Photodynamic therapy (PDT) is a minimally invasive treatment choice whose clinical success in dermatology relies on the interaction between a photosensitizer, light of an appropriate wavelength, and tissue oxygen, leading to reactive oxygen species generation and selective cytotoxicity. This narrative review summarizes contemporary mechanisms and clinical evidence supporting PDT across neoplastic, inflammatory, infectious, and esthetic indications. A comprehensive literature search included randomized trials when available, systematic reviews, meta-analyses, and guideline and consensus documents, complemented by mechanistic and translational studies relevant to clinical outcomes. In premalignant and neoplastic disease, strongest evidence supports field-directed PDT for actinic keratosis and high efficacy in Bowen’s disease, with favorable cosmetic outcomes and acceptable recurrence patterns. PDT plays a more selective role in basal cell carcinoma, particularly superficial and selected nodular lesions, while its routine use as monotherapy in squamous cell carcinoma remains limited by higher recurrence. Beyond oncology, PDT shows expanding utility in acne via sebomodulatory and immunomodulatory effects, and in infectious dermatoses through broad antimicrobial activity and biofilm disruption with low resistance potential. Cosmetic applications, including photorejuvenation, benefit from protocol tailoring and combination strategies that enhance penetration and remodeling. Overall, PDT is evolving into an adaptable therapeutic framework best positioned within mechanism-oriented, multimodal algorithms.

Background: Guselkumab, a monoclonal antibody selectively targeting the p19 subunit of interleukin-23, is typically administered with an induction phase at weeks 0 and 4 followed by maintenance dosing every 8 weeks for the treatment of psoriatic arthritis (PsA). In Europe, a four-week regimen (Q4W) has recently been approved for patients at high risk of joint damage progression; however, real-world evidence on this intensified schedule remains limited. This study aimed to evaluate the effectiveness and safety of guselkumab Q4W in routine clinical practice. Methods: This retrospective observational study included 18 PsA patients at high risk of structural damage treated with guselkumab Q4W at two psoriasis referral centers in the Lazio region of Italy. Baseline and 24-week clinical data were collected from medical records. Outcome measures included Disease Activity Index for Psoriatic Arthritis (DAPSA), Minimal Disease Activity (MDA), pain Visual Analog Scale (VAS), Dermatology Life Quality Index (DLQI), and Psoriasis Area and Severity Index (PASI). Results: After 24 weeks, mean DAPSA decreased from 34.3 ± 7.6 to 3.7 ± 3.0; 55.6% of patients achieved DAPSA remission and 77.8% achieved MDA. Significant improvements were observed in pain VAS, DLQI, and PASI scores. All patients achieved PASI 75, while 83.3% achieved PASI 90 and PASI 100. Treatment was generally well tolerated, with no adverse events or new safety signals documented during follow-up. Conclusions: In our study, guselkumab Q4W showed marked multidomain effectiveness and a favorable safety profile, supporting its use in PsA patients at high risk of joint damage progression.