Is there any clinical evidence showing euphorbia peplus (petty spurge, radium weed, milkweed) has any helpful effects treating basal cell carcinoma?

Probably: Basal cell carcinoma, untreated, will destroy your face, or wherever it is located, and then eat into your brain, albeit slowly. Milkweed alkaloids may work like other immunomodulators that remove basal cell carcinoma, but i would opt first for surgery unless my lesion is inoperable.

Milkweed for BCC: Basal cell carcinoma (bcc) is primarily treated surgically. When caught and treated early, it is almost always cured, and recurrence rates should be very low. If you have been diagnosed with a bcc, get it treated by your local surgeon or dermatologist. Don't risk a recurrence or growth of the tumor.