Epigenetic therapy inhibits metastases by disrupting premetastatic niches.

Cancer recurrence after surgery remains an unresolved clinical problem1-3. Myeloid cells derived from bone marrow contribute to the formation of the premetastatic microenvironment, which is required for disseminating tumour cells to engraft distant sites4-6. There are currently no effective interventions that prevent the formation of the premetastatic microenvironment6,7. Here we show that, after surgical removal of primary lung, breast and oesophageal cancers, low-dose adjuvant epigenetic therapy disrupts the premetastatic microenvironment and inhibits both the formation and growth of lung metastases through its selective effect on myeloid-derived suppressor cells (MDSCs). In mouse models of pulmonary metastases, MDSCs are key factors in the formation of the premetastatic microenvironment after resection of primary tumours. Adjuvant epigenetic therapy that uses low-dose DNA methyltransferase and histone deacetylase inhibitors, 5-azacytidine and entinostat, disrupts the premetastatic niche by inhibiting the trafficking of MDSCs through the downregulation of CCR2 and CXCR2, and by promoting MDSC differentiation into a more-interstitial macrophage-like phenotype. A decreased accumulation of MDSCs in the premetastatic lung produces longer periods of disease-free survival and increased overall survival, compared with chemotherapy. Our data demonstrate that, even after removal of the primary tumour, MDSCs contribute to the development of premetastatic niches and settlement of residual tumour cells. A combination of low-dose adjuvant epigenetic modifiers that disrupts this premetastatic microenvironment and inhibits metastases may permit an adjuvant approach to cancer therapy.

Commissioning and validation of a single photon beam model in RayStation for multiple matched Elekta Linacs.

PURPOSE: A single treatment planning system (TPS) model for matched linacs provides flexible clinical workflows from patient treatment to intensity-modulated radiation therapy (IMRT) quality assurance (QA) measurement. Since general guidelines for building a single TPS model and its validation for matched linacs are not well established, we present our RayStation photon TPS modeling strategy for matched Elekta VersaHD linacs. METHOD: The four linacs installed from 2013 to 2020 were matched in terms of Percent Depth Dose (PDD), profile, output factor and wedge factors for 6-MV, 10-MV, 15-MV, and 6-MV-FFF, and maintained following TG-142 recommendations until RayStation commissioning. The RayStation single model was built to represent all four linacs within the tolerance limits recommended by MPPG-5.a. The comprehensive validation tests were performed for one linac following MPPG-5.a and TG-119 guidelines, and spot checks for the other three. Our TPS modeling/validation method was evaluated by re-analyzing the previous 103 patient-specific IMRT/volumetric modulated arc therapy (VMAT) QA measurements with the calculated planar doses by the single model in comparison with the analysis results using four individual Pinnacle TPS models. RESULTS: For all energies, our single model PDDs were within 1% agreement of the four-linac commissioning measurements. The MPPG-5.a validation tests from 5.1 through 7.5 and all TG-119 measurements passed within the recommended tolerance limits. The IMRT QA results (mean ± standard deviation) for RayStation single model versus Pinnacle individual models were 98.9% ± 1.3% and 98.0% ± 1.4% for 6-MV, 99.9% ± 0.1% and 99.1% ± 1.9% for 10-MV, and 98.2% ± 1.3% and 97.9% ± 1.8% for 6-MV-FFF, respectively. CONCLUSION: We successfully built and validated a single photon beam model in RayStation for four Elekta Linacs. The proposed new validation methods were proven to be both efficient and effective.

Alignment of multiradiation isocenters for megavoltage photon beam.

