This Phase 3, randomized, multi-center, evaluator-blind, vehicle-controlled study evaluates the efficacy and safety of reformulated Levulan Kerastick (aminolevulinic acid HCl 20%) combined with photodynamic therapy (PDT) for field-directed treatment of actinic keratosis (AK) on the face or bald scalp. Approximately 160 adult patients with 4-8 mild to moderate AK lesions will be randomized into four treatment arms based on two variables: type of treatment (active drug or vehicle) and incubation time. During the study, up to 2 PDT sessions may be administered depending on the clearance of lesions. The primary endpoint is complete clearance rate (CCR) at Week 12. Secondary endpoints include AK clearance rate (AKCR), partial clearance, change in total lesion count and area, recurrence rate, cosmetic response, and patient satisfaction. Safety assessments include adverse events, local skin reactions, vital signs, and lab tests.

Inclusion Criteria:

1. Patient is a male or non-pregnant female aged 18-85 years.
2. Patient must have 4-8 mild to moderate actinic keratosis (AK) lesions (Olsen grade 1 or 2) in a single treatment area (either face or bald scalp, not both), each ≤1 cm in diameter and spaced at least 1 cm apart. [The treatment area should be approximately 25 cm², excluding sensitive facial regions (eyes, lips, nostrils, ears, mouth)]
3. Patients must be in good general health, avoid sunbathing or tanning devices, and maintain a consistent skincare routine throughout the study.
4. Patient is willing to stop using all topical products within the Treatment Area within 8 hours of a scheduled clinic visit with an efficacy evaluation.
5. Women of childbearing potential must use highly effective contraception; men with such partners must use barrier methods unless surgically sterile.

Exclusion Criteria:

1. Presence of Olsen grade 3 AK, untreated or suspicious skin cancers, or significant skin abnormalities (e.g., psoriasis, eczema) in the treatment area.
2. Active herpes simplex infection or history of ≥2 outbreaks in the treatment area within the past year.
3. History of cutaneous photosensitization, porphyria, photodermatosis, or hypersensitivity to porphyrins.
4. Positive human immunodeficiency virus (HIV) test result, hepatitis B surface (HBS) antigen, or hepatitis C virus (HCV) test result
5. Immunosuppressed status (e.g., HIV, organ transplant) or known coagulation defects.
6. Use of specific topical therapies (e.g., retinoids, steroids, cryotherapy, PDT, chemical peels) on the treatment area within 2 days to 8 weeks prior to randomization.
7. Recent use of certain systemic therapies (e.g., corticosteroids, immune-modulators, retinoids, cytotoxic drugs) within 6 weeks to 6 months prior to randomization.
8. Recent use of hypericin or other phototoxic/photoallergic medications (e.g., tetracyclines, psoralenes, St. John's wort) within 8 weeks of randomization.
9. Clinically significant or unstable illnesses, or history of alcohol/drug abuse in the past year.
10. Pregnancy/lactation, recent or current participation in another clinical study, or any other condition deemed unsuitable by the Investigator.

The participant, investigator and outcomes assessor are blinded to treatment assignment, as both Levulan and Vehicle products look identical and cannot be differentiated from the application characteristics.