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A Different View of Sarcoma Statistics by Joan Darling, PhD
Joan Darling, Ph.D. Environmental Consultant Lincoln, Nebraska
[Editor’s Note: Joan is a biologist. She is also mother of two, one of whom is an 11-year survivor of alveolar rhabdomyosarcoma. Joan is an active volunteer for a variety of childhood cancer and sarcoma organizations and spends much of her spare time as Co-List manager of the ACOR Rhabdo-Kids mailing list, as a Patient Advocate for the Children's Oncology Group, and as a board member of the Sarcoma Alliance.]
Introduction This article was inspired by several on-line postings that raised questions about the frequency of childhood cancers or sarcomas. In almost all of these postings, people seemed to understand the annual risk of developing a cancer but did not understand the cumulative risk over a lifetime. This article thus is an effort to view sarcoma statistics in a different way than they are usually presented, and possibly stimulate others to look into the interesting but not always intuitive field of cancer statistics and their uses.
What is the Incidence of Sarcoma? Everyone who has had experience with sarcoma knows that it is an uncommon type of cancer. For example, when my daughter Ali was diagnosed with rhabdomyosarcoma (rhabdo), the most common type of pediatric soft tissue sarcoma, we were told that the odds of getting this cancer were approximately six in a million.
This number is supported by the NCI SEER statistics. Table 1 shows that among children under the age of 20, on average about five children out of every 1,000,000 will be diagnosed with rhabdo each year. Add in about half of the “unspecified” cases, and that agrees closely with the six in a million incidence. Certainly it is not a common disease.
Table 1: Age-Adjusted and Age-Specific SEER Cancer Incidence Rates, 2001-2004 (Rates are per 1,000,000 and are age-adjusted to the 2000 US Std Population. Source: National Cancer Institute, SEER Cancer Statistics Review, 1975-2004, Adapted from Table XXIX-1. For Kaposi's sarcoma, the number of cases was too low to calculate an incidence rate. [Editor's Note: Click on the hyperlink in the above citation to view the specific table sited in the SEER report. Links to the other tables that are referenced in this article are also provided in the text and citations that follow.]
Sarcomas are not common among any age group. In adults, sarcomas are estimated to be about one percent of all cancers, and approximately 12,000 people a year in the U.S. are diagnosed with sarcoma (NCI, Report of the Sarcoma Progress Review Group, Jan. 2004, page 1).
Sarcomas are relatively more common among children. Between 1,500 and 1,700 U.S. children are diagnosed with a bone or soft tissue sarcoma each year making up about 15 percent of cancers in children under the age of 20 (ibid), but pediatric cancers themselves are rare and make up only about 1 percent of cancer cases (SEER Cancer Statistics Review 1975-2004, Table I-10 Age Distribution at Diagnosis and Death).
Table 2 shows that the estimated number of new bone and soft tissue cancers (most comparable to sarcomas according to the way that adult cancers are classified) in the U.S. in 2006 was 12,290, approximately 1 percent of all cancers.
Table 2: Estimated New Cancer Cases and Deaths for 2006 (Source: National Cancer Institute, SEER Cancer Statistics Review, 1975-2003, Adapted from Table I-1.)
Numbers such as “six in a million” are difficult for many people to understand, however. A number like this may make it seem as though only a minutely small number of families in the country are grappling with some stage of the rhabdo experience. It might be more understandable to many if incidence statistics were presented in a different way.
How Many People in the United States Have Sarcoma? The incidence of many cancers is given as a lifetime risk. An oft-quoted fact about prostate cancer is that one of every 6 men will develop it sometime during their lifetime. In the same vein, one out of every 8 women will develop breast cancer during their lifetimes.
It is very difficult to find comparable lifetime risks for developing sarcomas. The 1,700 children or 12,000 total numbers of sarcoma cases, and the six in a million for rhabdomyosarcoma, all represent the risk for a single year. However, representing the risk over a lifetime rather than a single year provides two benefits in understanding the incidence of a cancer. First, it represents the actual odds of a person developing that cancer at some time during their life. Second and perhaps as importantly, it makes the risk easier to understand.
