Key Points
Understanding the Research
Why This Study Was Needed
How the Study Was Conducted
Who Was in the Study
Key Findings: What the Researchers Discovered
Safety and Side Effects
What This Means for Patients
Study Limitations
Recommendations for Patients
Frequently Asked Questions
Source Information

Key Points

In the ALASCCA randomized trial, 160 mg aspirin daily for three years roughly halved colorectal cancer recurrence in patients with PI3K pathway–altered tumors.
Among PIK3CA hotspot mutation patients, three-year recurrence was 7.7% with aspirin versus 14.1% with placebo.
Severe adverse events occurred in 16.8% of aspirin recipients versus 11.6% of placebo recipients, mainly bleeding risks.
Tumor genetic testing for PIK3CA, PIK3R1, and PTEN can identify whether a patient might benefit from aspirin after surgery.
Aspirin should only be started on your oncology team's advice and is not a replacement for standard surgery, chemotherapy, or surveillance.

Understanding the Research

Colorectal cancer remains a major global health burden, with approximately 1.9 million new cases diagnosed annually worldwide. Despite advances in surgical techniques, radiation therapy, and systemic drug treatments, recurrence rates remain substantial—metastatic recurrence (cancer that spreads to other organs) develops in 30 to 40% of patients even after apparently successful treatment.

Aspirin is one of the most extensively studied and widely used medications in the world. It has well-established pain-relieving (analgesic), anti-inflammatory, and heart-protective (cardioprotective) effects. In recent years, however, emerging evidence has suggested that aspirin may also play an important role in cancer prevention, particularly in colorectal cancer.

Both laboratory (preclinical) and clinical studies have shown that aspirin reduces the incidence of colorectal adenomas (precancerous polyps) and colorectal cancer itself, especially among high-risk individuals with hereditary colorectal cancer syndromes, such as Lynch syndrome. In addition, some studies have reported that using aspirin after a colorectal cancer diagnosis is associated with improvements in both disease-free survival (time without cancer returning) and overall survival.

However, the results across studies have been inconsistent. This inconsistency has led researchers to suspect that the benefit of aspirin may not apply to everyone, but rather may be restricted to molecularly defined subgroups—that is, patients whose tumors have certain specific genetic mutations.

Why This Study Was Needed

To understand the science behind this trial, it helps to know a little about how aspirin and certain gene changes interact. The anti-cancer effects of aspirin are believed to work primarily through the inhibition of an enzyme called cyclooxygenase-2 (COX-2), which is frequently overproduced (overexpressed) in colorectal cancer. COX-2 promotes tumor growth by increasing the production of a substance called prostaglandin E2 (PGE2), which then activates a cellular signaling pathway known as the PI3K (phosphatidylinositol 3-kinase) pathway—a chain of molecular events inside cells that controls cell growth, division, and survival.

Here is where the genetics come in: the PI3K pathway is genetically altered in approximately one third of colorectal cancers. These alterations most commonly involve mutations in three genes: PIK3CA, PIK3R1, and PTEN.

Laboratory and retrospective clinical data suggest that COX-2 may be a critical downstream "effector" of tumors with PIK3CA mutations—meaning that these mutated tumors rely heavily on COX-2 activity, making them particularly sensitive to aspirin's effects. This sensitivity is potentially driven by a positive feedback loop between PI3K signaling and COX-2 expression, where each activates the other, creating a cycle that aspirin can interrupt.

Several observational studies (studies that observe patients without randomly assigning treatments) have shown that aspirin improves survival among patients with PIK3CA-mutated tumors but offers no benefit in patients whose tumors do not carry these mutations (wild-type tumors). However, given the variability in existing clinical and experimental findings, researchers knew that a rigorous randomized controlled trial was urgently needed to establish whether this benefit was real. The ALASCCA trial was designed specifically to answer that question.

How the Study Was Conducted

The ALASCCA trial was a registry-based, multinational, double-blind, randomized, placebo-controlled trial. Let's unpack what that means in plain language:

Registry-based: The trial was integrated into the Swedish Colorectal Cancer Registry, a national database tracking cancer patients.
Multinational: It was conducted at 33 hospitals across four Nordic countries: Sweden, Norway, Denmark, and Finland.
Double-blind: Neither the patients nor their doctors knew who was receiving aspirin versus placebo (an inactive sugar pill).
Randomized, placebo-controlled: Patients were assigned by chance (like flipping a coin) to receive either aspirin or a matched placebo, ensuring that the two groups were comparable.

