>>16737367 (OP)
>My parents
You're obviously young
>I think it's sarcoma
Very unlikely at your age
>It itches
Makes it even more unlikely
Mostly likely cause of this lump is a simple cyst. It's filled with disgusting pus from a blocked skin duct.
Usually these go away on their own, but it takes many days and a brief reflare in size is not uncommon.
If the lump gets bigger than 2 inches (5 cm) across, you really do need to insist to your parents that they have it looked at, since most medical sites say this is a matter of concern.