The accurate measurement of the linear accelerator (linac) radiation isocenter is critical, especially for stereotactic treatment. Traditional quality assurance (QA) procedure focuses on the measurement of single radiation isocenter, usually of 6 megavoltage (MV) photon beams. Single radiation isocenter is also commonly assumed in treatment planning systems (TPS). Due to different flattening filters and bending magnet and steering parameters, the radiation isocenter of one energy mode can deviate from another if no special effort was devoted. We present the first experience of the multiradiation isocenters alignment on an Elekta linac, as well as its corresponding QA procedure and clinical impact. An 8 mm ball-bearing (BB) phantom was placed at the 6 MV radiation isocenter using an Elekta isocenter search algorithm, based on portal images. The 3D radiation isocenter shifts of other photon energy modes relative to the 6 MV were determined. Beam profile scanning for different field sizes was used as an independent method to determine the 2D multiradiation isocenters alignment. To quantify the impact of radiation isocenter offset on targeting accuracy, the 10 MV radiation isocenter was manually offset from that for 6 MV by adjusting the bending magnet current. Because our table isocenter was mechanically aligned to the 6 MV radiation isocenter, the deviation of the table isocentric rotation from the "shifted" 10 MV radiation isocenter after bending magnet adjustment was assessed. Winston-Lutz test was also performed to confirm the overall radiation isocenter positioning accuracy for all photon energies. The portal image method showed the radiation isocenter of the 10 MV flattening filter-free mode deviated from others before beam parameter adjustment. After the adjustment, the deviation was greatly improved from 0.96 to 0.35 mm relative to the 6 MV radiation isocenter. The same finding was confirmed by the profile-scanning method. The maximum deviation of the table isocentric rotation from the 10 MV radiation isocenter was observed to linearly increase with the offset between 6 and 10 MV radiation isocenter; 1 mm radiation isocenter offset can translate to almost 2 mm maximum deviation of the table isocentric rotation from the 10 MV radiation isocenter. The alignment of the multiradiation isocenters is particularly important for high-precision radiotherapy. Our study provides the medical physics community with a quantitative measure of the multiradiation isocenters alignment. A routine QA method should be considered, to examine the radiation isocenters alignment during the linac acceptance.

Characterization of a commercial bioluminescence tomography-guided system for pre-clinical radiation research.

BACKGROUND: Widely used Cone-beam computed tomography (CBCT)-guided irradiators have limitations in localizing soft tissue targets growing in a low-contrast environment. This hinders small animal irradiators achieving precise focal irradiation. PURPOSE: To advance image-guidance for soft tissue targeting, we developed a commercial-grade bioluminescence tomography-guided system (BLT, MuriGlo) for pre-clinical radiation research. We characterized the system performance and demonstrated its capability in target localization. We expect this study can provide a comprehensive guideline for the community in utilizing the BLT system for radiation studies. METHODS: MuriGlo consists of four mirrors, filters, lens, and charge-coupled device (CCD) camera, enabling a compact imaging platform and multi-projection and multi-spectral BLT. A newly developed mouse bed allows animals imaged in MuriGlo and transferred to a small animal radiation research platform (SARRP) for CBCT imaging and BLT-guided irradiation. Methods and tools were developed to evaluate the CCD response linearity, minimal detectable signal, focusing, spatial resolution, distortion, and uniformity. A transparent polycarbonate plate covering the middle of the mouse bed was used to support and image animals from underneath the bed. We investigated its effect on 2D Bioluminescence images and 3D BLT reconstruction accuracy, and studied its dosimetric impact along with the rest of mouse bed. A method based on pinhole camera model was developed to map multi-projection bioluminescence images to the object surface generated from CBCT image. The mapped bioluminescence images were used as the input data for the optical reconstruction. To account for free space light propagation from object surface to optical detector, a spectral derivative (SD) method was implemented for BLT reconstruction. We assessed the use of the SD data (ratio imaging of adjacent wavelength) in mitigating out of focusing and non-uniformity seen in the images. A mouse phantom was used to validate the data mapping. The phantom and an in vivo glioblastoma model were utilized to demonstrate the accuracy of the BLT target localization. RESULTS: The CCD response shows good linearity with < 0.6% residual from a linear fit. The minimal detectable level is 972 counts for 10 × 10 binning. The focal plane position is within the range of 13-18 mm above the mouse bed. The spatial resolution of 2D optical imaging is < 0.3 mm at Rayleigh criterion. Within the region of interest, the image uniformity is within 5% variation, and image shift due to distortion is within 0.3 mm. The transparent plate caused < 6% light attenuation. The use of the SD imaging data can effectively mitigate out of focusing, image non-uniformity, and the plate attenuation, to support accurate multi-spectral BLT reconstruction. There is < 0.5% attenuation on dose delivery caused by the bed. The accuracy of data mapping from the 2D bioluminescence images to CBCT image is within 0.7 mm. Our phantom test shows the BLT system can localize a bioluminescent target within 1 mm with an optimal threshold and only 0.2 mm deviation was observed for the case with and without a transparent plate. The same localization accuracy can be maintained for the in vivo GBM model. CONCLUSIONS: This work is the first systematic study in characterizing the commercial BLT-guided system. The information and methods developed will be useful for the community to utilize the imaging system for image-guided radiation research.

