The stage at which cancer is diagnosed differs depending on where people live, research by Macmillan Cancer Support shows.
For the first time, it shows information by groups of local doctors’ practices called GP Cluster Networks.
One in five people in these clusters in Cwm Taf were diagnosed at its most advanced stage - stage 4 - compared to 13 per cent of people in Powys between 2011 and 2015.
Cancer tends to be the most treatable the earlier it is diagnosed.
For the whole of Wales, almost 17% of new cancer patients registered with a GP were diagnosed with cancer at it's earliest stage.
However Abertawe Bro Morgannwg and Hywel Dda’s GP Cluster Networks have a lower percentage of their GP-registered cancer patients diagnosed at the earliest stage - particularly with lung and bowel cancers.
It also reveals the difficulties in planning cancer care and patients reaching services in rural area with more than a third of new cancer patients living in rural areas.
Richard Pugh, Head of Services for Macmillan Wales, says it's just one of the challenges facing the NHS.
The Welsh Government say the research will help GPs detect cancer earlier.
GPs in Wales made more than 90,000 referrals last year for investigation. Two health boards are piloting vague symptom clinics.
The research was in partnership with the Welsh Cancer Intelligence and Surveillance Unit.