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BACKGROUND: Multiple rounds of head computed tomography (CT) scans increase the risk of radiation-induced lens opacification. PURPOSE: To investigate the effects of CT eye shielding and topogram-based tube current modulation (TCM) on the radiation dose received by the lens and the image quality of nasal and periorbital imaging. MATERIAL AND METHODS: An anthropomorphic phantom was CT-scanned using either automatic tube current modulation or a fixed tube current. The lens radiation dose was estimated using cropped Gafchromic films irradiated with or without a shield over the orbit. Image quality, assessed using regions of interest drawn on the bilateral extraorbital areas and the nasal bone with a water-based marker, was evaluated using both a signal-to-noise ratio (SNR) and contrast-noise ratio (CNR). Two CT specialists independently assessed image artifacts using a three-point Likert scale. RESULTS: The estimated radiation dose received by the lens was significantly lower when barium sulfate or bismuth-antimony shields were used in conjunction with a fixed tube current (22.0% and 35.6% reduction, respectively). Topogram-based TCM mitigated the beam hardening-associated artifacts of bismuth-antimony and barium sulfate shields. This increased the SNR by 21.6% in the extraorbital region and the CNR by 7.2% between the nasal bones and extraorbital regions. The combination of topogram-based TCM and barium sulfate or bismuth-antimony shields reduced lens doses by 12.2% and 27.2%, respectively. CONCLUSION: Image artifacts induced by the bismuth-antimony shield at a fixed tube current for lenticular radioprotection were significantly reduced by topogram-based TCM, which increased the SNR of the anthropomorphic nasal bones and periorbital tissues.
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Encéfalo/diagnóstico por imagem , Processamento de Imagem Assistida por Computador/métodos , Cristalino , Proteção Radiológica/métodos , Tomografia Computadorizada por Raios X/métodos , Cabeça/diagnóstico por imagem , Neuroimagem/métodos , Imagens de Fantasmas , Doses de RadiaçãoRESUMO
Glioblastoma multiforme (GBM) is the most common and highly malignant primary brain tumor, which is virtually incurable due to its therapeutic resistance to radiation and chemotherapy. To develop novel therapeutic approaches for treatment of GBM, we examined the role of miR-378 on tumor growth, angiogenesis, and radiation response in ectopic and orthotopic U87 glioblastoma models. Cell and tumor growth rates, in vitro and in vivo radiation sensitivities, and tumor vascular density were evaluated in U87-GFP and U87-miR-378 tumor lines. Ectopic tumor response to radiation was evaluated under normal blood flow and clamp hypoxic conditions. Results show that in vitro, miR-378 expression moderately increased cell growth rate and plating efficiency, but did not alter radiation sensitivity. U87-miR-378 tumors exhibited a higher transplantation take rate than U87-GFP tumors. In vivo, under oxygenated condition, subcutaneous U87-miR-378 tumors receiving 25 Gy showed a tendency for longer tumor growth delay (TGD) than control U87-GFP tumors. In contrast, under hypoxic condition, U87-miR-378 xenografts exhibited substantially shorter TGD than U87-GFP tumors, indicating that under normal blood flow conditions, U87-miR-378 tumors were substantially more oxygenated than U87-GFP tumors. Intracranial multi-photon laser-scanning microscopy demonstrated increased vascular density of U87-miR-378 versus control U87-GFP tumors. Finally, miR-378 increased TGD following 12 Gy irradiation in U87 intracranial xenografts, and significantly prolonged survival of U87-miR-378 tumor-bearing mice (P = 0.04). In conclusion, higher miR-378 expression in U87-miR-378 cells promotes tumor growth, angiogenesis, radiation-induced TGD, and prolongs survival of orthotopic tumor-bearing hosts. Regulation of VEGFR2 by miR-378 significantly increased vascular density and oxygenation in U87 xenografts.
