"As planned from the beginning, different possibilities were considered following the chemo. I tell you that the fight is not over for me," Haller wrote in a Twitter post on Wednesday.
"I will have to undergo an operation to permanently end this tumour which keeps me away from the pitch. I want to thank you all for the messages of support and the strength you give me in this ordeal. Looking forward to seeing you all soon."
Dortmund signed Haller for an initial fee of €31 million in the close season after the striker scored 34 goals last season for Dutch champions Ajax -- including 21 in the league and 11 in the Champions League.