Doing undercarriage on dozer. Missed grab iron stepping from track to engine compartment, lost balance and fell. Left buttock struck tilt cylinder. Dr diagnosed deep bruise to left buttock. Dr. prescribed phys.therapy (determined to be ""med. treatment""). Employee did not report injury to mgmt. until 11/23/04. Received ""med. treatments"" from PT 12/02/04 which triggered reporting.