Anatomy And Bone Cancer?

[Gregor]

The bone is hard and there's no space to accommodate an expanding tumor.
This does cause a lot of pain for patients with bony mets, and so frequently radiotherapy is directed at the largest metastatic lesions in the bone in an effort to reduce the size of these mets, and thus reduce pain.
In the oncology community this is sometimes rather sardonically referred to as "spot-welding" because of the large amount of radiation directed at a small area.
There is no intent to cure the patient with this procedure, only an intent to reduce pain for a while.

[Dafydd]

As a bone cancer grows it crowds out the bone marrow and vessels inside the bone basically replacing it.
It is most likely more painful because it causes tremendous pressure to build up since unlike soft tissue, where most cancers grow, the area cannot expand.
In most other types of tumors, the area in and around the tumor itself has tremendous flexibility to accommodate the growth before causing pain, if it ever does.