A simple new alternative to the linear-quadratic model (and where the LQ model comes from)

There is still controversy over whether the oxygen enhancement ratio (OER) varies as a function of dose and cell cycle phase. In the present study, the OER has been measured as a function of survival level and cell cycle phase using volume flow cell sorting. This method allows both the separation of cells in different stages of the cycle from an asynchronously growing population, and the precise plating of cells for accurate measurements at high survival levels. We have developed a cell suspension gassing and sampling system which maintained an oxygen tension less than 20 ppm throughout a series of sequential radiation doses. For both radiation-resistant cells (CHO-K1) and a radiation-sensitive clone (CHO-xrs6), we could separate relatively pure populations of G1-phase, G1/S-boundary, S-, and G2-phase cells. Each cell line showed a typical age response, with cells at the G1/S-phase boundary being 4 (CHO-K1) to 12 (CHO-xrs6) times more sensitive than cells in the late S phase. For both cell lines, G1-phase cells had an OER of 2.3-2.4, compared to an OER of 2.8-2.9 for S-phase and 2.6-2.7 for G2-phase cells. None of these age fractions showed a dependence of OER on survival level. Asynchronously growing cells or cells at the G1/S-phase boundary had an OER similar to that of G1-phase cells at high survival levels, but the OER increased with decreasing survival level to a value near that of S-phase cells. These results suggest that the decrease in OER at high survival levels for asynchronous cells may be due to differences in the OERs of the inherent cell age subpopulations. For cells in one cell cycle stage, oxygen appears to have a purely dose-modifying effect.

Artemisinin has been shown to inhibit the growth of some human cancer cells. In this study, we investigated the radiosensitizing effects of artemisinin on cervical cancer cells and normal human fibroblast cells and also assessed some possible mechanisms for these effects. Two cervical cancer cell lines, HeLa and SiHa cells, and GM0639 normal human fibroblast cell line were treated with various concentrations of artemisinin plus radiation; the cell viability was tested using both 3-(4,5-dimethylthiazolyl-2-y1)-2, 5-diphenyltetrazolium bromide and clonogenic assays. Radiation dose-modifying factors were measured by clonogenic survival assay. Annexin V/propidium iodide assay for the evaluation of apoptosis and cell cycle phase were determined by flow cytometry, and the expression of the cell cycle-associated proteins Wee 1 and cyclin B1 were analyzed by Western blot analysis. Artemisinin showed higher cytotoxicity in cervical cancer cell lines, especially in SiHa cells, than in the normal cell line. In both clonogenic assay and apoptosis, artemisinin sensitized the HeLa cancer cells to the cytotoxicity of radiation, yielding a dose-modifying factor of 1.24, but not SiHa cancer cells and GM normal cells. At a dose of 110 nmol/L, artemisinin did not change the distribution of cell cycle in 3 tested cell lines, but artemisinin abrogated the radiation-induced G2 blockade. Analyses of G2-checkpoint-related proteins, the activation of Wee 1 and depression of cyclin B1 expression induced by radiation, could be restored to the control level by artemisinin. Given the unique cytotoxic profile of artemisinin on cancer cells and normal cells, artemisinin may be a potentially promising radiosensitizer through the regulation of the expression of G2 checkpoint-related proteins like Wee 1 and cyclin B1, and improve therapeutic ratios for the combination of artemisinin and ionizing irradiation in the treatment of patients with cervical cancer.

Effects of Keratinocyte Growth Factor on the Proliferation and Radiation Survival of Human Squamous Cell Carcinoma Cell Lines In Vitro and In Vivo

Posttransplant hematopoiesis in patients undergoing sibling allogeneic stem cell transplantation reflects that of their respective donors although with a lower functional capability

Modeling the tumor behavior in the host organ as function of time and radiation dose has been a major study in the previous decades. Here the effort in estimation of cancerous and normal cell proliferation and growth in glioblastoma multiform (GBM) tumor is presented. This paper introduces a new mathematical model in the form of differential equation of tumor growth. The model contains dose delivery amount in the treatment scheme as an input term. It also can be utilized to optimize the treatment process in order to increase the patient survival period. Gene expression programming (GEP) as a new concept is used for estimating this model. The LQ model has also been applied to GEP as an initial value, causing acceleration and improvement of the algorithm estimation. The model shows the number of the tumor and normal brain cells during the treatment process using the status of normal and cancerous cells in the initiation of treatment, the timing and amount of dose delivery to the patient, and a coefficient that describes the brain condition. A critical level is defined for normal cell when the patient's death occurs. In the end the model has been verified by clinical data obtained from previous accepted formulae and some of our experimental resources. The proposed model helps to predict tumor growth during treatment process in which further treatment processes can be controlled.

