Hadiyah N Green,1,2 Stephanie D Crockett,3 Dmitry V Martyshkin,1 Karan P Singh,2,4 William E Grizzle,2,5 Eben L Rosenthal,2,6 Sergey B Mirov11Department of Physics, Center for Optical Sensors and Spectroscopies, 2Comprehensive Cancer Center, 3Department of Pediatrics, Division of Neonatology, 4Department of Medicine, Division of Preventive Medicine, Biostatistics and Bioinformatics Shared Facility, 5Department of Pathology, 6Department of Surgery, Division of Otolaryngology, Head and Neck Surgery, The University of Alabama at Birmingham, Birmingham, AL, USAPurpose: Nanoparticle (NP)-enabled near infrared (NIR) photothermal therapy has realized limited success in in vivo studies as a potential localized cancer therapy. This is primarily due to a lack of successful methods that can prevent NP uptake by the reticuloendothelial system, especially the liver and kidney, and deliver sufficient quantities of intravenously injected NPs to the tumor site. Histological evaluation of photothermal therapy-induced tumor regression is also neglected in the current literature. This report demonstrates and histologically evaluates the in vivo potential of NIR photothermal therapy by circumventing the challenges of intravenous NP delivery and tumor targeting found in other photothermal therapy studies.Methods: Subcutaneous Cal 27 squamous cell carcinoma xenografts received photothermal nanotherapy treatments, radial injections of polyethylene glycol (PEG)-ylated gold nanorods and one NIR 785 nm laser irradiation for 10 minutes at 9.5 W/cm2. Tumor response was measured for 10–15 days, gross changes in tumor size were evaluated, and the remaining tumors or scar tissues were excised and histologically analyzed.Results: The single treatment of intratumoral nanorod injections followed by a 10 minute NIR laser treatment also known as photothermal nanotherapy, resulted in ~100% tumor regression in ~90% of treated tumors, which was statistically significant in a comparison to the average of all three control groups over time (P<0.01).Conclusion: Photothermal nanotherapy, or intratumoral nanorod injections followed by NIR laser irradiation of tumors and tumor margins, demonstrate the potential of NIR photothermal therapy as a viable localized treatment approach for primary and early stage tumors, and prevents NP uptake by the reticuloendothelial system.Keywords: photothermal cancer therapy, malignancy, cancer treatment, intratumoral, gold nanorods, nanoparticles, PEGylation, laser therapy