Can head trauma cause ADHD in an adult?

Frontal damage. The frontal lobes of the brain are where we focus and attend and control behavior. Serious brain trauma can damage these frontal brain centers. Persistent problems with attention and behavior control can be an outcome of such trauma.
Same but different. Closed or hemorrhagic head injury can damage frontal lobe function. The resulting syndrome is indistinguishable from inherited adhd. The same executive functions are disrupted, particularly organization and inhibition. We don't call it "adhd, " we call it tbi.
YES. Head trauma, chemotherapy and radiation are all potentially damaging to the prefrontal cortex and cause the symptoms. Adhd is genetic and also inhibits executive function.