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Neoplasias Encefálicas/metabolismo , Glioblastoma/metabolismo , MicroRNAs/metabolismo , Tolerância a Radiação , Animais , Linhagem Celular Tumoral , Proliferação de Células/efeitos da radiação , Xenoenxertos/efeitos da radiação , Humanos , Masculino , Camundongos Nus , Neovascularização Patológica/metabolismo , Receptor 2 de Fatores de Crescimento do Endotélio Vascular/metabolismoRESUMO
BACKGROUND: Ultra-high dose rate (FLASH) radiation has been reported to efficiently suppress tumor growth while sparing normal tissue; however, the mechanism of the differential tissue sparing effect is still not known. Oxygen has long been known to profoundly impact radiobiological responses, and radiolytic oxygen depletion has been considered to be a possible cause or contributor to the FLASH phenomenon. PURPOSE: This work investigates the impact of tissue pO2 profiles, oxygen depletion per unit dose (g), and the oxygen concentration yielding half-maximum radiosensitization (the average of its maximum value and one) (k) in tumor and normal tissue. METHODS: We developed a model that considers the dependent relationship between oxygen depletion and change of radiosensitivity by FLASH irradiation. The model assumed that FLASH irradiation depletes intracellular oxygen more rapidly than it diffuses into the cell from the extracellular environment. Cell survival was calculated based on the linear quadratic-linear model and the radiosensitivity related parameters were adjusted in 1 Gy increments of the administered dose. The model reproduced published experimental data that were obtained with different cell lines and oxygen concentrations, and was used to analyze the impact of parameter uncertainties on the radiobiological responses. This study expands the oxygen depletion analysis of FLASH to normal human tissue and tumor based on clinically determined aggregate and individual patient pO2 profiles. RESULTS: The results show that the pO2 profile is the most essential factor that affects biological response and analyses based on the median pO2 rather than the full pO2 profile can be unreliable and misleading. Additionally, the presence of a small fraction of cells on the threshold of radiobiologic hypoxia substantially alters biological response due to FLASH oxygen depletion. We found that an increment in the k value is generally more protective of tumor than normal tissue due to a higher frequency of lower pO2 values in tumors. Variation in the g value affects the dose at which oxygen depletion impacts response, but does not alter the dose-dependent response trends, if the g value is identical in both tumor and normal tissue. CONCLUSIONS: The therapeutic efficacy of FLASH oxygen depletion is likely patient and tissue-dependent. For breast cancer, FLASH is beneficial in a minority of cases; however, in a subset of well oxygenated tumors, a therapeutic gain may be realized due to induced normal tissue hypoxia.
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Neoplasias , Oxigênio , Humanos , Oxigênio/metabolismo , Tolerância a Radiação , Neoplasias/radioterapia , Radiobiologia , HipóxiaRESUMO
To determine if the radiation sensitivity of cells that survive acute high-dose radiation exposure used in stereotactic body radiation therapy (SBRT), differs from the sensitivity of non-irradiated cells and cells that survive multiple 2 Gy doses of radiation. Isogenic rodent and two human tumor cell lines were exposed to 14 × 2 Gy of radiation, or a single acute dose of 12 Gy. The most resistant cell line was also exposed to an acute dose of 15 Gy. One week after 12 Gy, and 4 days after 14 × 2 Gy, surviving cells were exposed to 0-8 Gy in 2 Gy doses and cell survival was assessed by colony formation. In addition, the colony forming efficiency of 12 Gy survivors was evaluated for 1 month postirradiation. For cells exposed to 15 Gy, the response of surviving cells to 6 Gy was determined for up to 35 days postirradiation and compared to the 6 Gy surviving fraction of control cells. The radiation sensitivity of cells that survived 12 Gy exposure, and cells that survived 14 fractions of 2 Gy irradiation did not differ from the response of unirradiated control cells. However, the growth rate and colony forming efficiency of 12 Gy survivors was transiently reduced for greater than 2 weeks postirradiation. In contrast to the unchanged sensitivity of 12 Gy surviving cells at day 7 postirradiation, 15 Gy survivors exhibited enhanced sensitivity to radiation for up to 21 days postirradiation and suggests a biological basis for SBRT.