The lifetime risk is cumulative; that is, the risk of being diagnosed with a cancer in one year is added to the risk of being diagnosed in the next year, and so on. For example, each year the odds of a child getting rhabdo are about six in a million. Multiply that by the 20 years of "childhood" and the odds of a child getting rhabdo are about 120 in a million, or just about one in 8,500. And since rhabdo makes up a little more than one-quarter of pediatric sarcomas, about 1 in 2,300 children under the age of 20 in the U.S. will be diagnosed with a sarcoma. These numbers represent the risk of a child being diagnosed with sarcoma by the age of 20 – 1 child in 8,500 for rhabdo, and 1 child in 2,300 for all sarcomas. These numbers also bring the risk into a scale that is easier to understand, as people can relate more to risks expressed in hundreds or thousands rather than those expressed in millions.
The same cumulative principle holds true for adult sarcomas. There are about 12,000 new cases of sarcoma annually and approximately 300 million people in the U.S., thus the odds of getting a sarcoma can be given as about 40 in a million for any one year. Although the risk of developing a sarcoma as an adult actually varies somewhat from year to year, as a very rough estimate of lifetime risk the yearly risk can be assumed to be the same every year (see “A Double Check”). Multiply the annual risk by, say, 70 years of lifespan, and the odds of being diagnosed with sarcoma are 2,800 in a million, or about one in every 350 people.
“When” counts as much as “How Many” There is another important sarcoma statistic that underscores the impact on society rather than considering the number of cases. That statistic is the age at which sarcomas are diagnosed. Cancers that affect younger people have disproportionately large impacts, because the younger the victim, the more potential there is to contribute to society. For example, despite the relatively small number of cases of childhood cancer, an extremely large number of years of life are lost if a child does not survive. The graphs below from SEER demonstrate this effect.
The left graph in Figure 1 below illustrates that despite the rarity of childhood cancer, 105,200 person-years of life are lost each year to childhood cancer, just for children up to the age of 14. The right graph in Figure 1 shows that for each child who does not survive cancer, 70 years of life are lost on average. Compare this to the 9 years lost on average for prostate cancer, or the 19 years for breast cancer. It is a loss to society when anyone’s life is cut short, but the death of a child from cancer results in an incomparably larger loss of the potential to contribute to society.
Figure 1: Years of Life Lost to Different Cancers (Source: NCI, SEER Cancer Statistics Review 1975-2004, Table I-19, NCHS public use data file for the total US and 2003 Life Tables. Updated on June 28, 2007 to reflect March 28, 2007 update to NCHS 2003 Life Table. Click on the Table I-19 link to see a larger image of this figure.)
Similar statistics for adult sarcomas are difficult to find since most adult cancer statistics are given by site of cancer and not by type of cancer. As a result, sarcomas are not grouped in an easy way to analyze, but it is very likely that if adult sarcomas were broken out separately, they would also show a disproportionately large loss of person-years. The reason is that unlike many other cancers sarcoma is a disease of young and old alike.
The NCI SEER project has compiled data that show the risk for developing cancer at a specific age. The SEER data indicates that a third of soft tissue cancers (again, not exactly the same as but most comparable to soft tissue sarcomas) and over half of bone sarcomas are diagnosed in people under 45 years of age. In comparison, less than one-tenth of all cancers occur in this age group (Table 3). Furthermore, the table shows that nearly half (45 percent) of bone sarcomas and over a fifth (21 percent) of soft tissue cancers are diagnosed in people under the age of 35, compared to less than 4 percent of all cancers. For four of the most common types of cancer, 0 percent of prostate cancers, less than 1 percent of lung cancers, 1 percent of colon cancers, and 2 percent of breast cancers are diagnosed in people less than 35.
Note that it is not clear whether the SEER information for soft tissue cancers includes Kaposi’s sarcoma, which is a disease mostly in relatively young men with AIDS, and a disease that has declined dramatically in recent years due to better HIV treatments. If Kaposi’s is included, then the average age of diagnosis for soft tissue cancers may increase in coming years.
Table 3: Age Distribution (%) of Incidence Cases by Site, 2000-2004 (Source: National Cancer Institute, SEER Cancer Statistics Review, 1975-2004, Table I-10.)
