These findings may help in planning and evaluating screening programs, clinical trials and other studies, the researchers say.

Using a new mathematical model, the scientists were also able to estimate the numbers of breast cancers detectable by mammography. This is a new approach to estimating the growth rate of tumors and the ability of mammograms to find them.

"There are enormous implications for the sensitivity of breast cancer screening programs," lead researcher Harald Weedon-Fekjr, of the Department of Etiological Research, Cancer Registry of Norway, said in a statement.

"We found that mammography screen test sensitivity increases sharply with increased tumor size, as one might expect. Detection rates are just 26 percent for a 5 millimeter tumor but increase to 91 percent once a tumor is 10 millimeter in size," he added.

The report was published in the May 8 issue of the online journal Breast Cancer Research.

In the study, Weedon-Fekjr, and colleagues tested their model using mammography results from 395,188 women aged 50 to 69.

The researchers found that the growth rate of tumors varied significantly between patients. About one in 20 tumors doubled in size, from 10 to 20 millimeters in just over a month. However, a similar number of tumors took more than six years to double in size.

Based on this finding, Weedon-Fekjr's team estimated that it takes an average of 1.7 years for tumors to double in size. Moreover, tumor growth appeared to be faster among younger women and slowed as women aged, the researchers noted.

"Tumor growth and test sensitivity estimates can be directly linked to tumor size in a full population study, resulting in very useful growth estimates directly connected to a biologically relevant measure," the researchers wrote.

"Tumor growth seems to vary greatly between tumors, with higher growth rates among younger women. Most tumors become visible at screening when they reach a diameter of 5 millimeters to 10 millimeters," they concluded.

One expert thinks this study again confirms the need for women to have a mammogram every year.

"This study continues to prove why we need to screen women every year, starting at age 40," said Debbie Saslow, director of breast and gynecologic cancer at the American Cancer Society.

This is another study that shows that tumors grow faster in younger women, Saslow said. "It just doesn't make sense to have guidelines that say younger women should be screened every one to two years and every year as they get older," she said.

More information

For more on breast cancer, visit the U.S. National Cancer Institute.

Then, again, the rest of the workforce isn't doing that good of a job either, according to the study published online in the Journal of the American Academy of Dermatology.

Only 15 percent of all workers in the study had ever received a skin exam in their lifetime; however, the percentages were significantly lower for people whose jobs frequently subject them to the sun's harmful ultraviolet (UV) rays. These occupations included farm operators and managers (10 percent), farm workers and other agricultural workers (7 percent), forestry and fishing occupations (3 percent), construction and mining (8 percent), and construction laborers (8 percent).

"When we analyzed the data by industry sectors, we concluded that agriculture, forestry, fishing and construction workers reported the lowest rate of skin exams in 2000," dermatologist Robert S. Kirsner, vice chairman of the department's of dermatology and cutaneous surgery at the University of Miami Miller School of Medicine, said in a prepared statement. "Although the number of agriculture, forestry and fishing workers reporting a skin exam increased from 2000 to 2005, from 4.2 percent to 13.6 percent, the prevalence of skin exams among construction workers stayed essentially the same, from 5.2 percent to 5.6 percent."

The findings were based on an analysis of 2000 and 2005 data taken from the National Health Interview Survey (NHIS), an annual, cross-sectional, in-person household survey of U.S. workers.

"As dermatologists, we know that the early detection of skin cancer by routine skin examinations is crucial in successfully treating this potentially life-threatening condition, particularly for workers routinely exposed to harmful ultraviolet light. This study shows that workers who need careful monitoring for skin cancer due to the nature of their jobs are less likely to receive skin exams than workers in low-risk occupations," Kirsner said.

He said the trend could be reversed by holding local community health fairs that include screening programs targeting high-risk workers.

More information

The American Academy of Dermatology has more about where you can receive a free skin cancer screening.

"This is the first paper that helps us understand what causes this childhood cancer," said lead researcher Dr. John M. Maris, director of the Center for Childhood Cancer Research at The Children's Hospital of Philadelphia. "We expected for decades that this cancer was a genetic disease, but we have had a hard time understanding what abnormalities in our genetic makeup lead to this cancer."

Neuroblastoma, a cancer of the peripheral nervous system that usually appears as a solid tumor in the chest or abdomen, is the most common solid tumor malignancy seen in early childhood. Among infants, it can disappear with minimal treatment, but in older children, it can be an aggressive cancer spreading throughout the body. Neuroblastoma accounts for 7 percent of all childhood cancers but causes 15 percent of all childhood cancer deaths. There are about 700 new cases diagnosed each year in the United States, the researchers said.