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Radiocirurgia , Humanos , Radiocirurgia/efeitos adversos , Doses de Radiação , Tolerância a Radiação , Sobrevivência Celular/efeitos da radiação , Relação Dose-Resposta à RadiaçãoRESUMO
Objective. Irradiation at FLASH dose rates (>40 Gy s-1) has received great attention due to its reported normal tissue sparing effect. The FLASH effect was originally observed in electron irradiations but has since been shown to also occur with both photon and proton beams. Several mechanisms have been proposed to explain the tissue sparing at high dose rates, including effects involving oxygen, such as depletion of oxygen within the irradiated cells. In this study, we investigated the protective role of FLASH proton irradiation on the skin when varying the oxygen concentration.Approach. Our double scattering proton system provided a 1.2 × 1.6 cm2elliptical field at a dose rate of â¼130 Gy s-1. The conventional dose rate was â¼0.4 Gy s-1. The legs of the FVB/N mice were marked with two tattooed dots and fixed in a holder for exposure. To alter the skin oxygen concentration, the mice were breathing pure oxygen or had their legs tied to restrict blood flow. The distance between the two dots was measured to analyze skin contraction over time.Main results. FLASH irradiation mitigated skin contraction by 15% compared to conventional dose rate irradiation. The epidermis thickness and collagen deposition at 75 d following 25 to 30 Gy exposure suggested a long-term protective function in the skin from FLASH irradiation. Providing the mice with oxygen or reducing the skin oxygen concentration removed the dose-rate-dependent difference in response.Significance. FLASH proton irradiation decreased skin contraction, epidermis thickness and collagen deposition compared to standard dose rate irradiations. The observed oxygen-dependence of the FLASH effect is consistent with, but not conclusive of, fast oxygen depletion during the exposure.
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Terapia com Prótons , Prótons , Camundongos , Animais , Terapia com Prótons/métodos , Oxigênio , Pele , Fótons , Dosagem RadioterapêuticaRESUMO
Ultra-high dose rate irradiation has been reported to protect normal tissues more than conventional dose rate irradiation. This tissue sparing has been termed the FLASH effect. We investigated the FLASH effect of proton irradiation on the intestine as well as the hypothesis that lymphocyte depletion is a cause of the FLASH effect. A 16 × 12 mm2 elliptical field with a dose rate of ~120 Gy/s was provided by a 228 MeV proton pencil beam. Partial abdominal irradiation was delivered to C57BL/6j and immunodeficient Rag1-/-/C57 mice. Proliferating crypt cells were counted at 2 days post exposure, and the thickness of the muscularis externa was measured at 280 days following irradiation. FLASH irradiation did not reduce the morbidity or mortality of conventional irradiation in either strain of mice; in fact, a tendency for worse survival in FLASH-irradiated mice was observed. There were no significant differences in lymphocyte numbers between FLASH and conventional-dose-rate mice. A similar number of proliferating crypt cells and a similar thickness of the muscularis externa following FLASH and conventional dose rate irradiation were observed. Partial abdominal FLASH proton irradiation at 120 Gy/s did not spare normal intestinal tissue, and no difference in lymphocyte depletion was observed. This study suggests that the effect of FLASH irradiation may depend on multiple factors, and in some cases dose rates of over 100 Gy/s do not induce a FLASH effect and can even result in worse outcomes.
