11/30/2009
Study predicts number of people living with brain tumors, underscores need for increased awareness.
A Lay Summary from "Prevalence estimates for primary brain tumors in the United States by age group, gender, behavior, and select histology" by Porter KR, McCarthy BJ, Freels S, Kim Y, Davis F. Poster (#PSS7) presented at the International Association of Cancer Registries Annual Meeting, New Orleans, June 3-5, 2009 (p. 117 of the Program and Book of Abstracts). Manuscript accepted for publication in Neuro-Oncology.
The National Brain Tumor Society recently commissioned and sponsored a study by leading brain tumor epidemiologists through the Central Brain Tumor Registry of the United States (CBTRUS) to tell us what they could about the number of brain tumor patients living in the US. It may seem like it would be easy to answer this question. After all, we can find out how many new cases are diagnosed because laws require that hospitals register new brain tumor cases in a national database (like other cancers). This measurement of new cases is called incidence.
But, as it turns out, it can be a challenge to understand how many people are actually living with brain tumors in any given time period. In a disease like brain tumors, where some folks may live for years following their diagnosis and others may not, incidence does not tell the full story of how many people actually have the disease at a certain point in time.
To better understand the burden of the disease (or how much it affects our health care system and individuals' lives) we need to understand the disease prevalence. Prevalence is determined not only by looking at the number of new cases but also how long people survive with the disease.
One previous study, nearly 10 years old (Davis et al., 2001), focused on brain tumor prevalence but a lot has changed in that time for both incidence (now non-malignant tumors are tracked more systematically than they were then; CBTRUS, 2008) and survival (survival rates have improved; Horner et al., 2009). NBTS sponsored this current study in order to understand the number of people living with brain tumors and to estimate the prevalence rate for 2010.
The prevalence rates for 2004 and 2010 are estimated from statistical models, and they are based on assumptions made concerning the incidence and survival of brain tumors, as well as population estimates and survival in the general population.
Key Findings
2004 Prevalence Rate Estimates
The overall prevalence rate in 2004 was estimated to be 209.0 per 100,000. Prevalence for non-malignant tumors was 166.5 per 100,000. There was a range of 25.1 to 59.9 per 100,000 for malignant brain tumors depending on the assumptions used for statistical modeling.
2010 Prevalence Rate Estimates
The overall prevalence rate was estimated to be 221.8 per 100,000 in 2010. Prevalence for non-malignant tumors was 177.3 per 100,000. For malignant tumors, there was a prevalence range of 27.1 to 61.9 per 100,000 for malignant tumors depending on the model assumptions used.
Prevalence rate differences: post-2001 study, and between 2004 and 2010
Prevalence rate estimates were higher than those reported in the previous study (Davis et al., 2001). This can be attributed to substantial changes in laws governing collecting and reporting of non-malignant brain tumors, having 20 years of follow-up, and modest improvements in survival rate estimates since the last study was conducted. The prevalence rates for 2004 provide higher quality estimates than were previously available.
The slight increase in prevalence rate estimates between 2004 and 2010 is because estimated improvements in survival have been incorporated into the statistical models. Any additional changes to or improvements in survival or increased incidence which occurred after this data was collected could result in an under-reporting of prevalence.
Differences between malignant and non-malignant
In this study, the prevalence rate estimate for non-malignant tumors is much greater than for malignant tumors. This can be explained by the longer survival time associated with non-malignant tumors, particularly meningiomas which are well represented in this sample.
Differences between genders
There is a higher prevalence of brain tumors in women than men, due to histological differences in tumor incidence between genders (CBTRUS, 2008. See www.cbtrus.org for incidence and survival estimates). Meningiomas, the largest non-malignant tumor group in this study, affect more women than men and also have a longer survival time. Gliomas affect men more often and have a shorter survival time, further contributing to this gap.
Pediatric vs. adults
Brain tumors are about five times more common in adults than in children and this difference contributes to major differences in prevalence rate estimates (278.1 per 100,000 compared with 35.4 per 100,000).
Frequently Asked Questions
What is a prevalence rate?
Prevalence is the proportion of new and existing disease cases in the population during a specific time period. Another way to look at this is it is a composite of the estimates using incidence and survival which reflects the number of people living with the disease. To calculate prevalence (how many people are living with brain tumors at a given time), you need to include information about disease incidence (how many new cases are diagnosed), and survival (how long, on average, people live with the disease).
Think of the prevalence rate like a fraction. You have a number on top which is "how many people are living with a brain tumor during a specified time period (such as in 2004)" and a number on the bottom which is "the total number of people in the population who could possibly be living with a brain tumor (i.e. everyone in that population) during that same time period (i.e. 2004)." In order to take account of things like how long various folks live, the top number is actually incidence and survival combined to come up with a number that represents how many are living with brain tumors at any point during that time period. But this number is only meaningful when compared with the total population of people who could have possibly gotten a brain tumor (the bottom half of our fraction).
