Purpose: Deep tissue abscesses remain a serious cause of morbidity, mortality, and hospital stay despite development of percutaneous drainage and increasing use of perioperative antibiotics. The goal of this study was to examine the feasibility of methylene blue (MB) mediated photodynamic therapy (PDT) for treatment of infected abscesses with intracavity MB delivery, using computed tomography (CT) imaging data from a representative abscess patient population and Monte Carlo simulation of light delivery. Methods: This retrospective study included all adult subjects that received percutaneous abscess drainage between 1 January 2014 and 31 December 2014 at our institution whose abscess was confirmed by abdominal CT imaging less than 1 week preprocedure (n = 358). Of these, 40 subjects were further analyzed with Monte Carlo simulation. Abscess volumes were segmented from CT images, and imported into the Monte Carlo simulation space. Monte Carlo simulations were performed with a single fiber placement for each abscess, with the optical power at which a fluence rate of either 4 or 20 mW/cm 2 was achieved for 95% of the abscess wall recorded. Subjects for which a fluence rate of 4 mW/cm 2 was attainable in 95% of the abscess wall with a maximum input optical power of 2000 mW were considered eligible for MB-PDT. Results: 42.5% of subjects would have been eligible for MB-PDT, with 17.5% attaining the higher threshold of 20 mW/cm 2 in 95% of the abscess wall, given a 1% Intralipid concentration within the abscess cavity and the assumed abscess wall optical properties. The mean optical power necessary was 680 AE 580 mW for the 4 mW/cm 2 threshold, and 1100 AE 600 mW for the 20 mW/cm 2 threshold. Abscess surface area and threshold optical power were correlated (Spearman q = 0.73, P = 0.001), with larger abscesses requiring higher optical power. Of the subjects who were not eligible for MB-PDT, abscess volumes (150 AE 120 vs 62 AE 41 cm 3 , P = 0.0049) and surface areas (320 AE 200 vs 140 AE 70 cm 2 , P = 0.0015) tended to be larger than for those who were eligible. There were no significant differences in eligibility, optical power required, or abscess volume or surface area based on abscess location. For all eligible subjects, at the optical power necessary to achieve 4 mW/cm 2 in 95% of the abscess wall, 2.5 AE 3.7% (0%-13.2%) of the wall experienced a fluence rate greater than or equal to 400 mW/cm 2 . At the 20 mW/cm 2 threshold, 8.8 AE 11.4% (0%-31.1%) of the wall surpassed this 400 mW/cm 2 level. If subjects with greater than 5% of the wall exceeding 400 mW/cm 2 are treated as ineligible, overall eligibility becomes 32.5% for the 4 mW/cm 2 threshold and 10.0% for the 20 mW/cm 2 threshold. Conclusions: Assuming that the subjects analyzed were representative of the overall patient population, over 150 patients that received percutaneous abscess drainage during the study period would have been eligible for MB-PDT at the time of drainage, with smaller abscesses being more amenable for treatment. This technique could potent...

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