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BACKGROUND: Patients with metastatic HER2/neu-positive (HER2/neu +) breast cancer (BC) often experience treatment resistance, disease recurrences and metastases. Thus, new approaches for improving the treatment of HER2/neu + BC to prevent metastatic dissemination are urgently needed. Our previous studies have shown that losartan, an angiotensin receptor blocker, increases tumor perfusion and decreases hypoxia in a number of tumor models. Hypoxia reduces the efficacy of radiation and increases metastases. We therefore hypothesized that by modifying tumor stroma and increasing oxygenation, losartan will improve the outcome of radiotherapy and inhibit disease progression in a highly metastatic HER2/neu + murine BC model. METHODS: We established a metastatic HER2/neu + murine BC line (MCa-M3C) and used it to generate mammary fat pad isografts in syngeneic female FVB/N mice. Starting on day 3 after orthotopic tumor implantation, we administered a 7-day losartan treatment (40 mg/kg BW, gavage daily); or a 7-day losartan treatment followed by 20 Gy single dose local irradiation (S-IR) on day 10 (tumor size ~ 100 mm3), or 20 Gy local fractionated (5 × 4 Gy daily) irradiation (F-IR) on days 10-14. We analyzed tumor-growth delay (TGD), development of spontaneous lung metastases, animal survival, tumor vascular density, and tumor hypoxia. RESULTS: Treatments with S-IR, F-IR, Losartan + S-IR, or Losartan + F-IR resulted in a significantly increased TGD (8-16 days) in MCa-M3C tumors versus controls. However, the combination of Losartan + S-IR and Losartan + F-IR further enhanced tumor response to radiation alone by increasing TGD an additional 5 to 8 days for both single and fractionated dose irradiation (P < 0.01), decreasing lung metastasis (Losartan + IR vs. Control, P < 0.025), and increasing animal survival (Losartan + IR vs. Control, P = 0.0303). In addition, losartan treatment significantly increased tumor vascularity (P = 0.0314) and decreased pimonidazole positive (hypoxic) area (P = 0.0002). CONCLUSIONS: Combining losartan with local irradiation significantly enhanced tumor response, at least in part via reduced tumor hypoxia presumably due to increased tumor perfusion. Our findings suggest that combining losartan with radiotherapy is a potential new treatment strategy for local control and inhibiting metastasis in HER2 + BC.
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Bloqueadores do Receptor Tipo 1 de Angiotensina II/uso terapêutico , Losartan/uso terapêutico , Neoplasias Pulmonares/prevenção & controle , Neoplasias Mamárias Experimentais/terapia , Animais , Quimiorradioterapia , Feminino , Neoplasias Pulmonares/secundário , Neoplasias Mamárias Experimentais/patologia , Camundongos , Dosagem Radioterapêutica , Receptor ErbB-2/genética , Taxa de Sobrevida , Resultado do Tratamento , Células Tumorais Cultivadas , Hipóxia Tumoral/efeitos dos fármacosRESUMO
Cancer has become a leading cause of death and constitutes an enormous burden worldwide. Radiation is a principle treatment modality used alone or in combination with other forms of therapy, with 50%-70% of cancer patients receiving radiotherapy at some point during their illness. It has been suggested that traditional radiotherapy (daily fractions of approximately 1.8-2 Gy over several weeks) might select for radioresistant tumor cell sub-populations, which, if not sterilized, give rise to local treatment failure and distant metastases. Thus, the challenge is to develop treatment strategies and schedules to eradicate the resistant subpopulation of tumorigenic cells rather than the predominant sensitive tumor cell population. With continued technological advances including enhanced conformal treatment technology, radiation oncologists can increasingly maximize the dose to tumors while sparing adjacent normal tissues, to limit toxicity and damage to the latter. Increased dose conformality also facilitates changes in treatment schedules, such as changes in dose per treatment fraction and number of treatment fractions, to enhance the therapeutic ratio. For example, the recently developed large dose per fraction treatment schedules (hypofractionation) have shown clinical advantage over conventional treatment schedules in some tumor types. Experimental studies suggest that following large acute doses of radiation, recurrent tumors, presumably sustained by the most resistant tumor cell populations, may in fact be equally or more radiation sensitive than the primary tumor. In this review, we summarize the related advances in radiotherapy, including the increasing understanding of the molecular mechanisms of radioresistance, and the targeting of these mechanisms with potent small molecule inhibitors, which may selectively sensitize tumor cells to radiation.