Ultra-sensitive single pixel bioluminescence tomography for in vivo cell tracking.

We presented a single-pixel bioluminescence tomography (SPBLT) to monitor single or few cells in live animal. Simulations are proposed to validate the capability of SPBLT in detecting weak bioluminescence signal emitted from cells in vivo.

Bioluminescence tomography system for in vivo irradiation guidance.

We constructed a bioluminescence tomography(BLT) to localize soft tissue targets for preclinical radiotherapy study. With the threshold and margin designed for target volume, BLT can provide opportunity to perform conformal irradiation to malignancy.

Quantitative Bioluminescence Tomography for In Vivo Volumetric-Guided Radiotherapy.

Several groups, including ours, have initiated efforts to develop small-animal irradiators that mimic radiation therapy (RT) for human treatment. The major image modality used to guide irradiation is cone-beam computed tomography (CBCT). While CBCT provides excellent guidance capability, it is less adept at localizing soft tissue targets growing in a low image contrast environment. In contrast, bioluminescence imaging (BLI) provides strong image contrast and thus is an attractive solution for soft tissue targeting. However, commonly used 2D BLI on an animal surface is inadequate to guide irradiation, because optical transport from an internal bioluminescent tumor is highly susceptible to the effects of optical path length and tissue absorption and scattering. Recognition of these limitations led us to integrate 3D bioluminescence tomography (BLT) with the small animal radiation research platform (SARRP). In this chapter, we introduce quantitative BLT (QBLT) with the advanced capabilities of quantifying tumor volume for irradiation guidance. The detail of system components, calibration protocol, and step-by-step procedure to conduct the QBLT-guided irradiation are described.

Deep-Tissue Activation of Photonanomedicines: An Update and Clinical Perspectives.

With the continued development of nanomaterials over the past two decades, specialized photonanomedicines (light-activable nanomedicines, PNMs) have evolved to become excitable by alternative energy sources that typically penetrate tissue deeper than visible light. These sources include electromagnetic radiation lying outside the visible near-infrared spectrum, high energy particles, and acoustic waves, amongst others. Various direct activation mechanisms have leveraged unique facets of specialized nanomaterials, such as upconversion, scintillation, and radiosensitization, as well as several others, in order to activate PNMs. Other indirect activation mechanisms have leveraged the effect of the interaction of deeply penetrating energy sources with tissue in order to activate proximal PNMs. These indirect mechanisms include sonoluminescence and Cerenkov radiation. Such direct and indirect deep-tissue activation has been explored extensively in the preclinical setting to facilitate deep-tissue anticancer photodynamic therapy (PDT); however, clinical translation of these approaches is yet to be explored. This review provides a summary of the state of the art in deep-tissue excitation of PNMs and explores the translatability of such excitation mechanisms towards their clinical adoption. A special emphasis is placed on how current clinical instrumentation can be repurposed to achieve deep-tissue PDT with the mechanisms discussed in this review, thereby further expediting the translation of these highly promising strategies.

Mobile bioluminescence tomography-guided system for pre-clinical radiotherapy research.

