Period of Asbestos Cancer

Latency Period of Asbestos Lung Cancer

All asbestos-related diseases have a long latency period from the time of initial asbestos exposure to the development of an illness. Asbestos-related lung cancer is no different, typically taking between 15 and 35 years to develop. Studies show that latency is affected by the level of asbestos exposure and cocarcinogens such as cigarettes.

Lung cancer development begins long after asbestos fibers have reached the lungs. Once asbestos fibers are inhaled, they attach to the lung tissue because of their jagged-like structure. When the fibers have caused enough irritation, inflammation and genetic damage, tumor formation begins.

For doctors to attribute lung cancer to asbestos exposure, many say at least 10 years must pass from the time of exposure to lung cancer development. Multiple studies suggest lung cancer is most likely to develop between 30 and 35 years after exposure to asbestos, with the onset of cancer decreasing near the 40-year mark.

Helsinki Criteria

The Helsinki Criteria were established in 1997 to help doctors determine if lung cancer and other pleural diseases are asbestos-related. If a person's lung cancer is diagnosed as asbestos-related, it must fit two criteria, as laid out by the Helsinki Criteria. The first addresses the latency period:

• Lung cancer must develop at least 10 years after initial exposure to asbestos.
  
  Asbestos-related lung cancer has a minimum latency period of 10 years. So, if a person is diagnosed within a decade of his or her first exposure to asbestos, the exposure is ruled out as a possible contributing factor. If a patient developed lung cancer 10 or more years after initial exposure, he or she fits the first criterion. To prove that asbestos contributed to the development of lung cancer, the patient must fit only one of the remaining Helsinki Criteria:

1. Diagnosis of asbestosis. 
2. Higher than normal asbestos fibers in the lung tissue. The number of asbestos fibers is measured per gram of dry lung tissue and must coincide with the amount seen in asbestosis patients. The exact amount varies based on type and length of asbestos fibers. 
3. Higher than normal exposure to asbestos, measured in fibers/mL-year (f/mL-yr). "Fibers per milliliter" refers to the number of fibers in each milliliter of air. The patient must have been exposed to levels of airborne asbestos equal to 25 f/mL-yr. To reach this threshold in a one-year work period, for example, the patient must have been exposed to a level of 25 f/mL. Such a high level is typically only found in asbestos manufacturing and asbestos insulation work. To reach 25 f/mL-yr in a five-year period, an individual must have been exposed to asbestos at a level of 5 f/mL. This level is typical of shipbuilding and construction work.

In 2004, researchers reviewed new study results that had emerged since the Helsinki Criteria first were formed. Researchers concluded that the 17-year-old criteria still held. They did, however, amend the list to answer a common question.

Researchers more closely considered the role of smoking in asbestos-related lung cancer, stating that a second cause of cancer complicates the issue. Most lung cancer cases are linked to smoking, and some are further linked to asbestos. The authors are careful to note that the secondary attribution is difficult to determine. Because asbestos and smoking work together synergistically, researchers suggest it is especially difficult to distinguish smoking-related cases from those caused by both smoking and asbestos.




The Helsinki Criteria at work: Suppose a man is diagnosed with lung cancer and worked as a shipbuilder 30 years earlier for several years. The individual fits the first criterion: his first exposure to asbestos was more than 10 years before his diagnosis. He also fits one of the remaining criteria: his occupation exposed him to asbestos at a level of at least 25 f/mL-yr. Because of these two risk factors, doctors would conclude that asbestos caused or contributed to the man's lung cancer.

Criticism of the Helsinki Criteria

The main critique of the Helsinki Criteria is a result of contradictory hypotheses relating asbestos exposure to lung cancer. There are three distinct and conflicting hypotheses about asbestos causing lung cancer:

1. Asbestos exposure only increases the risk of lung cancer when it has caused asbestosis. 
2. Asbestos exposure increases the risk of lung cancer when it is enough to cause asbestosis, despite whether it actually led to an asbestosis diagnosis. This is the hypothesis favored by the Helsinki Criteria. 
3. Asbestos exposure always increases the risk of lung cancer, absent of any threshold needed for asbestosis.