Hypoxia is one of the most important factors influencing clinical outcome after radiotherapy. Improved knowledge of factors affecting the levels and distribution of oxygen within a tumor is needed. The authors constructed a theoretical 3D model based on histological images to analyze the influence of vessel density and hemoglobin (Hb) concentration on the response to irradiation. The pancreases of a Rip-Tag2 mouse, a model of malignant insulinoma, were excised, cryosectioned, immunostained, and photographed. Vessels were identified by image thresholding and a 3D vessel matrix assembled. The matrix was reduced to functional vessel segments and enlarged by replication. The steady-state oxygen tension field of the tumor was calculated by iteratively employing Green's function method for diffusion and the Michaelis-Menten model for consumption. The impact of vessel density on the radiation response was studied by removing a number of randomly selected vessels. The impact of Hb concentration was studied by independently changing vessel oxygen partial pressure (pO(2)). For each oxygen distribution, the oxygen enhancement ratio (OER) was calculated and the mean absorbed dose at which the tumor control probability (TCP) was 0.99 (D(99)) was determined using the linear-quadratic cell survival model (LQ model). Decreased pO(2) shifted the oxygen distribution to lower values, whereas decreased vessel density caused the distribution to widen and shift to lower values. Combined scenarios caused lower-shifted distributions, emphasising log-normal characteristics. Vessel reduction combined with increased blood pO(2) caused the distribution to widen due to a lack of vessels. The most pronounced radiation effect of increased pO(2) occurred with tumor tissue with 50% of the maximum vessel density used in the simulations. A 51% decrease in D(99), from 123 to 60 Gy, was found between the lowest and highest pO(2) concentrations. Our results indicate that an intermediate vascular density region exists where enhanced blood oxygen concentration may be beneficial for radiation response. The results also suggest that it is possible to distinguish between diffusion-limited and anemic hypoxia from the characteristics of the pO(2) distribution.

Objective.Nowadays the linear-quadratic model (LQ) is the most used model to estimate the biological effective dose (BED) and the equivalent dose in 2 Gy fractions (EQD2) for different fractionation regimens. Nevertheless, it is debated of applicability to use LQ model for hypofractionation. The objective of this study is to evaluate the LQ model in comparison with other radiobiological models concerning the adequacy of biological equivalent dose in 2 Gy fractions assessment across various hypofractionation regimens.Methods.The study was conducted for two cases: the prostate gland in the pelvic region and squamous cell carcinoma (SCC) in the head and neck region. Five radiobiological models including the LQ model, modified linear-quadratic (MLQ), linear-quadratic-linear (LQL), universal survival curve (USC), and Pade linear-quadratic (PLQ) models were compared for tumor control probability (TCP) andEQD2predictions. Published clinical outcomes (including local control, disease-free survival, and overall survival rates) were analyzed to identify clinically equivalent fractionation regimens. The radiobiological models were then evaluated by comparing calculatedEQD2andTCPvalues with clinical data for these equivalent regimens.Results:Modified radiobiological models showed that the LQ model overestimates the dose in hypofractionation. The dose limit at which the LQ model is applicable depends on the localization and type of tumor: for the prostate gland the value was 4.3 Gy, for the head and neck region 8.5 Gy.Conclusions:The applicability of the LQ model in hypofractionation depends on the tumorα/βvalue: the LQ model more sensitive to locations with lowα/βvalues and, conversely, less sensitive to locations with highα/βvalues. Among the alternatives, the MLQ model is recognized as the most practical alternative, combining a small number of parameters with resistance to variations. While modified models show efficacy, further clinical validation is needed to balance tumor control with normal tissue toxicity risks.

IntroductionTumor hypoxia is associated with poor treatment outcome. Hypoxic regions are more radioresistant than well‐oxygenated regions, as quantified by the oxygen enhancement ratio (OER). In optimization of proton therapy, including OER in addition to the relative biological effectiveness (RBE) could therefore be used to adapt to patient‐specific radioresistance governed by intrinsic radiosensitivity and hypoxia.MethodsA combined RBE and OER weighted dose (ROWD) calculation method was implemented in a FLUKA Monte Carlo (MC) based treatment planning tool. The method is based on the linear quadratic model, with α and β parameters as a function of the OER, and therefore a function of the linear energy transfer (LET) and partial oxygen pressure (pO2). Proton therapy plans for two head and neck cancer (HNC) patients were optimized with pO2 estimated from [18F]‐EF5 positron emission tomography (PET) images. For the ROWD calculations, an RBE of 1.1 (RBE1.1,OER) and two variable RBE models, Rørvik (ROR) and McNamara (MCN), were used, alongside a reference plan without incorporation of OER (RBE1.1).ResultsFor the HNC patients, treatment plans in line with the prescription dose and with acceptable target ROWD could be generated with the established tool. The physical dose was the main factor modulated in the ROWD. The impact of incorporating OER during optimization of HNC patients was demonstrated by the substantial difference found between ROWD and physical dose in the hypoxic tumor region. The largest physical dose differences between the ROWD optimized plans and the reference plan was 12.2 Gy.ConclusionThe FLUKA MC based tool was able to optimize proton treatment plans taking the tumor pO2 distribution from hypoxia PET images into account. Independent of RBE‐model, both elevated LET and physical dose were found in the hypoxic regions, which shows the potential to increase the tumor control compared to a conventional optimization approach.

