Intracapillary HbO2 saturation in tumor tissue of DS-carcinosarcoma during normoxia

Role of Hypoxia in Photodynamic Therapy of Tumors

The photodynamic therapy of tumors is based on a photosensitization reaction that produces oxygen-derived cytotoxic species. The availability of oxygen is therefore a necessary condition to obtain the desired effect. However, most tumors develop regions that have outgrown their vascular supply, and therefore present severe hypoxia. In many hypoxic, yet viable areas, oxygen partial pressures almost two orders of magnitude lower that in normal tissues have been measured by other authors. It is here suggested that hypoxic cells are resistant to the therapy and hence are a source of postirradiation recurrence of the tumors. Methods are reviewed and discussed that can be used to: (a) improve the tumor oxygenation status prior to, or during irradiation; (b) destroy hypoxic cells; and, (c) allow the reoxygenation of the tumor by using fractionated irradiation protocols which increase tumor photosensitivity. Hyperthermia, a therapy to which hypoxic cells are particularly sensitive, is discussed. Cellular and vascular parameters that should be considered when discussing the synergism between hyperthermia and photodynamic therapy are listed. The new research field of hypoxia mapping by nondestructive, noninvasive, imaging techniques is briefly discussed.

Validation of Perfusion Quantification with 3D Gradient Echo Dynamic Contrast-Enhanced Magnetic Resonance Imaging Using a Blood Pool Contrast Agent in Skeletal Swine Muscle

The purpose of our study was to validate perfusion quantification in a low-perfused tissue by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with shared k-space sampling using a blood pool contrast agent. Perfusion measurements were performed in a total of seven female pigs. An ultrasonic Doppler probe was attached to the right femoral artery to determine total flow in the hind leg musculature. The femoral artery was catheterized for continuous local administration of adenosine to increase blood flow up to four times the baseline level. Three different stable perfusion levels were induced. The MR protocol included a 3D gradient-echo sequence with a temporal resolution of approximately 1.5 seconds. Before each dynamic sequence, static MR images were acquired with flip angles of 5°, 10°, 20°, and 30°. Both static and dynamic images were used to generate relaxation rate and baseline magnetization maps with a flip angle method. 0.1 mL/kg body weight of blood pool contrast medium was injected via a central venous catheter at a flow rate of 5 mL/s. The right hind leg was segmented in 3D into medial, cranial, lateral, and pelvic thigh muscles, lower leg, bones, skin, and fat. The arterial input function (AIF) was measured in the aorta. Perfusion of the different anatomic regions was calculated using a one- and a two-compartment model with delay- and dispersion-corrected AIFs. The F-test for model comparison was used to decide whether to use the results of the one- or two-compartment model fit. Total flow was calculated by integrating volume-weighted perfusion values over the whole measured region. The resulting values of delay, dispersion, blood volume, mean transit time, and flow were all in physiologically and physically reasonable ranges. In 107 of 160 ROIs, the blood signal was separated, using a two-compartment model, into a capillary and an arteriolar signal contribution, decided by the F-test. Overall flow in hind leg muscles, as measured by the ultrasound probe, highly correlated with total flow determined by MRI, R = 0.89 and P = 10⁻⁷. Linear regression yielded a slope of 1.2 and a y-axis intercept of 259 mL/min. The mean total volume of the investigated muscle tissue corresponds to an offset perfusion of 4.7mL/(min ⋅ 100cm³). The DCE-MRI technique presented here uses a blood pool contrast medium in combination with a two-compartment tracer kinetic model and allows absolute quantification of low-perfused non-cerebral organs such as muscles.

Tumor oxygenation: an appraisal of past and present concepts and a look into the future : Arisztid G. B. Kovách Lecture

Since 1970, the multifactorial pathogenesis of the deficient and heterogeneous oxygenation of transplanted murine tumors and of human cancers (including parameters determining oxygen delivery, e.g., blood flow, diffusion geometry, oxygen transport capacity of the blood) has been investigated in vivo. Hypoxia and/or anoxia was quantitatively assessed and characterized using microtechniques and special preclinical tumor models. Hypoxia subtypes were identified, and critical supply conditions were theoretically analyzed. In the 1980s, first experiments on humans were carried out in cancers of the rectum and of the oral cavity. In the 1990s, the clinical investigations were carried out on cancers of the breast and of the uterine cervix, clearly showing that hypoxia is a hallmark of locally advanced human tumors. In multivariate analysis, hypoxia was found to be an independent, adverse prognostic factor for patient survival due to hypoxia-driven malignant progression and hypoxia-associated resistance to anticancer therapy.

