Rastreo cáncer de mama: el debate no se agota. Superan los beneficios a los daños...o viceversa?
No se va a terminar tan fácil el debate
Box 1: Benefit in reduction of cause specific mortality
For 10 000 women aged 50-70 the incidence of breast cancer is 2 per 1000 per year, or 200 for 10 000 women over 10 years observation
Median period at risk = 5 years
Treated with modern systemic therapy, 90% survival = 20 deaths5
If screening risk reduction is constant at 20% then breast cancer deaths avoided = 4 (Marmot estimate1) or 15% reduction = 3 (Nordic Cochrane Centre estimate2)
That is, 2500 to 3300 women screened for 10 years to avoid one breast cancer death.
Box 2: Harms caused by overdiagnosis
Best case scenario
I estimate overdiagnosis is 129 per 10 000 women screened, or 4000 every year in the UK (as in Marmot review1).
Therefore:
Cumulative mortality = 3.28% (lung cancer) and 6.16% (myocardial infarction)
Radiotherapy relative risk = 1.78 and 1.27
Excess deaths per 10 000 screened = 1.65 and 1.07
Total excess deaths = 2.72
I used 2008 age and sex specific lung cancer mortality rates from Cancer Research UK to calculate cumulative mortality.11 I used 1998 age and sex specific myocardial infarction mortality from the Health Technology Assessment report12 adjusted downwards by 30% to reflect declining mortality from myocardial infarction to estimate rates in 2008, based on trends in the US13 and UK. I calculated cumulative lung cancer and myocardial infarction mortality (adjusted for intercurrent mortality14) for a 30 year period (similar to Marmot review); 100 women aged 50, followed for the next 30 years. Radiotherapy is used as adjuvant treatment in 50% of these overdiagnosed cases. Detrimental effects of radiotherapy have been assumed to start immediately (without any lag) and last for the entire 30 year period. This will not inflate the excess deaths caused by radiotherapy because the effect is applied only to the overdiagnosed cases and not the entire population. I used point estimates from the Early Breast Cancer Trialists’ Collaborative Group overview to estimate detrimental effects of radiotherapy.
Worst case scenario
Excess deaths per 10000 screened = 5.60 (lung cancer) and 1.07 (myocardial infarction)
Total excess deaths = 9.25
This scenario differs from the best case scenario in these assumptions:
Estimate of overdiagnosis is 274 per 10 000 women screened, or 8500 every year in the UK. These figures are estimated by applying rates from Bleyer and Welch2 to NHSBSP figures. Bleyer and Welch estimate that 31% all breast cancers in the US (60% of which are screen detected) are overdiagnosed—that is, 50% all screen detected cancers are overdiagnosed. From 1 April 2009 to 31 March 2010, UK NHSBSP (England, Wales, Northern Ireland and Scotland) diagnosed 17013 cases.15 Radiotherapy is used as adjuvant treatment in 80% of these overdiagnosed cases.
Balance of benefit versus harm (best and worst case scenarios)
For every 10 000 women invited for screening, 3 to 4 breast cancer deaths are avoided at the cost of 2.72 to 9.25 deaths from the long term toxicity of radiotherapy.
Box 1: Benefit in reduction of cause specific mortality
For 10 000 women aged 50-70 the incidence of breast cancer is 2 per 1000 per year, or 200 for 10 000 women over 10 years observation
Median period at risk = 5 years
Treated with modern systemic therapy, 90% survival = 20 deaths5
If screening risk reduction is constant at 20% then breast cancer deaths avoided = 4 (Marmot estimate1) or 15% reduction = 3 (Nordic Cochrane Centre estimate2)
That is, 2500 to 3300 women screened for 10 years to avoid one breast cancer death.
Box 2: Harms caused by overdiagnosis
Best case scenario
I estimate overdiagnosis is 129 per 10 000 women screened, or 4000 every year in the UK (as in Marmot review1).
Therefore:
Cumulative mortality = 3.28% (lung cancer) and 6.16% (myocardial infarction)
Radiotherapy relative risk = 1.78 and 1.27
Excess deaths per 10 000 screened = 1.65 and 1.07
Total excess deaths = 2.72
I used 2008 age and sex specific lung cancer mortality rates from Cancer Research UK to calculate cumulative mortality.11 I used 1998 age and sex specific myocardial infarction mortality from the Health Technology Assessment report12 adjusted downwards by 30% to reflect declining mortality from myocardial infarction to estimate rates in 2008, based on trends in the US13 and UK. I calculated cumulative lung cancer and myocardial infarction mortality (adjusted for intercurrent mortality14) for a 30 year period (similar to Marmot review); 100 women aged 50, followed for the next 30 years. Radiotherapy is used as adjuvant treatment in 50% of these overdiagnosed cases. Detrimental effects of radiotherapy have been assumed to start immediately (without any lag) and last for the entire 30 year period. This will not inflate the excess deaths caused by radiotherapy because the effect is applied only to the overdiagnosed cases and not the entire population. I used point estimates from the Early Breast Cancer Trialists’ Collaborative Group overview to estimate detrimental effects of radiotherapy.
Worst case scenario
Excess deaths per 10000 screened = 5.60 (lung cancer) and 1.07 (myocardial infarction)
Total excess deaths = 9.25
This scenario differs from the best case scenario in these assumptions:
Estimate of overdiagnosis is 274 per 10 000 women screened, or 8500 every year in the UK. These figures are estimated by applying rates from Bleyer and Welch2 to NHSBSP figures. Bleyer and Welch estimate that 31% all breast cancers in the US (60% of which are screen detected) are overdiagnosed—that is, 50% all screen detected cancers are overdiagnosed. From 1 April 2009 to 31 March 2010, UK NHSBSP (England, Wales, Northern Ireland and Scotland) diagnosed 17013 cases.15 Radiotherapy is used as adjuvant treatment in 80% of these overdiagnosed cases.
Balance of benefit versus harm (best and worst case scenarios)
For every 10 000 women invited for screening, 3 to 4 breast cancer deaths are avoided at the cost of 2.72 to 9.25 deaths from the long term toxicity of radiotherapy.