Validated cancer health literacy instruments have been used in the past. You may want to contact the primary author of the attached article (Levent Dumenci). Researchers are generally happy to share instruments used by them.
Levent Dumenci, Department of Social and Behavioral Health, Virginia Commonwealth University, 830
East Main Street, P.O. Box 980149, Richmond, VA 23289, USA. [email protected].
Not sure if you are familiar with the Health Literacy Tool Shed which has quite a few validated tools that may suit your needs. Check out this link: http://healthliteracy.bu.edu/chlt-30
Dumenci, L., Matsuyama, R., Riddle, D.L., Cartwright, L.A., Perera, R.A., Chung, H., Simonoff, L.A. (2014). Measurement of Cancer Health Literacy and Identification of Patients with Limited Cancer Health Literacy, Journal of Health Communication, 19(2):205-224.
Rivera-Vasquez, O., Mabiso, A., Hammad, A., & Williams, K.P. (2009). A community-based approach to translating and testing cancer literacy assessment tools. Journal of Cancer Education, 24:319–325.