Incidence of sarcomas in young people unfortunately is paralleled by deaths from sarcomas. As can be seen in Table 4, over a third (36 percent) of deaths from bone cancers and about a fifth (19 percent) of deaths from soft tissue cancers occur in people under the age of 45, compared to less than 5 percent of deaths from cancers overall.
Table 4: Age Distribution (%) of Deaths by Site, 2000-2004 (Source: National Cancer Institute, SEER Cancer Statistics Review, 1975-2004, Table I-12.)
Another way of looking at this is to consider the MEDIAN age at which cancer is diagnosed. The median is a type of average which represents the age at which half the people are younger at diagnosis and half are older. The following lists the median age of diagnosis of all cancers, four of the most common cancers, and bone sarcomas and soft tissue cancers (Median Age Data Source: National Cancer Institute, SEER Cancer Statistics Review, 1975-2004), Adapted from Table I-11,)
Assuming that most of the bone and soft tissue cancers are sarcomas, it appears that the average age of a patient diagnosed especially with a sarcoma, particularly a bone sarcoma, is considerably younger than the age of most cancer diagnoses.
The young age at which many sarcomas are diagnosed has important implications. The younger the age of diagnosis, the more years of worry about relapse or dealing with side effects of treatment. A bone sarcoma survivor diagnosed at the median age of 39 has thirty-eight more years of life to look forward to, if they live to the average U.S. lifespan of 77 years. A colon cancer survivor diagnosed at the median age of 71 would have six more years of life, on average. Thus, research that improves sarcoma survival has a disproportionately large result if success is measured in the average number of years of life saved.
Another way of measuring this impact on society is Cancer Prevalence, which is the number of people alive a certain number of years after diagnosis. SEER provides limited data for bone and soft tissue cancers, estimating that 107,000 people in the U.S. are alive who have been diagnosed within the last 28 years (see SEER US Estimated Prevalence Counts on 1/1/2004 Bones and Joints Cancer and Soft Tissue including Heart Cancer). Unfortunately for these cancers SEER does not provide data for complete prevalence, in other words all the people alive after a diagnosis no matter when the diagnosis was made.
Survivor health issues are particularly true of children with sarcomas who required multi-modal therapy including relatively high doses of chemotherapy and radiation. The Children’s Oncology Group has developed the Long-Term Follow-Up Guidelines for Survivors of Childhood, Adolescent, and Young Adult Cancers for this very reason.
Survival Statistics are also Important Pediatric sarcomas have a lower survival rate than childhood cancer overall. Table 5 summarizes relative five-year survival rates of pediatric sarcomas for the period 1996-2003. The average five-year survival rate for pediatric cancers is 78 percent, but for sarcomas it is considerably less.
Table 5: 5-Year Relative Survival Rates, 1996-2003 (Source: NCI, SEER Cancer Statistics Review, 1975-2004, Adapted from Table XXIX-6.) Please note that these statistics lump many risk factors and do not reflect newer treatments; thus, they should not be used to assess an individual’s survival potential.
One possible reason for lower survival rates is the fact that many sarcomas are diagnosed in adolescents and young adults in ages that fall into what has been termed the “teen gap” in treatment success (see Adolescent and Young Adult Cancer Care by Dr. Karen Albritton in December 2004 issue of ESUN). The advances in cancer treatment for children and adults have not been matched for adolescents and young adults. There are several possible causes for this, including increased delay in diagnosis and lack of insurance, but one reason may be that this age group has different disease characteristics and in the past has had low enrollment in clinical trials, limiting the opportunity to improve treatments. In the past few years however there has been a concerted effort to include this group in clinical trials.
The following figure shows the average annual percent change in survival from 1975 to 1997 for all cancers and soft tissue sarcomas by age at diagnosis. The survival rate appears to have declined for all groups from age 15 through 44, while increasing for younger and older groups.
Figure 2: Average Annual Percent Change (AAPC) in 5-year Survival, 1975-1997 (Source: Cancer Epidemiology in Older Adolescents and Young Adults 15 to 29 Years of Age, Including SEER Incidence and Survival: 1975-2000, Soft Tissue Sarcomas (Chapter 7), Figure 7.23, Page 91. Click on the Chapter 7 and scroll to page 91 to see a larger image of this figure.)< | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
