Is there a volume or size criteria where you feel that standard doses for palliative radiation (8Gy/1fx-30Gy/10fx) are not effective?

At the risk of running off the mathematical rails, I thought about this question from a completely non-clinical viewpoint (from a clinical viewpoint, yes, I would probably use a bit more than 30/10 for a big 10 cm mass).

Here are a few assumptions. First, the D-sub-zero (D0) for many tumor systems happens to be about 3 Gy. The D0 is the (sparsely ionizing radiation) dose necessary to achieve ~63% tumor cell kill or ~37% tumor cell survival (Hall, 5th ed, p.46). Second, about a billion (1E9) viable tumor cells are contained within an "average" 1 cm spherical tumor volume. Third, a 10 cm spherical tumor volume has about 1,000 times the volume of a 1 cm tumor volume, so it would contain roughly 1E12 tumor cells versus 1E9 tumor cells in a 1 cm...