Assuming that the figure you've given is a PSA (prostate specific antigen) reading, it's high but not massively so. For men aged 50 to 69, a reading above 3 nanograms per milliletre is regarded as 'raised':
https://www.nhs.uk/conditions/prostate-cancer/should-i-have-psa-test/
For those who're are older, the threshold can be a bit higher.
However men with prostate cancer can often have FAR higher PSA levels. (Barquentine posted about his father-in-law on AB recently, whose PSA level was 312 nanograms per milliletre before starting treatment for his prostate cancer. I've got prostate cancer myself and, although I was never given a starting figure, the consultant was pleased when my PSA level fell to 35. The last time I was given a figure it was down to 12 but I've had two courses of chemo since then, so it's probably now down into single figures again).
A level of 7.8 might merit further investigation but it's certainly no reason to panic. Prostate cancer is incredibly common, with 4 out of every 5 men over 80 having it, but very few people actually die from it as it can be cured completely if it's not spread beyond the prostate or treated to hold it back if it's spread into the patient's bones or beyond.