read as an estimate of the probability that this completed trial detected a real effect. For time-to-event outcomes, the stated calculation convention concerns total events; for other outcomes, it concerns participants per arm.

General power calculations depend on the assumed effect, outcome variability or event frequency, significance level, and design. If any of these differ from the assumptions, the planning result may not apply. The registry does not report a trial-specific sample-size justification, so the planning illustrations should not be used to fill that gap.

Why distinguish event classifications?

Adverse events can be described by severity grade and by seriousness, which are separate dimensions. Grade 3–5 toxicity is the primary endpoint definition; serious adverse events are a different category in the posted safety summary. A count of participants with serious events cannot be substituted for the primary severe-toxicity proportion, even if the measures concern related safety experiences. General interpretation also requires attention to the event definition, the population at risk, and the observation period. The registry’s safety summary and primary endpoint definition do not provide enough information to reconcile those classifications participant by participant.

10. Exact Estimates for Each Arm

No two-arm comparison is shown for this trial, but each arm's result can still be summarized with exact interval estimates computed from the posted counts. The Clopper-Pearson interval is exact and conservative; the Wilson interval is a common closer-to-nominal alternative; the Bayesian credible interval uses a uniform Beta(1, 1) prior. For small groups the three can differ noticeably.

R code and output

arm    <- c("Treatment (Palbociclib, Letrozole or Fulvestrant)")
n      <- c(90)
events <- c(1)
for (i in seq_along(n)) {
  ci <- binom.test(events[i], n[i])$conf.int          # Clopper-Pearson exact interval
  cr <- qbeta(c(0.025, 0.975), 1 + events[i], 1 + n[i] - events[i])   # Bayesian, uniform Beta(1,1) prior
  wi <- (function(x, n) { p <- x / n; z <- qnorm(0.975); d <- 1 + z^2 / n
    c((p + z^2 / (2 * n) - z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d, (p + z^2 / (2 * n) + z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d) })(events[i], n[i])
  cat(sprintf("ARM|%s|%d|%d|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f\n", arm[i], n[i], events[i], 100 * events[i] / n[i],
              100 * ci[1], 100 * ci[2], 100 * wi[1], 100 * wi[2], 100 * cr[1], 100 * cr[2]))
}

ARM  Treatment (Palbociclib, Letrozole or Fulvestrant)  90  1  1.1  0.0  6.0  0.2  6.0  0.3  6.0

11. Multiplicity, Missing Data and Analysis Populations

The registry lists 11 outcome measures, including the primary severe-toxicity endpoint and secondary measures such as dose changes, treatment failure, adherence, response, progression-free survival, overall survival, and overall treatment utility. It does not report a multiplicity strategy for interpreting results across these outcomes or across the proposed associations with pretreatment characteristics. Without such information, the registry record does not establish how family-wise false-positive risk or the exploratory nature of multiple analyses would be handled.

The primary endpoint population is defined as eligible patients who did not withdraw before treatment and had adverse-event data available. This specifies who is considered evaluable, but broader missing-data procedures are not reported in the registry record. The record does not describe how missing event information, incomplete follow-up, or missing covariate values would be handled in endpoint or regression analyses. Nor does it report the number excluded from the primary endpoint population or reasons for exclusion.

Enrollment is listed as 93, while the safety summary lists 92 participants at risk for serious and other adverse events. This distinction should be retained rather than assuming the same denominator for every endpoint. The reason the counts differ is not reported in the registry record. A separate count of evaluable participants for the primary endpoint, and the handling of participants not meeting that definition, are also not reported in the registry record.

The registry lists 0 analysis records. This means results for the planned statistical analyses are not available as analysis records in the registry. It does not establish whether analyses were conducted outside the registry. Consequently, no multiplicity-adjusted results, missing-data sensitivity analyses, or analysis-population comparisons can be described from the record.

12. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: neutrophil count decreased (10.9%), white blood cell decreased (8.7%), anemia (6.5%), fatigue (6.5%), nausea (5.4%).

The posted safety summary identifies 92 participants at risk for serious adverse events, of whom 35 were affected. For the separately labelled “other” adverse-event category, 92 were at risk and 92 were affected. The summary also records 4 deaths. These figures describe the safety group in the registry; they do not state the causal relationship of events or deaths to treatment. Reasons for death and attribution are not reported in the registry record.

The listed serious-event terms include neutrophil count decreased (10.9%), white blood cell decreased (8.7%), anemia (6.5%), fatigue (6.5%), nausea (5.4%), and dyspnea (5.4%). The registry’s posted summary does not identify the denominator for these percentages. They should therefore be read as reported term frequencies, not combined or recalculated into a new overall rate.

Serious adverse events and grade 3–5 toxicities are not interchangeable measures. Seriousness concerns regulatory event criteria, while grade describes toxicity severity. The primary endpoint is a grade-based proportion over 6 months; the posted safety figures are not a reported value for that endpoint. Event-level detail, overlap between participants with different events, and attribution are not reported in the registry record.

