By all means, massage your breasts, but I'm not so sure your hypothesis holds up. The paucity of cancers of skeletal muscle is more likely due to cell type (terminally differentiated without much turnover) rather than vascular supply. Carcinomas of the lung and colon are some of the most prevalent (probably still would be even taking environmental factors into account) and those organs get plenty of blood supply. Leukemias also aren't rare and are literally bathed in blood flow.