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Extremely high-dose-rate irradiation, referred to as FLASH, has been shown to be less damaging to normal tissues than the same dose administrated at conventional dose rates. These results, typically seen at dose rates exceeding 40 Gy/s (or 2,400 Gy/min), have been widely reported in studies utilizing photon or electron radiation as well as in some proton radiation studies. Here, we report the development of a proton irradiation platform in a clinical proton facility and the dosimetry methods developed. The target is placed in the entry plateau region of a proton beam with a specifically designed double-scattering system. The energy after the double-scattering system is 227.5 MeV for protons that pass through only the first scatterer, and 225.5 MeV for those that also pass through the second scatterer. The double-scattering system was optimized to deliver a homogeneous dose distribution to a field size as large as possible while keeping the dose rate >100 Gy/s and not exceeding a cyclotron current of 300 nA. We were able to obtain a collimated pencil beam (1.6 × 1.2 cm2 ellipse) at a dose rate of â¼120 Gy/s. This beam was used for dose-response studies of partial abdominal irradiation of mice. First results indicate a potential tissue-sparing effect of FLASH.
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Terapia com Prótons/métodos , Animais , Feminino , Camundongos , Camundongos Endogâmicos C57BL , Método de Monte Carlo , Dosagem Radioterapêutica , Reprodutibilidade dos TestesRESUMO
In this study, we evaluated the role of tumor cell and tumor stroma sensitivity as determinants of radiation-induced tumor growth delay. A DNA double-strand break repair-defective DNA-PKcs(-/-) tumor cell line and its radioresistant DNA-PKcs(+/+)-transfected counterpart were used to initiate tumors in nude and hypersensitive severe combined immunodeficient (SCID) mice. Insertion of the human DNA-PKcs(+/+) gene substantially increased the intrinsic radioresistance of the DNA-PKcs(-/-) tumor cells and substantially decreased tumor response to radiation in both nude and hypersensitive SCID mice. Tumor cell radiosensitivity was the major determinant of tumor response in nude mice. In SCID mice, both tumor cell sensitivity and radiation-induced stromal damage contributed to response. The relative contribution of host and tumor cell sensitivity on tumor response was unchanged for single doses of 1 x 15 and 6 x 3 Gy-fractionated dose irradiation.
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Neoplasias Experimentais/radioterapia , Tolerância a Radiação/fisiologia , Animais , Processos de Crescimento Celular/efeitos da radiação , Dano ao DNA , DNA de Neoplasias/efeitos da radiação , Proteína Quinase Ativada por DNA/genética , Proteínas de Ligação a DNA/genética , Humanos , Camundongos , Camundongos Nus , Camundongos SCID , Neoplasias Experimentais/enzimologia , Neoplasias Experimentais/genética , Neoplasias Experimentais/patologia , Proteínas Nucleares/genética , Células Estromais/enzimologia , Células Estromais/patologia , Células Estromais/fisiologia , Células Estromais/efeitos da radiação , TransfecçãoRESUMO
PURPOSE: Variations in the radiosensitivity of tumor cells within and between tumors impact tumor response to radiation, including the dose required to achieve permanent local tumor control. The increased expression of DNA-PKcs, a key component of a major DNA damage repair pathway in tumors treated by radiation, suggests that DNA-PKcs-dependent repair is likely a cause of tumor cell radioresistance. This study evaluates the relative biological effect of spread-out Bragg-peak protons in DNA-PKcs-deficient cells and the same cells transfected with a functional DNA-PKcs gene. MATERIALS AND METHODS: A cloned radiation-sensitive DNA-PKcs-deficient tumor line and its DNA-PKcs-transfected resistant counterpart were used in this study. The presence of functional DNA-PKcs was evaluated by DNA-PKcs autophosphorylation. Cells to be proton irradiated or x-irradiated were obtained from the same single cell suspension and dilution series to maximize precision. Cells were concurrently exposed to 6-MV x-rays or mid 137-MeV spread-out Bragg peak protons and cultured for colony formation. RESULTS: The surviving fraction data were well fit by the linear-quadratic model for each of 8 survival curves. The results suggest that the relative biological effectiveness of mid spread-out Bragg peak protons is approximately 6% higher in DNA-PKcs-mediated resistant tumor cells than in their DNA-PKcs-deficient and radiation-sensitive counterpart. CONCLUSION: DNA-PKcs-dependent repair of radiation damage is less capable of repairing mid spread-out Bragg peak proton lesions than photon-induced lesions, suggesting protons may be more efficient at sterilizing DNA-PKcs-expressing cells that are enriched in tumors treated by conventional fractionated dose x-irradiation.
