Evaluation of Non-inferiority and Tolerability of the Device PHOS-ISTOS

Inclusion Criteria: * Clinical diagnosis of minimum 10 previously untreated not pigmented, non-hyperkeratotic AK lesions of Grade I and II of the forehead and/or scalp (according to Olsen et Al. JAAD 1991, cf. REF 31) where other therapies are unacceptable or considered medically less appropriate with a symmetrical repartition on both side of the forehead and/or scalp. The diagnosis of AK will be determined upon clinical evaluation (i.e. visual inspection and palpation) by the investigator. * No treatment for the AKs in the previous 30 days. * Symmetrical areas in terms of number and severity of lesions. The axis of symmetry between the two areas will be defined by the investigator according to the distribution of lesions. * The two areas to be treated should not be coalescing. A minimum distance of 10 mm is required between the lesions located on the 2 symmetrical areas. A minimum distance of 2 mm is required between the lesions on the same side. * Minimum 5 lesions with similar dimensions at both symmetrical areas will be treated. If the number of lesions is \>7, only 7 lesions on each side will be considered. Exclusion Criteria: * Patients with porphyria. * Patients immunosuppressed for idiopathic, disease specific or therapeutic reasons. * Use of topical corticosteroids to lesional areas within 2 weeks before PDT. * Patients receiving local treatment (including cryotherapy and curettage-electrocoagulation, any PDT treatment) in face / scalp area within the last 30 days. * Patients receiving topical treatment (including imiquimod, 5-FU and diclofenac, Picato) in face / scalp area within the last 30 days.in * Use of topical retinoids or alpha-hydroxy acids, systemic retinoids, chemotherapy or immunotherapy within 30 days of PDT. * Pigmented AK lesion(s). * Known allergy to Metvixia/Metvix, a similar PDT compound or excipients of the cream including arachis oil, or to peanut or soya. * Participation in other clinical studies either currently or within the last 30 days. * Female subjects must be of either: * Non-childbearing potential, i.e. post-menopausal or have a confirmed clinical history of sterility (e.g. the subject is without a uterus) or, * Childbearing potential, provided there is a confirmed negative urine pregnancy test or blood analysis prior to study treatment, to rule out pregnancy. * Any condition which may be associated with a risk of poor protocol compliance. * Patients currently receiving regular ultraviolet radiation therapy.