They could be powered by little rectennas, think RFID tag. Hit the bots with a radio frequency and it indict a charge and they power on and do their thing. Turn the radio off and they go inert.
So they could be injected into a tumor and powered by an external radio source.
In they're only powered by light a fiber optic filament could be injected in the activation site. So long as the light passes through enough tissue to power them you don't need to stick a halogen bulb up someone's butt.
A tuned antenna is correlated to frequency, a shitty antenna can be any size. In a microscopic robot needing likely µA of current an untuned antenna could be sufficient for power.
But that's me speculating, I don't know how to power an injectable micro robot. If I did the link might have been about me! RF might be completely inappropriate for power.
So they could be injected into a tumor and powered by an external radio source.
In they're only powered by light a fiber optic filament could be injected in the activation site. So long as the light passes through enough tissue to power them you don't need to stick a halogen bulb up someone's butt.