Intratumour heterogeneity in microvessel oxyhaemoglobin saturations

The aim of the present study was to examine intratumour heterogeneity in microvessel oxyhaemoglobin (HbO2) saturations in human melanoma xenografts. The HbO2 saturations, measured with a cryospectrophotometric micromethod, were found to vary substantially within single tumours. All tumours showed a decrease in overall HbO2 saturation from the periphery towards the centre, although the profiles could vary substantially between individual tumours. Local differences could be large, and many tumours had HbO2 saturations spanning from 0 to 100% in peripheral regions. In central tumour regions, low saturations were prevailing although subregions of intermediate and high saturations could also be found.

CCD imaging in cryospectrophotometric determination of microvascular oxyhemoglobin saturations

A microspectrophotometric imaging method has been developed for localized measurements of intravascular oxyhemoglobin (HbO2) saturations in microvessels from sections of quick-frozen tissue. HbO2 saturation was calculated from the absorption spectrum of red blood cells measured at five selected wavelengths in the 520- to 570-nm range. We combined the use of narrow-bandwidth interference filters and a CCD camera mounted on a microscope to obtain one gray image of the sample at each wavelength. Each pixel is a quantitative measure of transmitted light intensity from the tissue sample at that location. A linear calibration curve for blood frozen in vitro (humans and mice) and in vivo (mice) was obtained using a multicomponent analysis. Oxy- and deoxyhemoglobin were assumed to be the only hemoglobin components present. A constant term compensates for light loss due to scattering on red blood cells and ice crystals. The standard error in single measurements of HbO2 saturation was 5%. The present method allows off-line analysis of the HbO2 saturation distribution within a microvessel network and offers new possibilities for comparative morphological studies.

Intracapillary HbO2 saturations in murine tumours and human tumour xenografts measured by cryospectrophotometry: relationship to tumour volume, tumour pH and fraction of radiobiologically hypoxic cells

Frequency distributions for intracapillary HbO2 saturation were determined for two murine tumour lines (KHT, RIF-1) and two human ovarian carcinoma xenograft lines (MLS, OWI) using a cryospectrophotometric method. The aim was to search for possible relationships between HbO2 saturation status and tumour volume, tumour pH and fraction of radiobiologically hypoxic cells. Tumour pH was measured by 31P NMR spectroscopy. Hypoxic fractions were determined from cell survival curves for tumours irradiated in vivo and assayed in vitro. Tumours in the volume range 100-4000 mm3 were studied and the majority of the vessels were found to have HbO2 saturations below 10%. The volume-dependence of the HbO2 frequency distributions differed significantly among the four tumour lines; HbO2 saturation status decreased with increasing tumour volume for the KHT, RIF-1 and MLS lines and was independent of tumour volume for the OWI line. The data indicated that the rate of decrease in HbO2 saturation status during tumour growth was related to the rate of development of necrosis. The volume-dependence of tumour pH was very similar to that of the HbO2 saturation status for all tumour lines. Significant correlations were therefore found between HbO2 saturation status and tumour pH, both within tumour lines and across the four tumour lines, reflecting that the volume-dependence of both parameters probably was a compulsory consequence of reduced oxygen supply conditions during tumour growth. Hypoxic fraction increased during tumour growth for the KHT, RIF-1 and MLS lines and was volume-independent for the OWI line, suggesting a relationship between HbO2 saturation status and hypoxic fraction within tumour lines. However, there was no correlation between these two parameters across the four tumour lines, indicating that the hypoxic fraction of a tumour is not determined only by the oxygen supply conditions; other parameters may also be important, e.g. oxygen diffusivity, rate of oxygen consumption and cell survival time under hypoxic stress.

The detection and measurement of hypoxic cells in solid tumors

The presence of viable hypoxic cells in human cancers has concerned oncologists for years. Cells in tissues that are deficient in oxygen are relatively resistant to radiation inactivation and may not be accessible to some systemic chemotherapy. The premise that hypoxic tumor cells do, indeed, control the radiocurability of some cancers is supported by some clinical evidence. The presence of hypoxic regions within tumors can be directly and indirectly inferred from invasive procedures such as oxygen electrode techniques and histologic study, respectively, but such information does not significantly contribute to current prescriptions given by oncologists for tumor treatment. Novel procedures (some of which are noninvasive) for detecting hypoxic regions within solid tumors have been proposed and are based upon two recent developments: (1) the discovery that some radiosensitizing drugs become selectively bound by metabolism to the molecules of viable hypoxic cells, and (2) the growing availability of new imaging procedures based upon positron-emission tomography, single-photon emission tomography, and nuclear magnetic resonance spectroscopy. Preliminary research results from these novel procedures are reviewed, and the potential clinical impact of each is discussed.

