Study to Evaluate the Safety, Tolerability and Efficacy of BF-200 ALA (Ameluz®) in the Field-directed Treatment of Actinic Keratosis (AK) on the Extremities and Neck/Trunk With Photodynamic Therapy (PDT) Using a RhodoLED Lamp

Inclusion Criteria: 1. Willingness and ability of subjects to provide informed consent and sign the Health Insurance Portability and Accountability Act (HIPAA) form. A study-specific informed consent and HIPAA form must be obtained in writing prior to starting any study procedures. 2. 4 - 15 mild to moderate clinically confirmed AK lesions according to Olsen either on the extremities or on the neck/trunk with a diameter of ≥ 4 mm that must be present in the treatment field (defined as AK target lesions). In addition, non-target AK lesions may be present in the treatment field, including up to two severe AK lesions ≥ 4 mm. For each severe AK lesion (≥ 4 mm), a biopsy must be taken for confirmation of diagnosis. The treatment field (continuous or in several patches) totaling approx. either 80 cm², 160 cm² or 240 cm2 must be within one effective illumination area of the BF-RhodoLED® XL but may require up to three illuminations with the BF-RhodoLED®. All AK target lesions and, if applicable, severe AK lesions ≥ 4 mm located in the treatment field should be clearly distinguishable, without restrictions on the distance between lesions, and should have a minimal distance of 1 cm to the border of the treatment field. 3. All sexes, ≥ 18 years of age. 4. Willingness to undergo a 2 mm punch biopsy for each (up to two) severe AK lesion ≥ 4 mm, if applicable, at the screening visit. 5. Willingness and ability to comply with study procedures, particularly willingness to receive up to 2 PDTs within approximately 12 weeks. 6. Subjects with good general health or with clinically stable medical conditions will be permitted to be included in the study. 7. Willingness to stop the use of moisturizers and any other non-medical topical treatments within the treatment field at least 24 h prior to the next clinical visit. 8. Acceptance to abstain from extensive sunbathing and the use of a solarium or tanning beds during the treatment phase. 9. For female subjects with reproductive potential: Negative serum pregnancy test. 10. For female subjects with reproductive potential: Effective contraception at screening visit and throughout the treatment phase of the study (until Visit 4 or Visit 6). Exclusion Criteria: 1. Any known history of hypersensitivity to ALA, porphyrins, or excipients of BF-200 ALA. 2. History of soy or peanut allergy. 3. Sunburn or other possible confounding skin conditions (e.g., wounds, irritations, bleeding, or skin infections) inside or in close proximity (\< 2 cm distance) to the treatment field. 4. Clinically significant (cs) medical conditions making implementation of the protocol or interpretation of the study results difficult or impairing subject's safety such as: 1. Presence of photodermatoses or porphyria 2. Metastatic tumor or tumor with high probability of metastasis 3. Infiltrating skin neoplasia (suspected or known) 4. Unstable cardiovascular disease (New York Heart Association class III, IV) 5. Unstable hematologic (including myelodysplastic syndrome), hepatic, renal, neurologic, or endocrine condition 6. Unstable collagen-vascular condition 7. Unstable gastrointestinal condition 8. Immunosuppressive condition 9. Presence of clinically significant inherited or acquired coagulation defect 5. Clinical diagnosis of atopic dermatitis, Bowen´s disease (BD), basal cell carcinoma (BCC), eczema, psoriasis, rosacea, squamous cell carcinoma (SCC), other malignant or benign tumors inside or in close proximity (\< 2 cm distance) to the treatment field. 6. Presence of strong artificial pigmentation (e.g., tattoos) or any other abnormality that may impact lesion assessment or light penetration in the treatment field. 7. Any physical therapy such as cryotherapy, laser therapy, electrodessication, microdermabrasion, surgical removal of lesions, curettage, or treatment with chemical peels such as trichloroacetic acid inside or in close proximity (\< 10 cm distance) to the treatment field within 4 weeks prior to screening. 8. Any of the topical treatments defined below within the designated periods prior to screening: 1. Topical treatment with ALA or ALA esters (e.g., methyl aminolevulinic acid (MAL)) inside the treatment field within 3 months. 2. Topical treatment with immunomodulatory, cytostatic, or cytotoxic drugs inside or in close proximity (\< 10 cm distance) to the treatment field within 3 months. 3. Start of topical administration of a medication with hypericin or other drugs with phototoxic or photoallergic potential inside or in close proximity (\< 10 cm distance) to the treatment field within 4 weeks. Subjects may, however, be eligible if such medication was applied for more than 4 weeks prior to screening without evidence of an actual phototoxic/photoallergic reaction. 9. Any use of the systemic treatments within the designated periods prior to screening: 1. Cytostatic or cytotoxic drugs within 6 months. 2. Immunosuppressive therapies or ALA or ALA esters (e.g., MAL) within 12 weeks. 3. Drugs known to have major organ toxicity within 8 weeks. 4. Interferon or glucocorticosteroids within 6 weeks. 5. Start of intake of medication with hypericin or drugs with phototoxic or photoallergic potential within 8 weeks prior to screening. Subjects may, however, be eligible if such medication was taken in for more than 8 weeks prior to the screening visit without evidence of an actual phototoxic/photoallergic reaction. 10. Breast feeding women. 11. Suspicion of drug or alcohol abuse. 12. Subjects unlikely to comply with protocol, e.g., inability to return for visits, unlikely to complete the study, or inappropriate in the opinion of the investigator. 13. A member of study site staff or sponsor staff directly involved in the conduct of the protocol or a close relative thereof. 14. Receipt of any investigational drug or medical product within 8 weeks before screening or simultaneous participation in another clinical study. Reassessment of subjects is allowed once in case exclusion criterion 3 is met and eligibility can be achieved within 4 weeks. Reassessment can be done on the day of the actual treatment. Dosing day exclusion criteria: At Visit 2 (baseline, PDT-1) Subjects with sunburn or other possibly confounding skin conditions (e.g., wounds, irritations, bleeding, or skin infections) inside or in close proximity (\< 2 cm distance) to the treatment field. Reassessment of subjects is allowed once if the sunburn or other confounding skin conditions is/are expected to resolve within 2 weeks. At Visit 4 (PDT-2) Subjects with sunburn or other possibly confounding skin conditions (e.g., wounds, irritations, bleeding, or skin infections) inside or in close proximity (\< 2 cm distance) to the treatment field. Rescheduling of PDT-2 can be performed once at the earliest possibility after resolution, but rescheduling should not exceed 2 weeks.