The trial enrolled patients 18 to 80 years of age who had undergone radical (complete) surgical removal of their tumor and had either:

Stage II or III colon cancer (cancer of the large intestine), or
Stage I, II, or III rectal cancer (cancer of the rectum, the last section of the large intestine)

Cancer stage was assessed using the standard AJCC (American Joint Committee on Cancer) Cancer Staging Manual, 7th edition. Crucially, all patients' tumor tissue was genetically screened for specific mutations. Patients were eligible only if their tumors carried what the researchers defined as "PI3K pathway alterations," divided into two groups:

Group A alterations: Prespecified hotspot mutations (well-known, specific mutation sites) in exons 9 and 20 of the PIK3CA gene.
Group B alterations: Other moderate- or high-impact genetic variants in PIK3CA, PIK3R1, or PTEN—mutations with varying degrees of functional effect on the protein they encode.

Patients who had both Group A and Group B alterations were counted in the Group A cohort. The genomic profile was assessed using a custom assay developed specifically for this trial, testing both tumor tissue and whole blood samples.

Eligible patients were randomly assigned in a 1:1 ratio to receive either 160 mg of aspirin or a matched placebo once daily for 3 years. For context, 160 mg of aspirin is considered a low dose—lower than a standard pain-relief dose (typically 325–500 mg) but higher than the 81 mg "baby aspirin" commonly used for heart protection in the United States. Treatment began within 12 weeks after surgery.

Patients could also receive adjuvant chemotherapy (chemotherapy given after surgery to eliminate any remaining cancer cells) chosen by their local investigator according to each country's national treatment guidelines, administered alongside the aspirin or placebo.

Randomization was stratified (balanced) according to three factors: tumor location (colon or rectum), cancer stage (I, II, or III), and genetic alteration group (A or B). Patients were recruited from April 6, 2016, through July 19, 2021. Follow-up evaluations were conducted every 3 months, either in person or by telephone.

Surveillance for cancer recurrence included thoracic (chest) and abdominal computed tomography (CT) or magnetic resonance imaging (MRI), following national guidelines that recommend imaging at 1 year and 3 years after surgery. Any suspected recurrence was evaluated at the local investigator's discretion. Adherence (whether patients actually took their assigned pills) was monitored through pill counts at clinic visits and patient-reported adherence during telephone follow-ups.

Trial Oversight and Governance

An independent data and safety monitoring committee reviewed safety data twice a year (biannually). The trial protocol was approved by the Swedish Ethical Review Authority and local ethics committees at each trial site or in each participating country. All patients provided written informed consent. The study was funded by the Swedish Research Council and other organizations. Notably, Pfizer provided the aspirin free of charge but had no role in the design of the trial protocol or the conduct of the trial—eliminating industry bias in how the study was run.

What the Researchers Measured

The primary end point (the main question the study was designed to answer) was colorectal cancer recurrence—defined as locoregional recurrence (cancer returning near the original site), distant metastasis (cancer spreading to other organs), or death from colorectal cancer—assessed specifically in patients with Group A alterations, using a time-to-event analysis (measuring how long until recurrence occurred).

Secondary end points included:

Colorectal cancer recurrence in patients with Group B alterations
Disease-free survival (time from randomization to first occurrence of locoregional recurrence, distant metastasis, any new primary cancer, or death from any cause)
Overall survival (time from randomization to death from any cause)
Adverse events and severe adverse events

Severe adverse events were defined as grade 3 or higher according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 4.0—meaning serious, medically significant events that may require hospitalization or be life-threatening.

The original sample-size calculation estimated that 204 aspirin recipients and 204 placebo recipients with Group A alterations would provide 80% power to detect a hazard ratio of 0.5 for disease recurrence by year 3, assuming a 25% recurrence rate in the placebo group. This calculation was updated in 2020 based on the actual distribution of cancer stages among enrolled patients. The updated calculation estimated that 150 patients per group would provide 80% power to detect a hazard ratio of 0.36, assuming an 18% recurrence rate by year 3 in the placebo group.

Who Was in the Study

The study screened a very large population. Here is the step-by-step enrollment journey:

13,178 patients aged 18–80 years with clinical stage I, II, or III colorectal cancer and planned curative abdominal surgery were eligible for prescreening.
6,397 patients were assessed for eligibility; 2,614 were excluded (2,203 did not meet eligibility criteria; 411 declined to participate).
3,783 patients provided written informed consent and had tumor and blood samples obtained during surgery for molecular screening.
Of these, 2,680 were excluded: 1,877 had no somatic alterations in PI3K pathway genes, 530 could not have their PI3K alteration status determined, and 273 did not have molecular screening performed.
This left 1,103 patients (37.0%) with PI3K pathway alterations out of 2,980 patients with complete genomic data.

