A thermal cap that can fit a variety of head sizes is disclosed. The cap can include a shell having a fluid inlet and outlet, a sealing mechanism and removable sizing layers disposed within the shell. Depending upon the size of a patient's head, sizing layers can either be added to or removed from the outer shell to maintain a fluid circulation space between the head and the rigid shell and allow substantially even distribution of a thermal fluid about the scalp of the patient during operation. The shell is preferably rigid and an elastomeric member can seal the periphery of the cap to the patient's head to prevent leakage. Other types and aspects of thermal cap systems are also disclosed.