Comparison of Photon Versus Carbon Irradiation in Pancreatic Cancer

Curcumin (diferuloylmethane) is known to induce apoptosis in tumor cells. In asynchronous cultures, with time-lapse video-micrography in combination with quantitative fluorescence microscopy, we have demonstrated that curcumin induces apoptosis at G(2) phase of cell cycle in deregulated cyclin D1-expressed mammary epithelial carcinoma cells, leaving its normal counterpart unaffected. In our search toward delineating the molecular mechanisms behind such differential activities of curcumin, we found that it selectively increases p53 expression at G(2) phase of carcinoma cells and releases cytochrome c from mitochondria, which is an essential requirement for apoptosis. Further experiments using p53-null as well as dominant-negative and wild-type p53-transfected cells have established that curcumin induces apoptosis in carcinoma cells via a p53-dependent pathway. On the other hand, curcumin reversibly inhibits normal mammary epithelial cell cycle progression by down-regulating cyclin D1 expression and blocking its association with Cdk4/Cdk6 as well as by inhibiting phosphorylation and inactivation of retinoblastoma protein. In addition, curcumin significantly up-regulates cell cycle inhibitory protein (p21Waf-1) in normal cells and arrests them in G(0) phase of cell cycle. Therefore, these cells escape from curcumin-induced apoptosis at G(2) phase. Interestingly, these processes remain unaffected by curcumin in carcinoma cells where cyclin D1 expression is high. Similarly, in ectopically overexpressed system, curcumin cannot down-regulate cyclin D1 and thus block cell cycle progression. Hence, these cells progress into G(2) phase and undergo apoptosis. These observations together suggest that curcumin may have a possible therapeutic potential in breast cancer patients.

The treatment outcome of a given fractionated radiotherapy scheme is affected by oxygen tension and cell cycle kinetics of the tumor population. Numerous experimental studies have supported the variability of radiosensitivity with cell cycle phase. Oxygen modulates the radiosensitivity through hypoxia-inducible factor (HIF) stabilization and oxygen fixation hypothesis (OFH) mechanism. In this study, an existing mathematical model describing cell cycle kinetics was modified to include the oxygen-dependent G1/S transition rate and radiation inactivation rate. The radiation inactivation rate used was derived from the linear-quadratic (LQ) model with dependence on oxygen enhancement ratio (OER), while the oxygen-dependent correction for the G1/S phase transition was obtained from numerically solving the ODE system of cyclin D-HIF dynamics at different oxygen tensions. The corresponding cell cycle phase fractions of aerated MCF-7 tumor population, and the resulting growth curve obtained from numerically solving the developed mathematical model were found to be comparable to experimental data. Two breast radiotherapy fractionation schemes were investigated using the mathematical model. Results show that hypoxia causes the tumor to be more predominated by the tumor subpopulation in the G1 phase and decrease the fractional contribution of the more radioresistant tumor cells in the S phase. However, the advantage provided by hypoxia in terms of cell cycle phase distribution is largely offset by the radioresistance developed through OFH. The delayed proliferation caused by severe hypoxia slightly improves the radiotherapy efficacy compared to that with mild hypoxia for a high overall treatment duration as demonstrated in the 40-Gy fractionation scheme.

BackgroundThe increased resistance of hypoxic cells to ionizing radiation is usually believed to be the primary reason for treatment failure in tumors with oxygen-deficient areas. This oxygen effect can be expressed quantitatively by the oxygen enhancement ratio (OER). Here we investigate theoretically the dependence of the OER on the applied local dose for different types of ionizing irradiation and discuss its importance for clinical applications in radiotherapy for two scenarios: small dose variations during hypoxia-based dose painting and larger dose changes introduced by altered fractionation schemes.MethodsUsing the widespread Alper-Howard-Flanders and standard linear-quadratic (LQ) models, OER calculations are performed for T1 human kidney and V79 Chinese hamster cells for various dose levels and various hypoxic oxygen partial pressures (pO2) between 0.01 and 20 mmHg as present in clinical situations in vivo. Our work comprises the analysis for both low linear energy transfer (LET) treatment with photons or protons and high-LET treatment with heavy ions. A detailed analysis of experimental data from the literature with respect to the dose dependence of the oxygen effect is performed, revealing controversial opinions whether the OER increases, decreases or stays constant with dose.ResultsThe behavior of the OER with dose per fraction depends primarily on the ratios of the LQ parameters alpha and beta under hypoxic and aerobic conditions, which themselves depend on LET, pO2 and the cell or tissue type. According to our calculations, the OER variations with dose in vivo for low-LET treatments are moderate, with changes in the OER up to 11% for dose painting (1 or 3 Gy per fraction compared to 2 Gy) and up to 22% in hyper-/hypofractionation (0.5 or 20 Gy per fraction compared to 2 Gy) for oxygen tensions between 0.2 and 20 mmHg typically measured clinically in hypoxic tumors. For extremely hypoxic cells (0.01 mmHg), the dose dependence of the OER becomes more pronounced (up to 36%). For high LET, OER variations up to 4% for the whole range of oxygen tensions between 0.01 and 20 mmHg were found, which were much smaller than for low LET.ConclusionsThe formalism presented in this paper can be used for various tissue and radiation types to estimate OER variations with dose and help to decide in clinical practice whether some dose changes in dose painting or in fractionation can bring more benefit in terms of the OER in the treatment of a specific hypoxic tumor.

