The title
Tsar as we know today was originally derived from the Roman word
Caesar(in AD 68) and adopted by Ivan IV (the first Russian Tsar) in 1546 and continued to be used right upto 1917.
Tsesar was incorporated by the old Slavonic language (the first literary language of the Slavs developed in the 9th century by the Bulgarian monks Kiril and his brother Methodius).
For the sake of simplicity
Tsesar was later contracted to
Tsar' by other monks who penned religious manuscripts.
Originally the appellation
Tsesarevich was used specifically to identify the male heir apparent to the Imperial Crown. Similarily, this term was also contracted to
Tsarevich.
The title
Tsarevich is a general term used to identify the son of a Tsar (a word which was adopted into the English language directly from the French designation).
So while both
Tsarevich and
Tsesarevich have been used by historians, in my opinion it is really a matter of preference as whether the original titles are used or the now accepted modernizations.
Unfortunately few authors today choose to use the more formal original titles.