Thank you for the tag.
I completed my fellowship in HPM as a PGY4 after EM residency. I practice full-time at a large academic center in the palliative dept which is housed in the cancer institute. I'm faculty with the HPM fellowship at said institution, to OP's point, one of our graduating fellows had 20 years of experience as an attending prior to starting their HPM fellowship. No one is going to treat you as some green know-nothing. You will be respected for your experience and for being part of the team. Your fellowship attendings will teach you and sure enough, you will teach them. It is symbiotic since so many of us hail from different training vantage points. The concern that many have of "it is hard going back to being in training" mentality should only really apply in the sense of finances. The respect will still be there.
Of course the palliative care MD isn't directly managing all aspects of medical care but I don't see how I can provide even palliative care with a 20 year old understanding of management of even common medical conditions.
Yes and no. There are some palliative units where indeed you manage all facets of care as you are primary service. Depending on the culture of your dept that MIGHT be managing pure EoL patients with only symptom management -- or it could be managing somewhat medically complex "palliative patients" that need admission for XYZ. In my fellowship, our palliative unit functioned as the latter. I did not particularly feel confident or comfortable functioning essentially as an internist/hospitalist for these patients given my primary specialty being EM. Sure, I can manage them in the ED, but my primary field didn't encompass "so what comes next" (not that EM training should do otherwise). Overall, it was fine in fellowship as there was the attending for oversight (basically all IM primary specialty folks).
I took this knowledge and honest self-reflection to my subsequent job search with pointed questions about how places on the attending interview trail ran their IPU's (regarding management culture and admission criteria). I am perfectly comfortable managing the patients solo on my current IPU as these folks are not getting admitted to us for ongoing curative intent management of their melting tracheoesophageal-mediastinal-XYZ fistulas, acute on chronic hyponatremia, or thyroid crisis XYZ... Really we function more as an inpatient hospice and I am very happy with that. I offer our patients and their loved ones a "good death" or achieve stabilization of acute symptoms with the ability to return to their home/nursing facility. It is a good fit for my interests and training.
There are tons of institutions that don't have any form of IPU... it is all inpatient consults... or outpatient clinic... some institutions are all goals of care and very little symptom management.... others are more 50:50. It is important to know what you want because all the combinations are out there. But not all the combinations are out there in every geographic region.
In a palliative program worth it's salt you will get much more than primary palliative skills (basic GoC skills, standard opioid rotation, etc). You would ideally get exposure to more complex opioids (methadone, buprenorphine, etc) as well as adjuvants (ketamine, lidocaine, etc) for pain... and continue down the line for many various symptoms. I'm sure as psychedelics progress in clinical medicine our field will be among the early adopters. Really though the value of fellowship is the variety of patients under supervision and teaching of a HPM doc. Sure, you can read articles and textbooks without a fellowship...but going that route you don't get instant feedback and input on what is going well [or not well]. That honest specialty lens feedback is what will mold your practice.
Also, palliative jobs are requiring more and more being BE/BC in the field... so there is that.
Your background knowledge and experience in RadOnc would put you much ahead of most fellows with your understanding of different cancers, chemo regimens, radiation regimens, side effects, etc... than say a graduating EM resident (such as I). I'm sure you have experience with managing things like mucositis, I can't say I even knew much about it at all graduating residency. I sure do now though as I manage folks with it all the time. Yes, you will learn everything you need in fellowship to be [on paper] an HPM subspecialist regardless of what residency was pursued; however, that RadOnc foundation is nothing to shake a stick at!
The learning continues. It is a field one can easily practice into their 70's+ if so desired.
Happy to field any other questions you might have.