Due to low imaging contrast, a widely-used cone-beam computed tomography-guided small animal irradiator is less adept at localizing in vivo soft tissue targets. Bioluminescence tomography (BLT), which combines a model of light propagation through tissue with an optimization algorithm, can recover a spatially resolved tomographic volume for an internal bioluminescent source. We built a novel mobile BLT system for a small animal irradiator to localize soft tissue targets for radiation guidance. In this study, we elaborate its configuration and features that are indispensable for accurate image guidance. Phantom and in vivo validations show the BLT system can localize targets with accuracy within 1 mm. With the optimal choice of threshold and margin for target volume, BLT can provide a distinctive opportunity for investigators to perform conformal biology-guided irradiation to malignancy.

Fiducial-based image-guided SBRT for pancreatic adenocarcinoma: Does inter-and intra-fraction treatment variation warrant adaptive therapy?

PURPOSE: Variation in target positioning represents a challenge to set-up reproducibility and reliability of dose delivery with stereotactic body radiation therapy (SBRT) for pancreatic adenocarcinoma (PDAC). While on-board imaging for fiducial matching allows for daily shifts to optimize target positioning, the magnitude of the shift as a result of inter- and intra-fraction variation may directly impact target coverage and dose to organs-at-risk. Herein, we characterize the variation patterns for PDAC patients treated at a high-volume institution with SBRT. METHODS: We reviewed 30 consecutive patients who received SBRT using active breathing coordination (ABC). Patients were aligned to bone and then subsequently shifted to fiducials. Inter-fraction and intra-fraction scans were reviewed to quantify the mean and maximum shift along each axis, and the shift magnitude. A linear regression model was conducted to investigate the relationship between the inter- and intra-fraction shifts. RESULTS: The mean inter-fraction shift in the LR, AP, and SI axes was 3.1 ± 1.8 mm, 2.9 ± 1.7 mm, and 3.5 ± 2.2 mm, respectively, and the mean vector shift was 6.4 ± 2.3 mm. The mean intra-fraction shift in the LR, AP, and SI directions were 2.0 ± 0.9 mm, 2.0 ± 1.3 mm, and 2.3 ± 1.4 mm, respectively, and the mean vector shift was 4.3 ± 1.8 mm. A linear regression model showed a significant relationship between the inter- and intra-fraction shift in the AP and SI axis and the shift magnitude. CONCLUSIONS: Clinically significant inter- and intra-fraction variation occurs during treatment of PDAC with SBRT even with a comprehensive motion management strategy that utilizes ABC. Future studies to investigate how these variations could lead to variation in the dose to the target and OAR should be investigated. Strategies to mitigate the dosimetric impact, including real time imaging and adaptive therapy, in select cases should be considered.

Quantitative Bioluminescence Tomography-Guided Conformal Irradiation for Preclinical Radiation Research.

PURPOSE: Widely used cone beam computed tomography (CBCT)-guided irradiators in preclinical radiation research are limited to localize soft tissue target because of low imaging contrast. Knowledge of target volume is a fundamental need for radiation therapy (RT). Without such information to guide radiation, normal tissue can be overirradiated, introducing experimental uncertainties. This led us to develop high-contrast quantitative bioluminescence tomography (QBLT) for guidance. The use of a 3-dimensional bioluminescence signal, related to cell viability, for preclinical radiation research is one step toward biology-guided RT. METHODS AND MATERIALS: Our QBLT system enables multiprojection and multispectral bioluminescence imaging to maximize input data for the tomographic reconstruction. Accurate quantification of spectrum and dynamic change of in vivo signal were also accounted for the QBLT. A spectral-derivative method was implemented to eliminate the modeling of the light propagation from animal surface to detector. We demonstrated the QBLT capability of guiding conformal RT using a bioluminescent glioblastoma (GBM) model in vivo. A threshold was determined to delineate QBLT reconstructed gross target volume (GTVQBLT), which provides the best overlap between the GTVQBLT and CBCT contrast labeled GBM (GTV), used as the ground truth for GBM volume. To account for the uncertainty of GTVQBLT in target positioning and volume delineation, a margin was determined and added to the GTVQBLT to form a QBLT planning target volume (PTVQBLT) for guidance. RESULTS: The QBLT can reconstruct in vivo GBM with localization accuracy within 1 mm. A 0.5-mm margin was determined and added to GTVQBLT to form PTVQBLT, largely improving tumor coverage from 75.0% (0 mm margin) to 97.9% in average, while minimizing normal tissue toxicity. With the goal of prescribed dose 5 Gy covering 95% of PTVQBLT, QBLT-guided 7-field conformal RT can effectively irradiate 99.4 ± 1.0% of GTV. CONCLUSIONS: The QBLT provides a unique opportunity for investigators to use biologic information for target delineation, guiding conformal irradiation, and reducing normal tissue involvement, which is expected to increase reproducibility of scientific discovery.

