A meta-analysis in 2004 by Mason et al. showed topical NSAIDs to be effective and safe in treating acute painful conditions for 1 week. Whether this is also true of the patch is not known. This systemic review of 26 double-blind, placebo-controlled trials showed clinically significant efficacy in 19 of 26 trials, with a pooled relative benefit of 1.6 and number needed to treat of 3.8 vs. placebo to achieve an outcome of approximately 50% reduction in pain at 7 days. It is also well studied for ostearthritis and localized pain of rheumatic conditions.

Flector has not been adequately studied for pain of metastatic cancer. Oral doclofenac has  a morphine sparing effect, as do other NSAID. It can cause liver damage. How well it works locally has not been studied.  It is not a completely benign drug, especially for a debilitated cancer patient susceptible to side effects of NSAID or ir with known or undiagnosed liver metastases.

Flector® Patch (diclofenac epolamine 1.3%). Prescribing information. August 2011. King Pharmaceuticals, Inc.
Bristol, TN 37620. www.flectorpatch.com.

The Medical Letter on Drugs and Therapeutics,  1277a January 14, 2008(50): 1-2

Mason L, Moore RA, Edwards JE, Derry S, McQuay HJ. Topical NSAIDs for acute pain: a meta-analysis. BMC Fam Pract. 2004;5:10–9.

R. Björkman, A. Ullman, J. Hedne Morphine-sparing effect of diclofenac in cancer pain European Journal of Clinical Pharmacology January 1993, Volume 44, Issue 1, pp 1-5
 For Lay version see here

More on patches in pain management - Lidoderm