Maris' team found a common genetic variation of the gene 6p22 on chromosome 6, which doubles the risk of getting this disease. "This finding supports our assumption that there are a number of minor variations that work together -- in sort of a perfect storm -- to give a child this disease," he said. "This finding is the discovery of the first of these genetic variants."

Maris noted that this is the first time a childhood cancer has been found to be influenced by rather common genetic changes "that can be in you or me or anyone."

In addition, Maris said that having this particular genetic variation not only increases the risk of developing neuroblastoma, but also increases the risk of developing the more aggressive form of the disease. "This leads us to believe that the disease we call high-risk or low-risk neuroblastoma are really different diseases," he said.

The findings were published in the May 7 online edition of the New England Journal of Medicine.

For the study, Maris' team analyzed blood samples from 1,032 children with neuroblastoma and 2,043 children without the disease. The researchers honed in on three single nucleotide polymorphisms (SNPs) -- which are variations in DNA -- that were more common in patients with neuroblastoma than in patients without the disease. The three SNPs were clustered in the 6p22 region of chromosome 6. There are two genes in this region, but exactly what they do is unknown, the researchers said.

To confirm their findings, Maris' group analyzed blood samples from additional neuroblastoma patients and children without the disease. Among these additional patients, the researchers also found that variants in the 6p22 region were associated with increased risk for neuroblastoma.

"This finding gives us the motivation to continue this line of research to discover all of the different genetic variations that work together," Maris said. "We have already discovered additional variations."

Knowing the complete genetic influences on neuroblastoma may eventually lead to new treatments, he said.

Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, said that the new findings could one day lead to better diagnosis and treatment of the malignancy. "We still need to understand what these genes do, because little is known about these genes," he said.

Lichtenfeld added that, while the new research is important, it's still very preliminary. "Ultimately, what you want to do is to analyze the cancer and gain clues as to what the prognosis may be and what the appropriate treatment may be," he said. "This does not get us there, but it is one step along that pathway."

More information

For more on neuroblastoma, visit the American Cancer Society.

MONDAY, May 5 (HealthDay News) -- The overproduction of a protein may be what starts harmless colon polyps on their journey to becoming malignant tumors, Finnish researchers report.

The University of Helsinki research, published online in Cancer Cell, reveals that PROX1, a protein that controls formation of normal organs in embryos, is produced in excess during the early stages of cancer development. PROX1 even encourages tumor cell growth without additional signals from surrounding normal tissues.

The removal of PROX1 from cancer cells appears to reverse their malignant behavior, suggesting that future research may focus on the protein's use in colon cancer therapies.

Men and women face a lifetime risk of nearly 6 percent for the development of invasive colorectal cancer, making it one of the most common malignancies in the Western world. Past epidemiologic studies have cited obesity and several dietary factors -- including fat, red meat and a lack of vegetables and fiber -- as increasing the risk of the disease.

More information

The National Cancer Institute has more about colorectal cancer screening.

FRIDAY, May 2 (HealthDay News) -- Spanish researchers report they may have found a way to tell which suspicious prostate lesions are likely to develop into cancer.

The findings, published in the May 1 issue of Clinical Cancer Research, show a link between high-grade prostatic intraepithelial neoplasia (HG-PIN) lesions and the PTOV1 gene. The more PTOV1 the lesion expresses, the more likely cancer will develop. The report also backs the reverse -- that the lack of PTOV1 means a reduced risk of prostate cancer.

PTOV1 is a protein that researchers don't fully understand the function of, but they have previously found too much of it appears to promote the spread of cancer cells.

If subsequent studies confirm PTOV1 as a biomarker for prostate cancer, it could help men with the lesions avoid repeated needle biopsies.

"Those patients with a high PTOV1 score should undergo an immediate repeat biopsy," study author Rosanna Paciucci, a researcher at the Vall d'Hebron Hospital Research Institute in Barcelona, said in a prepared statement. But those with low PTOVI may not need to receive future "annoying and useless" biopsies, she said. "We estimate that we can save 40 percent of unnecessary biopsies -- those that are repetitively negative and contain HG-PIN lesions that do not develop into cancer."

HG-PIN, while present in most cancerous prostates, is a pre-malignant lesion and, given its association with other cancers, it is often repeatedly biopsied when found. Past studies have put the average risk of cancer being diagnosed in a HG-PIN biopsy at between 20 percent and 30 percent, the researchers said. However, none of these studies were to tell which lesions would progress to cancer, the researchers say.

Paciucci cautioned that her team's results need to be confirmed through a larger study group. "From this validation, we can expect to improve the current rate of early detection of cancer," she said.

More information

The U.S. National Cancer Institute has more about prostate cancer.