Modeling scatter-to-primary dose ratio for megavoltage photon beams.

PURPOSE: A three-parameter semiempirical model for scatter-to-primary dose ratio (SPR) is proposed to fit PDD (or TPR) and S(p) beam data. The SPR formula proposed in this study is more accurate than the previously published formula utilizing two parameters, especially for lower energy megavoltage photon beams, because the effect of backscattered photons is now taken into account. METHODS: Monte Carlo (MC) calculated SPR for photon energy spectrum between 60Co and 24 MV are used to evaluate the accuracy of the models. Based on fitting the MC data, the dependence of the SPR parameters (a0, w0,d0) with the attenuation coefficients of the photon beams is obtained and they were incorporated into the authors' optimization routine. The ability of the optimization routine to fit measured clinic data is examined for photon energies ranging from 60Co to 25 MV for all major cobalt and linear accelerator manufacturers. RESULTS: The authors' model successfully fits the measured photon beam data for field size (E/3-40 cm), where E is photon energy in MV and for clinically usable depths, d(max) to 20 cm for 60Co, d(max) to 30 cm for 4 MV, and d(max) to 40 cm for 6 MV and higher photon energies. The maximum error among these fits is better than 2% for photon energies above 60Co. CONCLUSIONS: The new SPR formula, along with the optimization routine, can serve as an efficient tool for performing quality control of x-ray beam data that conforms to AAPM Radiation Therapy Committee TG40 and Therapy Physics Committee TG142 reports on beam data requirement.

Development of a Mobile Fluorescence Tomography-guided System for Pre-clinical Radiotherapy Research.

We proposed to build a mobile fluorescence tomography (mFT) system as an image-guided platform for pre-clinical radiotherapy research. The mFT system is expected to localize functional target/tumor, guide irradiation, and provide longitudinal treatment assessment.

In vivo bioluminescence tomography-guided radiation research platform for pancreatic cancer: an initial study using subcutaneous and orthotopic pancreatic tumor models.

Genetically engineered mouse model(GEMM) that develops pancreatic ductal adenocarcinoma(PDAC) offers an experimental system to advance our understanding of radiotherapy(RT) for pancreatic cancer. Cone beam CT(CBCT)-guided small animal radiation research platform(SARRP) has been developed to mimic the RT used for human. However, we recognized that CBCT is inadequate to localize the PDAC growing in low image contrast environment. We innovated bioluminescence tomography(BLT) to guide SARRP irradiation for in vivo PDAC. Before working on the complex PDAC-GEMM, we first validated our BLT target localization using subcutaneous and orthotopic pancreatic tumor models. Our BLT process involves the animal transport between the BLT system and SARRP. We inserted a titanium wire into the orthotopic tumor as the fiducial marker to track the tumor location and to validate the BLT reconstruction accuracy. Our data shows that with careful animal handling, minimum disturbance for target position was introduced during our BLT imaging procedure(<0.5mm). However, from longitudinal 2D bioluminescence image(BLI) study, the day-to-day location variation for an abdominal tumor can be significant. We also showed that the 2D BLI in single projection setting cannot accurately capture the abdominal tumor location. It renders that 3D BLT with multiple-projection is needed to quantify the tumor volume and location for precise radiation research. Our initial results show the BLT can retrieve the location at 2mm accuracy for both tumor models, and the tumor volume can be delineated within 25% accuracy. The study for the subcutaneous and orthotopic models will provide us valuable knowledge for BLT-guided PDAC-GEMM radiation research.

