Hello and welcome to HCM, I read your query and understood your concern. Yes, phototherapy in office is different than sun exposure / tanning beds, since in office you d be exposed to a very narrow wavelength of light to yield benefit and exclude any phototoxic effects. Although sun exposure has also been found to have some benefit, it ll definitely be associated with untoward side effects like ageing / skin cancer. You can consider other options to light therapy and discuss with your physician. There are various other systemic and local drugs found to be effective for PLEVA depending upon how widespread the lesion is. Talk to your physician about considering oral antiinflammatory like dapsone or methotrexate, or topical immuno modulators. Topical tacrolimus/ pimecrolimus has shown pretty promising results with pityriasis lichenoides. Hope I helped.