The use of large language models (LLMs) such as ChatGPT for medical question-answering is becoming increasingly popular. However, there are concerns that these models may generate and amplify medical misinformation. Because cancer patients frequently seek to educate themselves through online resources, some individuals will likely use ChatGPT to obtain cancer treatment information. This study evaluated the performance and robustness of ChatGPT in providing breast, prostate, and lung cancer treatment recommendations that align with National Comprehensive Cancer Network (NCCN) guidelines. Four prompt templates were created to explore how differences in how the query is posed impacts response. ChatGPT output was scored by 3 oncologists and a 4th oncologist adjudicated in cases of disagreement. ChatGPT provided at least one NCCN-concordant recommendation for 102/104 (98%) prompts. However, 35/102 (34.3%) of these also included a recommendation that was at least partially non-concordant with NCCN guidelines. Responses varied based on prompt type. In conclusion, ChatGPT did not perform well at reliably and robustly providing cancer treatment recommendations. Patients and clinicians should be aware of the limitations of ChatGPT and similar technologies for self-education.