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The extracellular pH of tumor tissue is significantly lower than the extracellular pH of normal tissue, whereas the intracellular pH of both tissues is similar. In principle, extracellular acidity may be expected to enhance the intracellular uptake and cytotoxicity of weak acid chemotherapeutics that are membrane permeable in their uncharged state and inhibit the efficacy of weak bases. However, procedures for assessing the role of the gradient as a determinant of drug efficacy in vivo by altering the pH gradient may also alter drug availability and thus mask or exaggerate the effect of the gradient change. In the present study, we have altered the extracellular pH of tumors and compared the effect of the resultant pH gradient change on the efficacy of a weak acid versus a weak base. This experimental design gives rise to a change in the ratio of chlorambucil- to doxorubicin-induced tumor growth delay, independent of possible changes in drug availability. The extracellular pH of the 54A human tumor in NCr/Sed/nu/nu mice was altered by administration of 5 mg/g i.v. glucose. The resultant 0.2 pH unit increase in the tumor cell pH gradient gives rise to a predicted 2.3-fold increase in the ratio of chlorambucil to doxorubicin growth delay. The experimentally measured change in the growth delay ratio was 2.1. The results provide compelling evidence that the pH gradient in a determinant of the efficacy of weak electrolytes in the complex in vivo environment and may be exploited for the treatment of cancer.
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Antineoplásicos/uso terapêutico , Clorambucila/uso terapêutico , Doxorrubicina/uso terapêutico , Neoplasias/tratamento farmacológico , Animais , Espaço Extracelular/metabolismo , Glucose/farmacologia , Humanos , Concentração de Íons de Hidrogênio/efeitos dos fármacos , Masculino , Camundongos , Camundongos Endogâmicos , Transplante de Neoplasias , Neoplasias/química , Neoplasias/metabolismoRESUMO
Reports that a small subset of tumor cells initiate and sustain tumor growth, are resistant to radiation and drugs, and bear specific markers have led to an explosion of cancer stem cell research. These reports imply that the evaluation of therapeutic response by changes in tumor volume is misleading, as volume changes reflect the response of the sensitive rather than the resistant tumorigenic cell population. The reports further suggest that the marker-based selection of the tumor cell population will facilitate the development of radiation treatment schedules, sensitizers, and drugs that specifically target the resistant tumorigenic cells that give rise to treatment failure. This review presents evidence that contests the observations that cancer stem cell markers reliably identify the subset of tumor cells that sustain tumor growth and that the marker-identified population is radioresistant relative to the marker-negative cells. Experimental studies show that cells and tumors that survive large radiation doses are not more radioresistant than unirradiated cells and tumors, and also show that the intrinsic radiosensitivity of unsorted colony-forming tumor cells, in combination with the fraction of unsorted tumor cells that are tumor initiating, predicts tumor radiocurability.