Heterogeneous oxygenation of rectal carcinomas in humans: a critical parameter for preoperative irradiation?

Tissue oxygenation was measured in 10 patients with differentiated adenocarcinoma in a very localized region in the middle part of the rectum (grade I - II, clinical stage II) by means of a cryophotometric micromethod. The results obtained clearly show that the oxygenation of differentiated rectal adenocarcinoma is distinctly lower than that of the normal rectal mucosa; tissue hypoxia or even anoxia are a common feature in those tumors; There exist considerable inter- individual differences among tumors of the same clinical staging and histological grading; substantial intra- individual heterogeneities in the oxygenation are evident within the same tumor and even within neighbouring microareas of the tissue. These findings imply that the commonly used classifications do not allow any conclusions concerning the oxygenation status, and probably the radiosensitivity of a tumor, respectively.

Tumour oxygenation under normobaric and hyperbaric conditions

Tumour oxygenation during exposure to normobaric and hyperbaric oxygen was assessed by means of a cryophotometric micromethod and a specially constructed pressure chamber. The tumours investigated were DS-carcinosarcomas implanted subcutaneously into the thigh of Sprague-Dawley rats. The animals were anaesthetised and put on a heating pad. Mean arterial blood pressure was monitored throughout the entire time of pressurisation. Cryobiopsies were removed from the tumours after exposure to O2 for 30 min at 1 bar (atmospheric pressure), 2, 3, or 4 bar. Oxyhaemoglobin saturation (HbO2) values of single red blood cells in tumour microvessels were determined photometrically in the frozen tissue samples as a quantitative measure for tumour oxygenation. Frequency distribution curves of HbO2 showed that there was a marked improvement of the O2 supply to tumour tissue in O2 atmospheres at 1 bar compared with exposure to air at 1 bar. Pressurisation in O2 atmospheres up to 2 and 3 bar did not substantially alter the distribution curve of the HbO2 values in comparison with the data sampled at 1 bar O2 exposure, yet led to a significant drop in the occurrence of HbO2 values below 5% saturation. Thus, pressurisation particularly raises those tissue O2 partial pressures (pO2) that are in a range where the radiosensitivity is critically influenced by the pO2. Pressurisation up to 4 bar caused a shift of the HbO2 distribution curve to values significantly higher than those from tumours exposed to O2 at 1 bar, with no values below 35% saturation. Using these data and a previously published model for computing tissue pO2 values it can be shown that radiobiological hypoxia is unlikely to occur in DS-carcinosarcomas exposed to an O2 atmosphere at 4 bar under the conditions chosen. Hyperbaric oxygenation is recommended as an efficient adjuvant of X irradiation, particularly in superficial tumours.

Oxygenation of malignant tumors after localized microwave hyperthermia

The oxyhemoglobin saturation (HbO2) of single red blood cells within tumor microvessels (diameter: 3-12 micrometers) of DS-Carcinosarcoma was studied using a cryophotometric micromethod. In untreated control tumors (mean tissue temperature approx. 35 degrees C) the measured values scattered over the whole saturation range from zero to 100 sat. %, the mean being 51 sat. %. Upon heating at 40 degrees C for 30 min, the oxygenation of the tumor tissue significantly improved as compared with control conditions. After 40 degrees C-hyperthermia a mean oxyhemoglobin saturation of 66 sat. % was obtained. In contradistinction to this, after 43 degrees C-hyperthermia the tumor oxygenation was significantly lower and reached a mean HbO2 saturation value of 47 sat. %. A further temperature rise to 45 degrees C caused the oxygenation to drop drastically (mean oxyhemoglobin saturation value: 24 sat. %). This is due to a severe restriction of nutritive blood flow. The changes in tumor oxygenation after hyperthermia seem to be predominantly mediated through changes in tumor blood flow, including tumor microcirculation, which showed a similar temperature dependence. Metabolic effects probably play a minor role in the oxyhemoglobin saturation distribution within tumor microvessels.