[Riot]

That's very sad news. Deep condolences to her owners.

Some information: Lymphosarcoma is very rare in young animals. There is probably nothing "causal" to it (that we are aware of), it simply happens (like bone cancer in young humans). It's very sad to diagnose this in young animals as you don't have many options to try, and lymphosarc in young animals is very difficult to put into remission.

In older animals, however, it's one of the most successful, rewarding and easiest cancers to treat into a good remission (generally speaking).

It's difficult sometimes to find growths and masses on the internal organs of horses, simply due to their size. Dogs and cats (and humans) we can ultrasound all their organs readily and thoroughly at virtually any facility, take biopsies either via ultrasound-guided or directly through an easy, quick surgery. You can get to a diagnosis of lymphosarcoma quickly and readily.

With horses it nearly always takes referral to a facility where MRI or advanced imaging or surgical procedures can be done, to get a definitive diagnosis. It's harder to get to or verify a diagnosis easily or readily, simply due to their size.