ORIGINAL ARTICLE Interstitial PDT using diffuser fiber—investigation in phantom and in vivo models Mirian D. Stringasci 1 & Thereza C. Fortunato 1 & Lilian T. Moriyama 1 & José Dirceu Vollet Filho 1 & Vanderlei S. Bagnato 1 & Cristina Kurachi 1 Received: 18 May 2016 /Accepted: 27 April 2017 # Springer-Verlag London 2017 Abstract Photodynamic therapy (PDT) has been used for local treatment of several types of tumors. Light penetration of biological tissue is one limiting factor in PDT, decreasing the success rates of the treatment of invasive and solid tumors. In those cases, a possible solution is to use interstitial PDT, in which both diffuser optical fibers are inserted into the tumor. The uniformity of the diffuser emission plays a crucial role in planning the delivery of the appropriate light fluence and in ensuring treatment success. In this study, we characterized a diffuser optical fiber concerning its homogeneity. We showed that the diffuser emission can be inhomogeneous and that the necrosis generated by interstitial PDT using such a diffuser for illumination is asymmetrical in volume as a result. This ob- servation has relevant consequences in achieving success in PDT and phototherapies in general, as the delivered light fluence depends on adequate previous knowledge of the irra- diation profile. Keywords Photodynamic therapy . Interstitial . iPDT . Diffuser optical fiber . Heterogeneous irradiation profile Introduction Photodynamic therapy (PDT) has increasingly been used in the local treatment of both superficial malignant and potentially malignant lesions. PDT involves the use of a pho- tosensitizer (PS), which is either topically or systemically ad- ministered. After waiting for a time interval, depending on the photosensitizer and/or pro-drug administration, the biodistribution and accumulation occur preferentially in the tumor tissue. The light, at an appropriate wavelength, is deliv- ered to the target tissue. Excited PS molecules transfer energy to the molecular oxygen, resulting in the production of cyto- toxic reactive oxygen species that are able to cause tumor cell death [1]. PDT light delivery can be performed superficially or inter- stitially [1, 2]. Light dosimetry is one of the key aspects of PDT. A threshold dose must be achieved in all tumor volumes in order to induce tumor death by necrosis, apoptosis, or au- tophagy. Theories such as radiation transport and Kubelka– Munk models describe light propagation in turbid media [3] and in biological tissues and have been used to predict the light distribution and then to calculate the dose of light (deliv- ered light energy per volume) of treatment [4, 5]. The use of surface illumination in PDT results in significant tumor response; however, the limitation of light penetration into the tissue prevents the treatment of bulky or deep tumors since the delivery of sufficient light for treatment using the presently approved photosensitizers is limited to a thickness up to 5–10 mm for wavelengths in the red region of the visible spectrum [6]. For a higher volumetric treatment, it is possible to use in- terstitial PDT (iPDT), in which the tumor is treated from in- side out through the insertion of one or multiple optical fibers into the lesion. In this application, either flat-ended or cylin- drical diffuser fibers are used [7, 8]. The light delivered direct- ly to the tumor volume minimizes damage to adjacent healthy tissues [9, 10]. iPDT has been used since 2000s for the treatment of pros- tate cancer [11]. In this application, determination of the * Mirian D. Stringasci mirianstringasci@gmail.com 1 Sao Carlos Institute of Physics, University of São Paulo, Trabalhador Sao-Carlense Street, Number 400, São Carlos, São Paulo CEP: 13566-590, Brazil Lasers Med Sci DOI 10.1007/s10103-017-2225-7