Cancer results from mutations that accrue in the genome of normal cells over time. These types of mutations include small mutations such as point mutations, and small insertions and deletions. Larger structural changes in chromosomes include large insertions, deletions, inversions, and translocations.
A traditional way to look at structural variation and ploidy is through karyotyping. One of the most common types used is spectral karyotyping (e.g. SKY) where chromosomal-specific fluorescent probes are used with fluorescence microscopy.