Câncer vs. cancro vs. Caranguejo
These three Portuguese words share an etymological root related to the concept of a crab, but their usage depends entirely on regional dialect (Brazilian vs. European Portuguese) and context (medical vs. astrological vs. zoological).
Câncer
B1In Brazilian Portuguese, Câncer is the standard term for the malignant disease as well as the fourth astrological sign of the zodiac. In European Portuguese, this distinct spelling is often reserved solely for the Latin scientific name or the specific zodiac constellation, though it is less common in daily speech.
O médico explicou os tratamentos disponíveis para o câncer.
(The doctor explained the treatments available for the cancer.)
Meu irmão é do signo de Câncer.
(My brother is of the Cancer sign.)
A campanha de conscientização sobre o câncer de mama começa em outubro.
(The breast cancer awareness campaign starts in October.)
Ela leu o horóscopo de Câncer no jornal de hoje.
(She read the Cancer horoscope in today's newspaper.)
Cancro
B2In European Portuguese, cancro is the standard everyday word for the malignant disease. In Brazilian Portuguese, however, cancro is rarely used for the general disease; instead, it specifically refers to a chancre (a venereal ulcer) or certain plant diseases.
Em Portugal, a luta contra o cancro é uma prioridade nacional.
(In Portugal, the fight against cancer is a national priority.)
O paciente recuperou totalmente do cancro do pulmão.
(The patient fully recovered from lung cancer.)
Este produto ajuda a combater o cancro cítrico nas plantações de laranja.
(This product helps fight citrus canker in orange plantations.)
O jardineiro notou um cancro no tronco da macieira.
(The gardener noticed a canker on the apple tree trunk.)
Caranguejo
B1This word primarily refers to the aquatic animal, the crab, in all Portuguese-speaking countries. However, in European Portuguese, Caranguejo is also the common name used for the Zodiac sign, whereas Brazilians would use Câncer for the sign.
As crianças brincaram de perseguir o caranguejo na areia.
(The children played at chasing the crab in the sand.)
Dizem que as pessoas de Caranguejo são muito sensíveis.
(They say that Cancer people are very sensitive.)
Pedimos uma porção de patolas de caranguejo no almoço.
(We ordered a portion of crab claws for lunch.)
O signo de Caranguejo é regido pela Lua.
(The sign of Cancer is ruled by the Moon.)
Summary
To summarize based on region: In Brazil, use Câncer for the disease and the zodiac sign, and use Caranguejo only for the animal. In Portugal, use cancro for the disease, and Caranguejo for both the animal and the zodiac sign.