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Neoplasias/radioterapia , Células-Tronco Neoplásicas/efeitos da radiação , Humanos , Tolerância a Radiação , Radiobiologia , Dosagem RadioterapêuticaRESUMO
Accumulating evidence suggests that ubiquitin E3 ligases are involved in cancer development as their mutations correlate with genomic instability and genetic susceptibility to cancer. Despite significant findings of cancer-driving mutations in the BRCA1 gene, estrogen receptor (ER)-positive breast cancers progress upon treatment with DNA damaging-cytotoxic therapies. In order to understand the underlying mechanism by which ER-positive breast cancer cells develop resistance to DNA damaging agents, we employed an estrogen receptor agonist, Erb-041, to increase the activity of ERß and negatively regulate the expression and function of the estrogen receptor α (ERα) in MCF-7 breast cancer cells. Upon Erb-041-mediated ERα down-regulation, the transcription of an ERα downstream effector, BCA2 (Breast Cancer Associated gene 2), correspondingly decreased. The ubiquitination of chromatin-bound BCA2 was induced by ultraviolet C (UVC) irradiation but suppressed by Erb-041 pretreatment, resulting in a blunted DNA damage response. Upon BCA2 silencing, DNA double-stranded breaks increased with Rad51 up-regulation and ataxia telangiectasia mutated (ATM) activation. Mechanistically, UV-induced BCA2 ubiquitination and chromatin binding were found to promote DNA damage response and repair via the interaction of BCA2 with ATM, γH2AX and Rad51. Taken together, this study suggests that Erb-041 potentiates BCA2 dissociation from chromatin and co-localization with Rad51, resulting in inhibition of homologous recombination repair.
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The purpose of this study was to determine the relative biological effectiveness (RBE) along the axis of two range-modulated proton beams (160 and 230 MeV). Both the depth and the dose dependence of RBE were investigated. Chinese hamster V79-WNRE cells, suspended in medium containing gelatin and cooled to 2 °C, were used to obtain complete survival curves at multiple positions throughout the entrance and 10 cm spread-out Bragg peak (SOBP). Simultaneous measurements of the survival response to (60)Co gamma rays served as the reference data for the proton RBE determinations. For both beams the RBE increased significantly with depth in the 10 cm SOBP, particularly in the distal half of the SOBP, then rose even more sharply at the distal edge, the most distal position measured. At a 4 Gy dose of gamma radiation (S = 0.34) the average RBE values for the entrance, proximal half, distal half and distal edge were 1.07 ± 0.01, 1.10 ± 0.01, 1.17 ± 0.01 and 1.21 ± 0.01, respectively, and essentially the same for both beams. At a 2 Gy dose of gamma radiation (S = 0.71) the average RBE values rose to 1.13 ± 0.03, 1.15 ± 0.02, 1.26 ± 0.02 and 1.30 ± 0.02, respectively, for the same four regions of the SOBP. The difference between the 4 Gy and 2 Gy RBE values reflects the dose dependence of RBE as measured in these V79-WNRE cells, which have a low α/ß value, as do other widely used cell lines that also show dose-dependent RBE values. Late-responding tissues are also characterized by low α/ß values, so it is possible that these cell lines may be predictive for the response of such tissues (e.g., spinal cord, optic nerve, kidney, liver, lung). However, in the very small number of studies of late-responding tissues performed to date there appears to be no evidence of an increased RBE for protons at low doses. Similarly, RBE measurements using early responding in vivo systems (mostly mouse jejunum, an early-responding tissue which has a large α/ß â¼ 10 Gy) have generally shown little or no detectable dose dependence. It is useful to compare the RBE values reported here to the commonly used generic clinical RBE of 1.1, which assumes no dependence on depth or on dose. Our proximal RBEs obviously avoid the depth-related increase in RBE and for doses of 4 Gy or more, the low-dose increase in RBE is also minimized, as shown in this article. Thus the proximal RBE at a 4 Gy dose of 1.10 ± 0.01, quoted above, represents an interesting point of congruence with the clinical RBE for conditions where it could reasonably be expected in the measurements reported here. The depth dependence of RBE reported here is consistent with the majority of measurements, both in vitro and in vivo, by other investigators. The dose dependence of RBE, on the other hand, is tissue specific but has not yet been demonstrated for protons by RBE values in late-responding normal tissue systems. This indicates a need for additional RBE determination as function of dose, especially in late-responding tissues.
