Yoga in general can be a special gift for anyone recovering from serious illnesses such as a partial mastectomy (what you call a "lumpectomy.") I suspect that what you need now is not Yin Yoga, but rather restorative yoga. Yin Yoga generally targets the connective tissues of the lower body: it is not restorative yoga but restorative yoga often incorporates aspects of Yin Yoga. But there may be some Yin Yoga postures that can help you a bit.
First, let's look at yoga in general and your specific situation. Yoga helps to mobilize our inner resources. If you like the poetic images of the East, yoga will stimulate the Chi (or Prana) to flow more freely, which in turn nourishes and heals all tissues of the body. The way we work with our breath in yoga also helps to turn on the parasympathetic nervous system which promotes healing and turns off the stress systems of the body, which are so easily activated through fear when we are sick. (For more on the breath and the PNS read the
Yin-side of Breathing posted in my
Newsletter #5.) Physiologically, we also want to gently stimulate the tissues harmed during your surgery and gentle movements can assist with that.
Before giving you specific suggestions, I would suggest you create an advisory team consisting of your doctor, your surgeon and a caring yoga therapist: let them each know about the other's thoughts and then ask what their recommendations are. You are the head of the team though: once you have listened to everyone, then make your decision as to how much and how deeply you want to practice.
If you want to incorporate some specific Yin Yoga postures for the upper body, and if your yoga therapist agrees that these are okay for you, check out the article on
Upper Body Yin in
Newsletter #6. For more yang yoga suggestions, check out this post on
Yoga Journal.
Over time, keep checking back in with your advisory team and update your program. Work with attention and intention. Probably more important than the physical work will be your breath work and the practice of being present, which is your meditation. Meditating on health will work wonders for you.
I hope this helps. Let us know how it goes: others may learn a great deal from your journey.
Cheers
Bernie