Cancer trial design

Observational studies are generally the only type of study that can ethically be carried out if the focus is on harm. For example, we can’t randomise people to smoke cigarettes or not, since we know tobacco intake is harmful. 

Observational studies

Observational studies aim to understand the epidemiology of a particular condition. For example, finding the prevalence of a particular disease or detecting associations between certain exposures and disease. 

There are three main types of observational studies – cross-sectional, case-control and cohort studies.

Cross-sectional studies

Cross-sectional studies are a snapshot in time. For example:

• the prevalence of pancreatic cancer in each of the Australian states
• the past smoking habits, and current diagnoses of lung cancer among Australians
• the number of people in Australia with diabetes who are obese and the number of people in Australia with diabetes who are not obese.

Case-control studies

Case-control studies select participants based on their disease status. There are two groups: cases, which have the condition of interest, and controls, which do not. Their past exposure to something is then investigated. It’s important that the exposure is in the past so that the exposure came before the disease. 

A case-control study looking at pancreatic cancer and vitamin D intake is an example. The cases are pancreatic cancer patients. Similar people without cancer are the controls. Vitamin D intake is the exposure of interest in this study. All participants are asked about past vitamin D intake to determine if it affects the development of pancreatic cancer in any way. 

Cohort studies

Cohort studies select participants based on their exposure but who do not yet have a condition of interest. Participants are followed up over time to see if they develop the condition.

An advantage of a cohort study is that it looks forward in time and can consider other variables that may be associated with both the disease and exposure – called confounders. The disadvantages are that they can be expensive. Since they can take a long time, participants are often lost to follow-up. 

Interventional studies

Interventional studies aim to test a new drug or treatment in patients with a condition of interest. Cancer trials mainly use randomised controlled trials (RCTs).

Randomised controlled trials (RCTs)

RCTs are a type of intervention study that has two groups of patients: 

• 1 group receives the new treatment or drug
• 1 group receives the control or placebo.

Participants are randomised, which means they randomly placed into one of the groups. The patients are then given the intervention and follow up over time.

RCTs are the gold standard of clinical trials, because they provide the best evidence for whether a new treatment works or not, and reduce bias. Researchers are often blinded, which means they don’t know which patients are in each group. Sometimes the participants are blinded as well, so they don’t know which group they’re in. Blinding researchers and participants further reduces bias.