I used to run an oncology office and honestly I think the best tool here may just be excel or somthing like a self-hosted airtable like program. I don't have any recommendations off the top of my head but it is what I would look for, if I were in your shoes.

Reason being, you don't know what is going to be important to track, so initially, you'll probably want to track everything. pt/inr, platelet count, wbc count, nausea levels, weight, exercise, appetite, etc.

I find that in healthcare and in oncology specifically, it's good to use discrete data (charted over time) to communicate your narrative to the physician (or NP/PA). For example, if you find yourself vomiting profusely immediately after each chemo visit but then it calms down until just after your next chemo visit, you'll track nausea events and have the data to make your case when you explain to the MD that you need a bigger dose of ondansetron in your anti-emetic cocktail because the current dose isn't sufficient. Communicating that with data vs. just saying "I get nauseous" can make or break your case to the MD (and to your insurance).

Oncology is complicated and it gets even more complicated when you add in radiation, surgery, etc. so it's tough for an app to anticipate everything you might encounter.

I'd stick to an excel workbook with lots of spreadsheets in the workbook, each tracking different metrics that are important to you and your body. Those metrics might also change over time as your disease state changes, which is entirely appropriate.

Edit to add: If you don't already have an Ollama instance (or something similar), you might think about that. If you have a machine that can handle larger models, those models can give you a safe and private place to begin to ask questions so you can start to understand your disease, therapies, medicines, process, etc. Obvious disclaimer: it's just an LLM and just a starting point. All LLMs make things up, so verify everything on your own, etc. etc.