In Vivo Bioluminescence Tomography Center of Mass-Guided Conformal Irradiation.

PURPOSE: The cone-beam computed tomography (CBCT)-guided small animal radiation research platform (SARRP) has provided unique opportunities to test radiobiologic hypotheses. However, CBCT is less adept to localize soft tissue targets growing in a low imaging contrast environment. Three-dimensional bioluminescence tomography (BLT) provides strong image contrast and thus offers an attractive solution. We introduced a novel and efficient BLT-guided conformal radiation therapy and demonstrated it in an orthotopic glioblastoma (GBM) model. METHODS AND MATERIALS: A multispectral BLT system was integrated with SARRP for radiation therapy (RT) guidance. GBM growth curve was first established by contrast CBCT/magnetic resonance imaging (MRI) to derive equivalent sphere as approximated gross target volume (aGTV). For BLT, mice were subject to multispectral bioluminescence imaging, followed by SARRP CBCT imaging and optical reconstruction. The CBCT image was acquired to generate anatomic mesh for the reconstruction and RT planning. To ensure high accuracy of the BLT-reconstructed center of mass (CoM) for target localization, we optimized the optical absorption coefficients µa by minimizing the distance between the CoMs of BLT reconstruction and contrast CBCT/MRI-delineated GBM volume. The aGTV combined with the uncertainties of BLT CoM localization and target volume determination was used to generate estimated target volume (ETV). For conformal irradiation procedure, the GBM was first localized by the predetermined ETV centered at BLT-reconstructed CoM, followed by SARRP radiation. The irradiation accuracy was qualitatively confirmed by pathologic staining. RESULTS: Deviation between CoMs of BLT reconstruction and contrast CBCT/MRI-imaged GBM is approximately 1 mm. Our derived ETV centered at BLT-reconstructed CoM covers >95% of the tumor volume. Using the second-week GBM as an example, the ETV-based BLT-guided irradiation can cover 95.4% ± 4.7% tumor volume at prescribed dose. The pathologic staining demonstrated the BLT-guided irradiated area overlapped well with the GBM location. CONCLUSIONS: The BLT-guided RT enables 3-dimensional conformal radiation for important orthotopic tumor models, which provides investigators a new preclinical research capability.

Reconstruction of in-vivo optical properties for human prostate using interstitial diffuse optical tomography.

A CW interstitial diffuse optical tomography has been developed to characterize the in-vivo optical properties of prostate gland during photodynamic therapy. The spatial distributions of light fluence rate can be described by the diffusion equation. Optical properties of the prostate are reconstructed by solving the inverse problem with an adjoint method. The 3D reconstructed in-vivo optical properties for a human prostate is illustrated and compared with the results generated by a well-established point-by-point method. Moreover, the calculated fluence rate using the reconstructed optical properties matches the measured data.

Fluence rate-dependent intratumor heterogeneity in physiologic and cytotoxic responses to Photofrin photodynamic therapy.

Photodynamic therapy (PDT) can lead to the creation of heterogeneous, response-limiting hypoxia during illumination, which may be controlled in part through illumination fluence rate. In the present report we consider (1) regional differences in hypoxia, vascular response, and cell kill as a function of tumor depth and (2) the role of fluence rate as a mediator of depth-dependent regional intratumor heterogeneity. Intradermal RIF murine tumors were treated with Photofrin PDT using surface illumination at an irradiance of 75 or 38 mW cm(-2). Regional heterogeneity in tumor response was examined through comparison of effects in the surface vs. base of tumors, i.e. along a plane parallel to the skin surface and perpendicular to the incident illumination. 75 mW cm(-2) PDT created significantly greater hypoxia in tumor bases relative to their surfaces. Increased hypoxia in the tumor base could not be attributed to regional differences in Photofrin concentration nor effects of fluence rate distribution on photochemical oxygen consumption, but significant depth-dependent heterogeneity in vascular responses and cytotoxic response were detected. At a lower fluence rate of 38 mW cm(-2), no detectable regional differences in hypoxia or cytotoxic responses were apparent, and heterogeneity in vascular response was significantly less than that during 75 mW cm(-2) PDT. This research suggests that the benefits of low-fluence-rate PDT are mediated in part by a reduction in intratumor heterogeneity in hypoxic, vascular and cytotoxic responses.