Among these 1,103 patients, 515 (17.3%) had Group A alterations (PIK3CA hotspot mutations in exon 9 or 20), and 588 (19.7%) had Group B alterations (other variants in PIK3CA, PIK3R1, or PTEN).

After reassessing eligibility criteria following surgery:

314 patients with Group A alterations were randomly assigned: 157 to aspirin, 157 to placebo.
312 patients with Group B alterations were randomly assigned: 156 to aspirin, 156 to placebo.

In the safety analysis (patients who actually received at least one dose of their assigned treatment), there were 155 aspirin recipients and 156 placebo recipients in Group A, and 154 aspirin recipients and 155 placebo recipients in Group B. Only a very small number of patients (1–2 in each group) were lost to follow-up.

Baseline characteristics were generally well balanced between the groups. Here is a profile of the 626 randomized patients:

51.8% were women (324 patients)
33.1% had rectal cancer (207 patients)
Median age: 66 years (range, 31 to 80)
Median time from surgery to randomization: 7.7 weeks (range, 1.6 to 23.3)
91.7% of patients (574 patients) were randomized within 12 weeks after surgery

Additional molecular testing of the randomized patients revealed important tumor characteristics:

Microsatellite instability-high (MSI-High)—a marker of a specific type of genetic instability—was found in 145 of 530 patients (27.4%).
BRAF V600E mutation (a common cancer-driving mutation) was present in 140 of 552 patients (25.4%).
KRAS mutation (another common cancer gene mutation) was present in 288 of 579 patients (49.7%).
NRAS mutation was present in 27 of 540 patients (5.0%).

Adherence to the study regimen—defined as taking at least 80% of assigned doses—was 66.1% in the aspirin group (207 of 313 patients) and 70.3% in the placebo group (220 of 313 patients). Adherence was slightly higher in men (69.9%) than in women (66.7%).

Key Findings: What the Researchers Discovered

The Primary End Point: Recurrence in Patients with Group A Alterations

The most important finding of the trial was dramatic: among patients with Group A alterations (PIK3CA hotspot mutations in exon 9 or 20), the estimated 3-year cumulative incidence of colorectal cancer recurrence was 7.7% with aspirin, compared with 14.1% with placebo.

This means that aspirin reduced the risk of recurrence by about half. The hazard ratio was 0.49 (95% confidence interval [CI], 0.24 to 0.98; P = 0.04). In plain language, this means that patients taking aspirin were 51% less likely to experience a recurrence during the follow-up period than those taking placebo.

What does the P value mean? A P value of 0.04 means there is only a 4% probability that this result occurred by random chance. In medical research, a P value below 0.05 (5%) is considered statistically significant—meaning the result is likely to be a real effect, not luck.

What does the confidence interval mean? The 95% confidence interval of 0.24 to 0.98 represents the range of plausible values for the true effect. Because the entire range is below 1.0 (with the upper boundary barely touching 0.98), this confirms that aspirin is consistently associated with a reduced recurrence risk, though the precise magnitude of benefit could range from a 2% reduction to a 76% reduction.

Secondary End Points

The researchers also examined outcomes in patients with Group B alterations (other mutations in PIK3CA, PIK3R1, or PTEN), and the results were similarly encouraging:

Recurrence in Group B patients: The estimated 3-year cumulative incidence of recurrence was 7.7% with aspirin versus 16.8% with placebo, corresponding to a hazard ratio of 0.42 (95% CI, 0.21 to 0.83)—a 58% relative risk reduction.
Disease-free survival in Group A patients: The estimated 3-year disease-free survival rate was 88.5% with aspirin versus 81.4% with placebo (hazard ratio, 0.61; 95% CI, 0.34 to 1.08). While the hazard ratio favored aspirin, the confidence interval includes 1.0, meaning this particular result did not reach conventional statistical significance.
Disease-free survival in Group B patients: The estimated 3-year disease-free survival rate was 89.1% with aspirin versus 78.7% with placebo, corresponding to a hazard ratio of 0.51 (95% CI, 0.29 to 0.88)—this result was statistically significant.

It is worth emphasizing that the recurrence findings in both groups were remarkably consistent: aspirin roughly halved the recurrence risk regardless of whether patients had the specific PIK3CA hotspot mutations (Group A) or other PI3K pathway alterations (Group B). The disease-free survival results also favored aspirin in both groups, with the Group B result reaching statistical significance.

The researchers noted that the secondary end point analyses were descriptive and did not include adjustments for multiple comparisons—meaning these results, while strongly suggestive, should be interpreted with some caution.

