I'd say that the only thing that reliably differentiates a _podcast_ from a _radio show_ is just that the podcast's method of delivery is RSS.
RSS can be polled as frequently or infrequently as you like, it's just a bit of XML hosted by a site which lists content or links to content. Tracking what's been read or not isn't done by the RSS feed or whoever hosts…
I'd say that the only thing that reliably differentiates a _podcast_ from a _radio show_ is just that the podcast's method of delivery is RSS.
RSS can be polled as frequently or infrequently as you like, it's just a bit of XML hosted by a site which lists content or links to content. Tracking what's been read or not isn't done by the RSS feed or whoever hosts…