Simulations of measured photobleaching kinetics in human basal cell carcinomas suggest blood flow reductions during ALA-PDT.

BACKGROUND AND OBJECTIVE: In a recently completed pilot clinical study at Roswell Park Cancer Institute, patients with superficial basal cell carcinoma (sBCC) received topical application of 20% 5-aminolevulinic acid (ALA) and were irradiated with 633 nm light at 10-150 mW cm(-2). Protoporphyrin IX (PpIX) photobleaching in the lesion and the adjacent perilesion normal margin was monitored by fluorescence spectroscopy. In most cases, the rate of bleaching slowed as treatment progressed, leaving a fraction of the PpIX unbleached despite sustained irradiation. To account for this feature, we hypothesized a decrease in blood flow during ALA-photodynamic therapy (PDT) that reduced the rate of oxygen transported to the tissue and therefore attenuated the photobleaching process. We have performed a detailed analysis of this hypothesis. STUDY DESIGN/MATERIALS AND METHODS: We used a comprehensive, previously published mathematical model to simulate the effects of therapy-induced blood flow reduction on the measured PpIX photobleaching. This mathematical model of PDT in vivo incorporates a singlet-oxygen-mediated photobleaching mechanism, dynamic unloading of oxygen from hemoglobin, and provides for blood flow velocity changes. It permits simulation of the in vivo photobleaching of PpIX in this patient population over the full range of irradiances and fluences. RESULTS: The results suggest that the physiological equivalent of discrete blood flow reductions is necessary to simulate successfully the features of the bleaching data over the entire treatment fluence regime. Furthermore, the magnitude of the blood flow changes in the normal tissue margin and lesion for a wide range of irradiances is consistent with a nitric-oxide-mediated mechanism of vasoconstriction. CONCLUSION: A detailed numerical study using a comprehensive PDT dosimetry model is consistent with the hypothesis that the observed trends in the in vivo PpIX photobleaching data from patients may be explained on the basis of therapy-induced blood flow reductions at specific fluences.

Photodynamic dose does not correlate with long-term tumor response to mTHPC-PDT performed at several drug-light intervals.

Meso-tetra-hydroxyphenyl-chlorin (mTHPC, Foscan), a promising photosensitizer for photodynamic therapy (PDT), is approved in Europe for the palliative treatment of head and neck cancer. Based on work in mice that investigated optimal tumor accumulation, clinical protocols with Foscan typically employ an interval of 96 h between systemic sensitizer administration and irradiation. However, recent studies in mouse tumor models have demonstrated significantly improved long-term tumor response when irradiation is performed at shorter drug-light intervals of 3 and 6 h. Using a previously published theoretical model of microscopic PDT dosimetry and informed by experimentally determined photophysical properties and intratumor sensitizer concentrations and distributions, we calculated photodynamic dose depositions following mTHPC-PDT for drug-light intervals of 3, 6, 24, and 96 h. Our results demonstrate that the singlet oxygen dose to the tumor volume does not track even qualitatively with tumor responses for these four drug-light intervals. Further, microscopic analysis of simulated singlet oxygen deposition shows that in no case do any subpopulations of tumor cells receive a threshold dose. Indeed, under the conditions of these simulations more than 90% of the tumor volume receives a dose that is approximately 20-fold lower than the threshold dose for mTHPC. Thus, in this evaluation of mTHPC-PDT at various drug-light intervals, any PDT dose metric that is proportional to singlet oxygen creation and/or deposition would fail to predict the tumor response. In situations like this one, other reporters of biological response to therapy would be necessary.