How do epidemiologists account for differences in survival between malignant and non-malignant types of tumors? Specific tumor types?
Because we know that tumor types are different, the researchers analyzed a variety of subgroups of the data alongside the more complete data set. In addition to "All Primary Brain Tumor" they also looked at the larger categories of "Malignant" and "Non-malignant" as well as looking at some specific tumor types. This allows us to see that, as we might expect, the prevalence of non-malignant is higher than of malignant given that people with non-malignant tumors tend to live longer. Even though specific tumor types may have similar rates of diagnosis, the survival time affects the prevalence.
Why is there a range for the prevalence?
The researchers used two different assumptions in their statistical models for malignant tumors to predict prevalence rates. One model made an assumption that brain tumor patients more than 20 years out from their tumor diagnosis would have a survival similar to anyone in the general population–in other words, that the effect of the tumor on survival by this point is minimal. Another model made the assumption that after 20 years post-diagnosis malignant tumor patients are no longer alive. The overall prevalence rate estimates were the sum of the non-malignant prevalence estimate and the average of the two malignant brain tumor prevalence rates, one assuming survival after 20 years post-diagnosis was similar to the general population and the alternative assuming survival 10 years post-diagnosis falls to zero. By calculating malignant prevalence estimates using two different survival assumptions, the true malignant prevalence rate is very likely to lie between these two estimates.
How certain are we about this data? What does it mean?
A model is never a perfect scenario as things can change over time. For example, as survival data changes, the prevalence will also change. The authors tried to account for this having happened already (with the estimated survival benefit of a recent therapy regimen) in their model for the 2010 prevalence rate estimates. If something more major happens to improve brain tumor therapy, it is likely we will see a change in prevalence from what was reported here. The advent of Avastin for brain tumor patients, for example, happened later than this data included here so it is likely we'll see some shift in future studies based on this change in treatment if it affects survival in a measurable way.
What is the take away message?
The incidence rate estimates in this study were higher than in the previous study (Davis et al., 2001). We cannot be sure exactly to what this can be attributed, although improvements in reporting, especially of the non-malignant tumors, play a role. More research is needed to look at causation. However, a continued upward trend in prevalence due to expected improvements in survival time and the aging of the US population (the number of newly diagnosed brain tumors increases with increasing age) is likely, and this will necessitate the adequate development of resources to support brain tumor patients in the survivorship phase of their disease.
Action Steps
In response to this study, several action steps can be taken by advocates in the brain tumor community.
- This is a critical baseline report that will allow future study of trends in changes of prevalence. Now that non-malignant tumors are being tracked systematically, it will be important to carry out a similar study of brain tumor prevalence on a regular basis and monitor the changes. This will allow for adequate planning of the healthcare system and necessary resources.
- Incidence rates should be monitored closely (especially with tumors diagnosed after 2004 when the new laws governing the collection of non-malignant tumors were put in place). If the incidence rate patterns significantly change, it will signal a need for research looking more closely at causation to better understand what is driving the changes in the number of newly diagnosed patients.
- This study underscores the fact that brain tumors are affecting more than 600,000 people each year, and far more than that are affected as caregivers and family members. The size and recent growth of this population supports the need for increased awareness regarding services and research specific to this disease.
- As survival improves it is imperative that the healthcare system be prepared to cope with the complex and ongoing needs of brain tumor survivors. Improvements in malignant brain tumor survival have been shown by the SEER registry in their Cancer Statistics Review (Horner et al., 2009). Meanwhile, the healthcare system is not equipped to serve the myriad of needs brain tumor patients have post-treatment so this is an area where advocacy is needed.
References
CBTRUS (2008). Statistical Report: Primary Brain Tumors in the United States, 2000-2004. Published by the Central Brain Tumor Registry of the United States.
Davis FG, Kupelian V, Freels S, McCarthy B, Surawicz T. Prevalence estimates for primary brain tumors in the United States by behavior and major histology groups. Neuro-Oncology. 3(3):152-158, 2001. (Posted to Neuro-Oncology [serial online], Doc. 00-056, June 5, 2001. URL neuro-oncology.mc.duke.edu)
Horner MJ, Ries LAG, Krapcho M, Neyman N, Aminou R, Howlader N, Altekruse SF, Feuer EJ, Huang L, Mariotto A, Miller BA, Lewis DR, Eisner MP, Stinchcomb DG, Edwards BK (eds). SEER Cancer Statistics Review, 1975-2006, National Cancer Institute. Bethesda, MD, http://seer.cancer.gov/csr/1975_2006/, based on November 2008 SEER data submission, posted to the SEER web site, 2009.
