Simple SummaryChronic lymphedema in the arm is a rather common side-effect of breast cancer treatment and its prevention is desired. The most important and evidence-based treatment of arm lymphedema is daily use of a compression sleeve. In our previous randomized controlled trial, it was shown that early treatment with a compression sleeve in compression class 1 in mild arm lymphedema can prevent progression for 6 months. The aim of the present study was to follow the progression/no progression for 12 months. It was revealed that the results from the 6-month intervention were persistent at 9- and 12-months follow-up. Therefore, it can be concluded that a compression sleeve (compression class 1) may be applied immediately after early diagnosis of lymphedema to prevent progression and to avoid mild arm lymphedema becoming chronic.AbstractBackground: In our previous randomized controlled trial (RCT), the progression/no progression of mild breast cancer-related arm lymphedema (BCRL) was examined among women randomized to a compression group (CG) with a compression sleeve (compression class (ccl) 1) or not (NCG) for 6 months. In the present prospective study, BCRL in the CG and NCG was followed for 12 months. Methods: At the end of the RCT, 33 women with mild BCRL were eligible in the CG and 37 in the NCG. The proportional differences in no progression/progression of BCRL were defined as a >2% increase from start of RCT or exceeding 10% in the lymphedema relative volume as measured by the water displacement method. In addition, changes in the lymphedema relative volume and tissue dielectric constant ratio, which measures local tissue water, were examined. At the end of the RCT (i.e., after 6 months), a one-month break of the compression treatment was made in the CG. If the lymphedema relative volume progressed by definition, the compression treatment was resumed and continued, with follow-up of all women at 9 and 12 months. Results: A larger proportion of women in the NCG showed progression (57%, 61%, 67%) compared to the CG (16%, 22%, 31%) at 6, 9, and 12 months (p< 0.001, 0.005, 0.012), respectively. Twelve (33%) women in the NCG did not progress at all. No changes of the lymphedema relative volume and local tissue water were found over time at any follow-ups, but were stable on a low level. Conclusions: To avoid the progression of mild BCRL into a chronic issue in the long-term, compression sleeve ccl 1 may be applied immediately after early diagnosis of mild BCRL.