The Helsinki Criteria specifically state that asbestosis is not necessary to prove a causal relationship, but that asbestosis must have been possible based on the amount of asbestos exposure. Researchers who disagree with this hypothesis therefore disagree with the Helsinki Criteria. Proponents of the first hypothesis argue that the criteria are too lenient, and supporters of the third hypothesis argue that they are too strict. Despite some opposition, the Helsinki Criteria are an accepted standard throughout the world.

Other Sets of Criteria to Determine Asbestos Association
Countries struggling with asbestos-related health problems lay out their own versions of the Helsinki Criteria. Panels of experts worldwide typically accept the Helsinki Criteria as a valid set of guidelines and make minor changes specific to each country. The American Thoracic Society (ATS), for example, lays out criteria to determine if a disease is caused by asbestos. The ATS states that, although its guidelines are outlined for nonmalignant diseases and not lung cancer, they coincide with the Helsinki Criteria.

The AWARD (Adelaide Workshop on Asbestos-Related Diseases) Criteria, another set of widely recognized standards, were laid out in 2000 to determine the utility of the Helsinki Criteria in Australia. In making the AWARD Criteria, Australia's experts made minor alterations to reflect the specific types of asbestos found in the country. Overall, the panel of experts deemed the Helsinki Criteria to be reasonable and applicable.

Asbestos-Related Lung Cancer and Asbestosis: After much debate, the medical community now believes the presence of asbestosis is a reliable diagnostic marker that a patient has received enough asbestos exposure to develop lung cancer. Some studies claimed asbestosis must be present for lung cancer to be associated with asbestos, while others stated the two can exist independently. Asbestosis and lung cancer are commonly associated because the risk for both rises in a parallel manner as asbestos fibers accumulate in lung tissue. Evidence shows the level of asbestos exposure required to develop asbestosis and lung cancer is similar.

Asbestos-Related Lung Cancer and Smoking

The risk for lung cancer is much higher among asbestos-exposed smokers because the combination of asbestos and smoking greatly impairs lung function. When someone smokes, the efficiency of cilia (responsible for cleaning the lungs) is reduced and the lungs' ability to dispel asbestos fibers is significantly weakened. Researchers propose that cigarette smoke fosters the accumulation of asbestos fibers, and with more fibers retained in the lungs the risk for lung cancer increases.

The combination of the effects from asbestos exposure and smoking is referred to as a multiplicative effect. According to the Agency for Toxic Substances and Disease Registry, a cigarette smoker exposed to asbestos is 50 to 84 times more likely to develop lung cancer.

Asbestos-exposed smokers can lower their risk for lung cancer by deciding to stop smoking, but unfortunately the effects of asbestos exposure are irreversible. Those who have smoked and been exposed to asbestos are advised to stop smoking immediately and receive annual screenings for lung cancer.
Prognosis and Survival Rates

The prognosis and survival rate for someone with asbestos lung cancer differs by type and subtype of the disease and also by the stage at which the cancer is found. Overall, though, the median survival for a limited stage small cell patient, with combination chemotherapy, is 16 to 22 months. For patients with extensive stage cancer, the median survival is nine to 11 months with the same treatment.

The prognosis and survival rate for someone with asbestos lung cancer differs by type and subtype of the disease and also by the stage at which the cancer is found. Overall, though, the median survival for a limited stage small cell patient, with combination chemotherapy, is 16 to 22 months. For patients with extensive stage cancer, the median survival is nine to 11 months with the same treatment.

A 2008 review of medical literature found that adenocarcinoma is almost always associated with longer survival times. A 2011 study agreed with these findings, reporting that adenocarcinoma patients lived a median of 8.4 months while all other NSCLC patients lived a median of 8.1 months. While the difference is small, researchers believe it to be significant.

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Types of Lung Cancer

There are two primary forms of asbestos lung cancer: small cell lung cancer and non-small cell lung cancer. Of the two, non-small cell lung cancer is less aggressive and more common, accounting for at least 80 percent of all lung cancer cases. Small cell lung cancer makes up less than 20 percent of cases, is more difficult to treat.

About 6 percent of small cell lung cancer patients live five years or longer, but early treatment can improve survival rates significantly. About 17 percent of non-small cell lung cancer patients live five years or longer, nearly three times the percentage for small cell lung cancer.