13. What the Result Does, and Does Not, Mean

The record supports a descriptive account of a single-group phase II study in adults aged 70 and older with estrogen receptor-positive, HER2-negative metastatic breast cancer. The posted safety summary reports serious adverse events in 35 of the 92 participants at risk, other adverse events in 92 of 92 at risk, and 4 deaths. It also lists selected serious-event terms with percentages. These registry entries characterize what the registry summary records; they do not independently establish that palbociclib or either endocrine therapy caused an event.

The registry’s primary endpoint is more specific than the overall safety summary: it concerns documented grade 3–5 toxicity, regardless of attribution, in the defined evaluable population over 6 months. Its proportion and 95% confidence interval are not reported in the registry record. Therefore, the serious-event count cannot answer the primary endpoint question or convey its precision.

The trial also cannot determine comparative outcomes between letrozole and fulvestrant, because participants were not randomized between those options and the choice was made at physician discretion. Nor does a single treatment group provide a randomized estimate against an alternative regimen or no treatment. Differences in patient characteristics, clinical decisions, and follow-up could influence descriptive outcomes.

Secondary outcomes are listed, but their results are not reported in the registry record. The record therefore does not support conclusions about response, progression-free survival, overall survival, adherence, or treatment utility. More broadly, the absence of posted analysis results means the planned tests and models cannot be evaluated for estimates, uncertainty, or robustness. The data described in the registry should be interpreted as safety reporting, not as a complete account of every planned outcome or a causal comparison.

14. Why This Trial Matters Statistically

This study addresses a statistical question in a defined population: how to describe severe toxicity during treatment among older adults with metastatic breast cancer. Age, disease characteristics, and treatment context define who the findings could describe. Clear reporting of the event definition, assessment period, and evaluable population is essential because a safety proportion can change meaning when its numerator or denominator changes.

The registry distinguishes a planned grade 3–5 toxicity endpoint from posted serious adverse-event counts. That distinction is statistically important. Measures that sound similar can answer different questions, and combining them would obscure the endpoint actually specified for the trial. The record also describes confidence intervals and exploratory associations, emphasizing that estimates should be interpreted alongside uncertainty and the limits of a modest sample.

The single-group design is suited to describing the treatment group’s recorded experience, but it does not supply a randomized counterfactual. The physician-directed choice between letrozole and fulvestrant adds another reason not to treat the treatment options as directly comparable. The statistical interpretation must therefore remain descriptive rather than comparative.

Finally, the registry lists multiple secondary outcomes and planned exploratory analyses but no analysis records. This highlights the difference between outcomes named in a protocol record and results available for assessment. For clinical researchers, that difference matters when judging completeness, precision, and the scope of conclusions that a registry entry can support.

15. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

Enrollment was 93, while the posted safety summary lists 92 participants at risk. The reason for this difference is not reported in the registry record. The number included in the primary endpoint population is also not reported, so the safety denominator cannot be assumed to equal the endpoint denominator.

The primary endpoint is specified as grade 3–5 toxicity over 6 months, but its observed proportion and confidence interval are not reported in the registry record. Serious adverse-event counts do not resolve this gap because seriousness and toxicity grade are different classifications. The summary also does not report attribution or the reasons for the 4 deaths.

The design is single-group, with no randomized comparator. Letrozole or fulvestrant was selected at physician discretion, so any apparent differences between those receiving each option could reflect patient selection or clinical decisions rather than treatment effects. The record does not report results for the listed efficacy and treatment-utility outcomes.

The registry describes a modest sample and exploratory logistic regression, but does not report the fitted model or its estimates. The statistical analysis details beyond the stated methods, a multiplicity strategy, missing-data procedures, and trial-specific sample-size justification are not reported in the registry record. These omissions limit assessment of precision, robustness, and generalizability.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Testing the Effects of Palbociclib and Fulvestrant or Letrozole for Patients 70 Years of Age of Older With Metastatic Breast Cancer — NCT03633331.
• Registry record last updated 2026-10-07; status last verified 2026-04.
• Results first posted 2021-10-15.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

19. Record Summary

NCT03633331 is a completed phase II, single-group trial sponsored by the Alliance for Clinical Trials in Oncology. It enrolled 93 patients aged 70 and older with estrogen receptor-positive, HER2-negative metastatic breast cancer. Participants received palbociclib with letrozole or fulvestrant, with the endocrine therapy choice made at physician discretion.

The primary endpoint was the proportion of evaluable patients with documented grade 3–5 toxicity during 6 months, regardless of attribution, with a planned 95% binomial confidence interval. The posted safety summary lists 92 participants at risk for serious events, 35 affected, and 4 deaths. It does not report the primary endpoint estimate or its interval. The record lists 11 outcome measures and 0 analysis records, so results for the planned analyses and secondary outcomes are not reported in the registry record.