The presence of oxygen in tumours has substantial impact on treatment outcome; relative to anoxic regions, well-oxygenated cells respond better to radiotherapy by a factor 2.5–3. This increased radio-response is known as the oxygen enhancement ratio. The oxygen effect is most commonly explained by the oxygen fixation hypothesis, which postulates that radical-induced DNA damage can be permanently ‘fixed’ by molecular oxygen, rendering DNA damage irreparable. While this oxygen effect is important in both existing therapy and for future modalities such a radiation dose-painting, the majority of existing mathematical models for oxygen enhancement are empirical rather than based on the underlying physics and radiochemistry. Here we propose a model of oxygen-enhanced damage from physical first principles, investigating factors that might influence the cell kill. This is fitted to a range of experimental oxygen curves from literature and shown to describe them well, yielding a single robust term for oxygen interaction obtained. The model also reveals a small thermal dependency exists but that this is unlikely to be exploitable.

In this study, the relationships between cellular oxygen enhancement ratios (OER) and nucleotide excision repair capability were examined using the UV20 mutant cell line (which has a defective ERCC1 gene). Using a clonogenic survival assay, the OER for the killing of wild-type AA8 cells was 3.2 +/- 0.1, whereas that for UV20 cells was only 2.35 +/- 0.05; the decreased OER of UV20 cells was the result of their significantly greater radiosensitivity relative to wild-type cells under hypoxic conditions. In AA8 cells, hypoxia protected against DNA double-strand break (dsb) induction (determined by pulsed-field gel electrophoresis) by a factor 3.5 +/- 0.3; i.e. to a similar extent that it modulated cell killing. However, this correlation was not apparent in UV20 cells, where hypoxia protected against dsb induction to a similar extent as in wild-type cells (approximately 3.2-fold). Stably transfected UV20 cells over-expressing a full-length ERCC1 cDNA clone displayed a normal OER (3.5 +/- 0.1) in addition to wild-type resistance to UV light. Our data suggest that the hypoxic radiosensitivity of UV20 cells is a direct result of their ERCC1 deficiency and reflects their inability to process some type of DNA damage (not dsbs) that is induced preferentially in hypoxic cells.

We have investigated the influence of hypoxia on the radiosensitivity of 4 early-passage tumor cell lines that were established from malignant glioma patients at our Institute. These cell lines were M006, M059J (a highly radiosensitive line), M059K (a radioresistant line derived from the same biopsy as M059J), and M010b. The GM637 human fibroblast cell line was used as a normal control. The oxygen enhancement ratios (OERs) for these cell lines, determined using a clonogenic survival assay, were approximately 3.6 (GM637), approximately 3.7 (M006), approximately 2.5 (M010b), approximately 2.1 (M059K), and approximately 3.5 (M059J). The broad range of OERs for these glioma lines was not related to cellular glutathione levels or to differences in intrinsic cellular radiosensitivity. Because studies with rodent cell lines indicate that defects in certain DNA repair genes, including ERCC1, can greatly influence cellular OERs, and because several such repair genes, including ERCC1, localize to a region of chromosome 19q that is close to a common deletion in human glioma, we reasoned that such deletions might contribute to the diverse OERs of these tumor cell lines. However, measurements of ERCC1 protein levels using immunofluorescence staining or Western blotting, of ERCC1 mRNA levels using Northern blotting, and of functional nucleotide excision repair capability using the UV/adenovirus reactivation assay, failed to indicate any deficit in these activities. Thus, although the effect of hypoxia on the radiosensitivity of different human glioma cell lines can vary widely, the mechanism of this effect remains unknown. The potential implications of this finding for radiation therapy, and especially for hypoxia imaging-guided intensity-modulated radiation therapy (IMRT) treatment planning, are discussed.