For a number of years I was a Vice President of a software firm  that developed retirement reporting programs for some of the nation's largest investment firms, insurance companies and banks. Due to my exposure to these firms I  was able to develop an understanding of their policies and approaches.
The general advice  offered to company participants (401K clients ) was never tap your retirement funds if at all possible.  
Consider this ... the funds in a retirement account are earning  some level of tax deferred income. Depending on your age there may be significant fees involved to if you withdraw early - which can reduce the actual withdrawal amounts. 
My free advice (worth every penny) would be - consider avoiding this road until you have some type of professional advice.
Based on my personal experience :
 
1)Speak with an attorney whose primary focus is  retirement planning, elder law etc. Ask what the next steps should be given your financial situation, age and treatment needs.
2) Meet with a personal  financial planner - one who is bound by fiduciary responsibility when offering advice - ( not a stockbroker.) and discuss your concerns.
3) ASAP  ... Meet with the social services department at your  hospital  and explain your situation. They often have internal resources or can open up doors for you to apply for assistance from drug companies, foundations and more.
A number of years ago my mother was in a situation similar to your own. The time I spent with an elder law attorney and the social services department of the hospital saved her signifiant sums of money and allow her to take advantage of treatment programs that appeared to be out of reach.
Once you tap into your retirement funds - it is difficult to replace those funds and the compound interest that was earned over a period of  years.
Please keep us updated as you can.
...   Dennis (blood-cancer.com TEAM)