Use The Side Effect Case
Dear Patient, I understand you are concerned at having xyz cancer. I also understand that many patients with xyz also have worms/ acid reflux, yada, Yada, Yada, (delete as required). I have to warn you that one side effect of this treatment is that tumours shrink/die/disappear while the heart burn, worms or whatever are being treated. Now may I ensure that you are kept free of worms heartburn whatever?
The civil service hate this approach but in my past experience in a different field they end up living with it and have even been bounced into following that approach - if the incentive is in the right place.
There is an ongoing battle over cheap and effective vs a too expensive to use treatment for if believe wet macular degeneration.