Deficiency is no hypothesis. There is proof: Scurvy, a disease caused by lack of vitamin C, sickened sailors who had no access to fresh food supplies in 16th to 18th century.

Regarding laetrile. Try it, keep a diary, and draw your own conclusions. It may be good for me, but make you sick, irrespective of biochemistry analysis.

Cancer cells may be destroyed by HCN, but if the patient restarts old habits then it will return. Then what? Endless HCN treatments? Maybe appreciate the integrated approach, trusting the body, supplying it with quality foods and keeping it free from toxins, too much stress and electro magnetic overdose (WiFi, GSM). Dr Gonzales had a patient move away from a cell tower, which improved her condition. 

 True, I should have clarified. I do believe vitamin deficiency causes diseases, I am just skeptical of B17 deficiency specifically