It is less effective when used on lower limbs and generally impractical for the posterior trunk due to inaccessibility.
Why is Imiquimod 5% cream (US brand: Aldara) used to treat a superficial basal cell carcinoma (sBCC) less effective on lower limbs than the rest of the body?
The papers studying the effectiveness of Imiquimod 5% cream (US brand: Aldara) used to treat sBCC I have found so far doesn't mentions the effectiveness on lower limbs in particular. E.g. https://www.accessdata.fda.gov/drugsatfda_docs/label/2004/20723s016lbl.pdf (mirror):