UPDATE 1-Melanoma patients live longer on Roche's Avastin

* First time a trial has shown melanoma survival benefit

* News may trigger early prescribing by melanoma specialists

* Could offer potential $500 million new market for Roche

(Adds background, analysts, more detail)

BERLIN, Sept 21 (Reuters) - Patients with melanoma, a notoriously difficult to treat cancer of the skin, live longer when given Roche's ROG.VX drug Avastin, according to a scientific abstract from Europe's top cancer meeting.

The median overall survival in a Phase II trial was 12.6 months for Avastin plus standard chemotherapy, significantly more than the 8.6 months seen with chemotherapy on its own.

“This is the first randomised placebo-controlled trial in metastatic melanoma to demonstrate a statistically significant and clinically meaningful improvement in OS (overall survival),” investigator Steven O’Day reported.

The finding, which will be reported in full at the ECCO-ESMO cancer conference in Berlin on Wednesday, is likely to boost interest in Avastin among melanoma specialists, who may use it on a so-called “off label” basis before formal approval.

While melanoma accounts for only a small percentage of skin cancers, it is responsible for most skin cancer deaths. The disease is characterised by the uncontrolled proliferation of pigment-producing skin cells called melanocytes.

Over-exposure to sunlight is to blame for at least two-thirds of cases as DNA in sunburnt skin cells becomes damaged, leading to the genetic mutations.

Avastin is already a blockbuster and a key driver for Roche, with global sales of 5.2 billion Swiss francs ($5.02 billion) in 2008 as a treatment for colon, lung and breast cancers.

Analysts said the results of its trial in melanoma represented the first ever survival benefit for a so-far unmet need in cancer.

It is possible Roche may decide to apply for a licence for Avastin in melanoma based on these results alone, they said, avoiding the need to go through full final phase trials and opening up a potential $500 million new market. ($1=1.036 Swiss Franc) (Reporting by Ben Hirschler and Kate Kelland; editing by David Cowell)