Partially necrotic tumors are common because a lot of them outgrow their blood supply. Also the new blood vessels they grow are usually leaky and don't deliver blood very well. That's one thing Avastin (bevacizumab) does--it reduces the leakage and normalizes the blood flow, which is actually a good thing since it means the other chemo (it is not used alone) can access the tumor better.
I don't recall when this tumor was found on your father but it was fairly large, correct? My husband has a tumor in a very similar location, against the sacrum. I am going to try to talk them into using radiation on the remnant. His surgeon said that there are a lot of blood vessels there so surgery isn't an option, but radiation works. However, it puts the small intestine at risk. His tumor has shrunk dramatically on folfiri/bev which should make radiation safer. Have they discussed trying to shrink his tumor before radiation?
D/H Dx 10/2019 RC age 61
Clinical T4bN2M1a (common iliac and para-aortic lymph nodes)
MSS KRAS G12D
CRT 11/19-1/20 FOLFOX 3/20-7/20
Pelvic exenteration w/LAR 8/20
ypT4bN0Mx G3 0/14 nodes LVI not seen PNI-
CEA 10/19:20, 1/20-11/20:1.6, 4.3, 3.4, 2.7, 2/21:9.0 3/21:18,40 4/21:28,19, 5/21:13.3,8.6
PET 3/21 recurrence in distal nodes, L5 vertebra, pelvis