The thorax was a part of human anatomy which Sontarans used to distinguish females from males. Jingo Linx observed that Sarah Jane Smith's thorax was of a "different construction" than a male, leading him to believe that human males and females were separate species. (TV: The Time Warrior) Similarily, Commander Skorr observed that Martha Jones had a weak thorax. (TV: The Sontaran Stratagem)
As part of a medical examination, Strax examined Clara Oswald's thorax. (TV: Deep Breath)
Bone spiders had a thorax. (PROSE: The Stone House)