[BykrBayb]

Russell is hopeful that Imlygic represents "a first step in the direction of a complete change in the game" in how we treat cancer. "We can't prematurely claim that we've achieved our ultimate goal, because we haven't; this really is a single step along that path," he said. "But it's a very important and very significant step."

It's designed to be injected into a tumor. This doesn't seem feasible for each and every cancer, but this does seem to me to be a major advance in cancer treatment.

[Moonman62]

It’s designed to be injected into a tumor. This doesn’t seem feasible for each and every cancer, but this does seem to me to be a major advance in cancer treatment.

...

Some systemic effects have been observed, plus some doctors may choose to use it off label. The downside is that during tests this therapy only showed a life extension of five months. It’s a start in the right direction I think.

[Ellendra]

It's designed to be injected into a tumor.

I was thinking about that part, and realized that with the way a virus replicates, injecting it into a tumor makes the most sense.

If the virus can only infect cancer cells, then that means it can only reproduce using cancer cells. From what I understand about viral engineering (take with a grain of salt), scientists can't engineer the body of the virus itself. What they do is print out the DNA* for the virus, and then inject that into an incubator cell. The DNA then does what virus DNA will do, which is to hijack the cell and force it to build virus bodies for it.

When the incubator cell bursts, it may only have made a few copies of the virus. (The exact number depends on a whole host of factors.) If you injected those copies directly into the bloodstream, it would be a race to see if one of them could find and infect a cancer cell before the immune system killed it. On the other hand, if you take the incubator cell before it bursts, and inject it directly into a tumor, then when the viruses emerge they'll be surrounded by eligible cells. They can infect and reproduce right off the bat, before the immune system realizes they're there. By the time they then spread to the bloodstream, there would be millions of them. They could flood the body in search of cancer cells before the immune system could ramp up enough to stop them.

Add in some mild immune-suppresing drugs, and the chances of the virus succeeding get even better.

*I'm assuming any engineered anti-cancer viruses will be DNA-based, since they mutate at a much slower rate than the RNA-based viruses.