Ah ha the old tinfoil hat. They don't work.
Given how the US has consistently engineered the world political stage, getting Saddam into power, supplying arms to the Taliban to fight the russians (the same arms that are being used to kill british, american, french, australian etc troops) Reagans terroist atrocities that were perpetrated under the name 'the war on drugs' and the fact that Venezuela was instrumental in getting oil to be allowed to be traded in euros instead of dollars which in turn caused many countries to follow suit which caused a massive decline in the US revenue that it recieved by selling dollars to those buying oil, I wouldn't put it past the yanks to pull something like this.
The point made by Hugo Chávez is not that a disproportionate number of current Latin American heads of government have developed cancer in their lifetime.
What he actually meant is that the number of prominent Latin American leaders who have been in political conflict with the governments of the United States in the latest years have developed cancer in the latest years unusually often.
The alleged cancer "epidemic" has apparently started with Fidel Castro in 2006. The time interval to take into account is 2006 to 2011.
Let's list the Latin American countries with a left-leaning anti-US government that took part in the ALBA alliance promoted by Chávez and Castro:
Venezuela, led by Hugo Chávez.
Cuba, led by Fidel Castro, then Raúl Castro.
Ecuador, led by Rafael Correa.
Nicaragua, led by Daniel Ortega.
Bolivia, led by Evo Morales.
Honduras, led by Manuel Zelaya, then overthrown by a pro-US coup.
Dominica, led by Roosevelt Skerrit.
San Vicente and the Granadines, led by Ralph Gonsalves.
Antigua and Barbuda, led by Baldwin Spencer.
Beside the ALBA countries, it makes sense to add the following South American governments who are unanimously regarded as friendly towards the ALBA and somewhat conflictual with the United States:
Brazil, led by Lula Da Silva, then Dilma Rousseff.
Argentina, led by Nestor Kirchner, then Cristina Kirchner.
Paraguay, led by Fernando Lugo.
Uruguay, led by José Mujica.
We have 16 candidates for the "cancer attack". 6 of them (Chávez, Fidel Castro, Lula, Roussef, Cristina Kirchner, Lugo) were diagnosed with cancer in the time interval. One of them died of heart failure, another one was removed with a coup.
Birth year of the people mentioned above:
Fidel Castro 1926.
Raúl Castro 1931.
Nestor Kirchner 1950.
Christina Kirchner 1953.
The males had an average age of 58 in 2006, the two women of 56. We can find out the incidence rate of cancer for US residents that age and sex at http://seer.cancer.gov/faststats/selections.php
, which should give a hint at the typical values. The latest data (2008) give an yearly incidence rate lower than 1% for males, lower than 0.8% for females. Since we are talking about 6 years (2006 to 2011), this means that less than 6% of men that age and less than 5% of women that age are expected to be diagnosed with cancer. In this sample, it happened to 29% of males and 100% of females. The sample is very small but the mismatch with the usual values is very high.
It sounds like Cavez could have some grounds for his suspicions.