Je comprends! However, the word subject has many meanings, and if you use it in this context at the beginning as well, the meaning gets a bit confused.

Thanks. I first avoided the repetition, then kept it because it's also there in French.

Marina, what do you think of: The topic is entirely relevant to the subject of your thesis.
To avoid the repetition of 'subject' and be more specific in its meaning.
'relevant' indicates a direct, significant connection to something, whereas 'related to' implies a broader, more general type of relationship.