Your client should first complete the PIDS and MEQ online questionnaires at //www.cet.org/assessment/, hosted by the nonprofit professional agency, the Center for Environmental Therapeutics. The questionnaire feedback will tell him whether his seasonal slumps have clinical severity, and — if so — how to treat them with light therapy. (His cancer status is irrelevant.) He can print out the results to discuss with his doctor and make decisions toward next steps. Appropriate lighting apparatuses are available at some pharmacies, but not most. Some, indeed, are selling “hyped” devices that have never seen clinical testing and are likely to be ineffective. See the same website for guidelines on choosing a light box — their “Suppliers” page — and their clinically tested recommendation on the “CET Store” page.