Thermal Management for Arcol HS50/HS100 50/100W Wirewound resistors
Well, here is the basis for the rating from Vishay-Dale's datasheet: 



30cm x 30cm x 3mm thick is probably considerably more than your PCB area, and the thermal conductivity of the PCB is not as good as solid aluminum so your heat spreading will not be as good. Even if it was (which it isn't), the temperature rise acceptable for an aluminium and ceramic resistor may be excessive for your PCB material. 

You might be able to get 10W continuous out of it. An alternative would be to buy a finned heatsink with specified °C/W rise and design for a reasonable rise in temperature at whatever you consider to be worst-case normal operating conditions. You could consider adding a fan, but that would require a power source.