Safety and Side Effects

Because aspirin is a blood thinner, it carries known risks, particularly gastrointestinal bleeding and bleeding in other parts of the body. The trial therefore carefully tracked adverse events.

The key safety finding: Severe adverse events occurred in 16.8% of aspirin recipients and 11.6% of placebo recipients. This means that patients taking aspirin had a somewhat higher rate of serious side effects—about 5 percentage points higher. While the absolute numbers are not enormous, this difference matters and highlights the importance of doctors weighing the benefits of recurrence prevention against the potential risks of bleeding and other serious complications in each individual patient.

Adverse events were carefully assessed in the safety population (all patients who received at least one container of aspirin or placebo). Events were graded according to standardized criteria, and local investigators who were unaware of treatment assignments evaluated them initially, with the principal investigator confirming them after the trial group assignments were revealed—a safeguard to ensure unbiased reporting.

What This Means for Patients

This study is genuinely practice-changing for several reasons. First, it provides the first randomized, placebo-controlled evidence that aspirin can reduce colorectal cancer recurrence in a genetically selected patient population. Prior to ALASCCA, the evidence for aspirin's benefit in PIK3CA-mutated colorectal cancer came mainly from observational studies, which can be influenced by various biases. This trial's rigorous double-blind, randomized design provides much stronger evidence.

Second, the treatment is remarkably simple, inexpensive, and accessible. Aspirin is available over the counter worldwide at very low cost. If confirmed by additional research, this could offer an affordable treatment option for colorectal cancer patients in both high-income and low-income countries.

Third, the study reinforces the concept of precision medicine—matching treatments to the genetic characteristics of an individual patient's tumor. Nearly 40% of patients in the trial (37.0%) had PI3K pathway alterations in their tumors, meaning a very substantial proportion of colorectal cancer patients could potentially benefit from this approach if genetic testing becomes part of routine care.

Fourth, there are possible broader implications. The finding that aspirin's benefit appears to extend to a wider range of PI3K pathway alterations (Group B), not just the specific PIK3CA hotspot mutations (Group A), suggests that the population of potentially eligible patients could be even larger than initially thought. However, the secondary nature of the Group B analyses means this requires confirmation.

The authors concluded that aspirin led to a significantly lower incidence of colorectal cancer recurrence than placebo among patients with PIK3CA hotspot mutations in exon 9 or 20 (Group A) and appeared to have a similar benefit among those with other somatic alterations in PI3K pathway genes (Group B).

Study Limitations

Like all medical research, this trial has important limitations that should be acknowledged:

Adherence was imperfect: Only 66.1% of aspirin recipients and 70.3% of placebo recipients took at least 80% of their assigned doses. This means some patients did not receive the full potential benefit, and the true treatment effect might be larger or smaller than observed depending on how adherence is analyzed.
Secondary end points were not adjusted for multiplicity: The results for Group B alterations and disease-free survival are described as descriptive, meaning they did not undergo the same rigorous statistical controls as the primary end point. While the results are strongly encouraging, they should not yet be treated as definitive.
Limited diversity of participants: The authors noted that Black patients were underrepresented in the trial, which may limit the generalizability of the findings to all populations.
Duration of follow-up: The primary analysis focused on 3-year recurrence and disease-free survival. Colorectal cancer can recur later than 3 years after surgery, and the trial's 5-year overall survival data were still being collected when the primary results were reported.
Sample size in subgroup analyses: The trial was powered primarily for the Group A analysis, and subgroups defined by cancer stage, tumor location, and other factors had relatively small numbers of patients, making those subgroup analyses less reliable.
The "number needed to treat" varies: While the relative risk reduction (~50%) was dramatic, the absolute benefit depends on a patient's baseline risk. The authors calculated the 3-year number needed to treat (NNT)—the number of patients who need to be treated to prevent one recurrence—according to cancer stage and tumor location. For lower-risk patients, more patients may need to be treated to prevent one recurrence, while for higher-risk patients, fewer.

