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Commentator Theta Sigma was a version of the Doctor who existed in an alternative timeline where Rassilon failed to finish the Eye of Harmony before his death. Consequently, Theta Sigma never left Gallifrey. He became a commentator rather than a renegade Time Lord. He appeared to be in his sixth incarnation. (AUDIO: Forever)

In the proper timeline, "Theta Sigma" was the Doctor's nickname at the Academy. (TV: The Armageddon Factor, The Happiness Patrol)