Which one is correct: 'Chemotherapy should be the first call of treatment for cancer' or Chemotherapy should be first call of treatment for cancer'?
In that context, drop the if, and only if, first acts as a marker to enumerate a list, as it does in the following examples (emphasis, mine):
He is first kind, no-nonsense, selfless, nice, sweet and helpful. He is later mean, evil, dangerous, cold-blooded, ruthless, ag[g]ressive, murderous, power-hungry and bad. - Tom and Jerry wiki, about the main antagonist
he is first happy for me finding a rewarding field for my career, and secondly, he is happy that it was his vision that caused the difference - Statistical Physics, High Energy, Condensed Matter and Mathematical Physics ... by C. H. Oh
first ordinal number 2.2 Firstly; in the first place (used to introduce a main point or reason) ‘first, it is wrong that the victims should have no remedy’ - ODO
I don't think your example can be read that way. There, "first" isn't marking an element of a list, but instead acts to qualify call (or the noun phrase headed by call).
first ordinal number 1 Coming before all others in time or order; earliest; 1st. ‘It is hoped to have everything in order for the first performance at the end of November.’ - ODO
This usage is marked by a determiner before the word first. So in your example, retain the.