Recommendations for Patients

Based on this study, here is what patients should know and discuss with their oncology team:

Do not start taking aspirin on your own: This trial tested a specific dose (160 mg daily) in a specific patient population (those with PI3K pathway alterations). Not all colorectal cancer patients have these genetic changes, and aspirin carries real bleeding risks. Your cancer care team is the right person to determine whether aspirin might be appropriate for you.
Ask about genetic testing of your tumor: If you have been diagnosed with colorectal cancer, ask your doctor whether your tumor has been or could be tested for PIK3CA, PIK3R1, and PTEN mutations. This information can help determine whether you might be one of the roughly one-third of patients whose tumors carry PI3K pathway alterations.
Consider the risk-benefit balance: While aspirin reduced recurrence risk dramatically, severe adverse events occurred in 16.8% of aspirin recipients compared with 11.6% of placebo recipients. Discuss your personal risk of bleeding (including any history of ulcers, bleeding disorders, or use of other blood-thinning medications) with your doctor.
Understand that this is one study: While the results are promising and statistically significant for the primary end point, the medical community will want to see confirmatory trials before aspirin becomes a standard recommendation. Stay informed and ask your doctor whether any follow-up studies have been published.
Maintain adherence if prescribed: If your doctor does recommend aspirin, taking it consistently matters. The trial found that recurrence rates were dramatically lower with aspirin, and the benefit depends on actually taking the medication as prescribed (at least 80% of doses).
Remember the basics still apply: Aspirin, if recommended, would be an addition to—not a replacement for—standard treatments including surgery, chemotherapy when indicated, and regular surveillance imaging at 1 and 3 years after surgery.

In summary, the ALASCCA trial provides the strongest evidence to date that a simple, inexpensive daily aspirin tablet can meaningfully reduce the risk of colorectal cancer recurrence in patients whose tumors carry PI3K pathway mutations. For these patients, the future of post-surgical care may soon include this widely available medication—a powerful example of how genetic testing can unlock the full potential of established drugs.

Frequently Asked Questions

Should I take aspirin after colorectal cancer surgery to lower the chance of recurrence?

Not on your own. In the ALASCCA trial, 160 mg aspirin daily for three years roughly halved recurrence risk in patients whose tumors had PI3K pathway alterations, especially PIK3CA hotspot mutations. But aspirin has bleeding risks. Ask your oncology team whether genetic testing and aspirin might be appropriate for your situation.

How can I find out if my colorectal tumor has the gene changes that aspirin might help?

Ask your doctor about testing tumor tissue for mutations in PIK3CA, PIK3R1, and PTEN. In the trial, specialized genetic screening identified PI3K pathway alterations in 37% of patients with complete genomic data. Testing your tumor can show whether you are part of the group that might benefit from aspirin after surgery.

What risks should I consider before taking daily aspirin after colorectal cancer surgery?

Aspirin is a blood thinner and can cause serious bleeding. In the trial, severe adverse events occurred in 16.8% of aspirin recipients versus 11.6% of placebo recipients. Your doctor will weigh your personal bleeding history, ulcers, or other blood thinners against the potential recurrence prevention benefit before recommending aspirin.

Does aspirin help every colorectal cancer patient, or only some?

Only patients whose tumors carry specific PI3K pathway alterations appeared to benefit in the ALASCCA trial. About one-third of colorectal cancers have these mutations. The trial enrolled only patients with these gene changes. For others, aspirin did not show the same effect and carries risks, so it is not for everyone.

What dose of aspirin was tested, and for how long after surgery?

The trial tested 160 mg of aspirin once daily for three years, starting within 12 weeks after surgery. This is a low dose, lower than standard pain-relief doses but higher than the 81 mg baby aspirin used for heart protection. Always follow your doctor's prescription rather than choosing a dose yourself.

Is taking aspirin now a standard recommendation after colorectal cancer surgery?

Not yet. The ALASCCA trial is one randomized study with strong results, but confirmatory trials are needed before aspirin becomes a routine recommendation. Discuss the findings with your oncology team and ask whether any newer studies have been published. Aspirin would be an addition to, not a replacement for, standard treatments.

Source Information

Original article title: Low-Dose Aspirin for PI3K-Altered Localized Colorectal Cancer

Authors: A. Martling, I. Hed Myrberg, M. Nilbert, H. Grönberg, F. Granath, M. Eklund, T. Öresland, L.H. Iversen, C. Haapamäki, M. Janson, K. Westberg, J. Segelman, U. Ersson, M. Prytz, E. Angenete, R. Bergström, M. Mayrhofer, B. Glimelius, and J. Lindberg, for the ALASCCA Study Group

Trial registration: ClinicalTrials.gov number, NCT02647099; EudraCT number, 2015-004240-19. Funded by the Swedish Research Council and others. Pfizer provided the aspirin free of charge but had no role in trial design or conduct.

This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and should not replace individualized medical advice from your healthcare provider. Always consult your doctor before starting, stopping, or changing any medication, including aspirin.

Low-Dose Aspirin After Colorectal Cancer Surgery: New Hope for Patients with PI3K Gene Changes

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