One; was to ensure that all types of cancers were to be treated by the "Radium-Based" methods (Cancer Inducing Radiation) and,
Second; it was necessary to introduce such an act to protect the profits of Pharmaceutical companies.
The following 2 links will provide you with the Cancer Act of 1939 for your own review.
http://www.legislation.gov.uk/ukpga/1939/13/pdfs/ukpga_19390013_en.pdf
http://www.legislation.gov.uk/ukpga/Geo6/2-3/13/contents
What has now been proven is: