What Happens During a PDT Treatment Session, Step by Step?

Introduction

Knowing exactly what happens at a photodynamic therapy (PDT) appointment can make the visit far less unfamiliar. While specifics vary somewhat between clinics — the exact photosensitizing agent, the incubation time, and the type of light source used all differ by protocol — every PDT session follows the same basic sequence: the skin is prepared, a photosensitizing agent is applied and given time to absorb, the area is exposed to a specific wavelength of light, and the skin is checked and cared for afterward.

This guide walks through each of those steps in the order they happen, so you know what to expect at every point in the appointment.

The session starts with the treatment area — often the face or scalp — being cleansed to remove oil, makeup, or sunscreen. If any of the AKs are thick or scaly, the clinician may gently scrape or curette the surface (a quick, in-office step) to remove built-up scale.

This preparation matters because it helps the photosensitizing agent penetrate evenly across the whole area, rather than being blocked by a layer of dead skin over the thicker lesions.

This is the longest part of the visit. The photosensitizing agent needs time to be absorbed selectively by the damaged cells before it can be activated by light — typically somewhere between about 30 minutes and a few hours, depending on the specific protocol your dermatologist uses. Some clinics cover the area with an occlusive dressing or foil during this time to keep the agent in place and out of ambient light; others leave it uncovered under low light conditions.

Incubation itself usually feels like nothing more than a mild tightness or awareness of the product on the skin. This is typically a waiting period — you may sit in a treatment room or a dimly lit waiting area — rather than an active part of the procedure. Daylight PDT protocols handle this step differently: after a brief preparation, the photosensitizer is applied and the patient goes outdoors into natural daylight for a set period instead of waiting indoors before a separate light device is used.

Immediately following light exposure, the treated area is typically red, swollen, and warm to the touch — a reaction that looks similar to a sunburn and is an expected sign that the treatment reached the intended cells. Some mild stinging or sensitivity often continues for a few hours afterward.

Before you leave, the treated skin is usually protected from light exposure (a bandage, dressing, or simply clothing and sunglasses depending on the area), and you'll be given specific instructions about avoiding direct sunlight — generally for 24 to 48 hours — since the skin remains photosensitive during that window.

Conclusion

A PDT session follows a predictable sequence — cleansing, photosensitizer application, an incubation wait, brief light exposure, and a sunburn-like reaction immediately afterward — even though the exact timing and light source vary between clinics and protocols. Most of the appointment is waiting, and the treatment step itself, while it can be uncomfortable, is short.

Knowing this sequence ahead of time makes it easier to plan your day around the appointment and to know what's normal once you're home.