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Apoptose/efeitos da radiação , Sobrevivência Celular/efeitos da radiação , Pulmão/citologia , Pulmão/fisiologia , Radioterapia de Alta Energia/métodos , Animais , Linhagem Celular , Cricetinae , Cricetulus , Relação Dose-Resposta à Radiação , Pulmão/efeitos da radiação , Terapia com Prótons , Dosagem RadioterapêuticaRESUMO
PURPOSE: Clinical proton beam therapy has been based on the use of a generic relative biological effectiveness (RBE) of 1.0 or 1.1, since the available evidence has been interpreted as indicating that the magnitude of RBE variation with treatment parameters is small relative to our abilities to determine RBEs. As substantial clinical experience and additional experimental determinations of RBE have accumulated and the number of proton radiation therapy centers is projected to increase, it is appropriate to reassess the rationale for the continued use of a generic RBE and for that RBE to be 1.0-1.1. METHODS AND MATERIALS: Results of experimental determinations of RBE of in vitro and in vivo systems are examined, and then several of the considerations critical to a decision to move from a generic to tissue-, dose/fraction-, and LET-specific RBE values are assessed. The impact of an error in the value assigned to RBE on normal tissue complication probability (NTCP) is discussed. The incidence of major morbidity in proton-treated patients at Massachusetts General Hospital (MGH) for malignant tumors of the skull base and of the prostate is reviewed. This is followed by an analysis of the magnitude of the experimental effort to exclude an error in RBE of >or=10% using in vivo systems. RESULTS: The published RBE values, using colony formation as the measure of cell survival, from in vitro studies indicate a substantial spread between the diverse cell lines. The average value at mid SOBP (Spread Out Bragg Peak) over all dose levels is approximately 1.2, ranging from 0.9 to 2.1. The average RBE value at mid SOBP in vivo is approximately 1.1, ranging from 0.7 to 1.6. Overall, both in vitro and in vivo data indicate a statistically significant increase in RBE for lower doses per fraction, which is much smaller for in vivo systems. There is agreement that there is a measurable increase in RBE over the terminal few millimeters of the SOBP, which results in an extension of the bioeffective range of the beam in the range of 1-2 mm. There is no published report to indicate that the RBE of 1.1 is low. However, a substantial proportion of patients treated at approximately 2 cobalt Gray equivalent (CGE)/fraction 5 or more years ago were treated by a combination of both proton and photon beams. Were the RBE to be erroneously underestimated by approximately 10%, the increase in complication frequency would be quite serious were the complication incidence for the reference treatment >or=3% and the slope of the dose response curves steep, e.g., a gamma(50) approximately 4. To exclude >or=1.2 as the correct RBE for a specific condition or tissue at the 95% confidence limit would require relatively large and multiple assays. CONCLUSIONS: At present, there is too much uncertainty in the RBE value for any human tissue to propose RBE values specific for tissue, dose/fraction, proton energy, etc. The experimental in vivo and clinical data indicate that continued employment of a generic RBE value and for that value to be 1.1 is reasonable. However, there is a local "hot region" over the terminal few millimeters of the SOBP and an extension of the biologically effective range. This needs to be considered in treatment planning, particularly for single field plans or for an end of range in or close to a critical structure. There is a clear need for prospective assessments of normal tissue reactions in proton irradiated patients and determinations of RBE values for several late responding tissues in laboratory animal systems, especially as a function of dose/fraction in the range of 1-4 Gy.
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Transferência Linear de Energia , Terapia com Prótons , Eficiência Biológica Relativa , Animais , Encéfalo/efeitos da radiação , Neoplasias Encefálicas/radioterapia , Humanos , Masculino , Especificidade de Órgãos , Neoplasias da Próstata/radioterapia , Prótons/efeitos adversos , Lesões por Radiação/etiologia , Reto/efeitos da radiação , Valores de Referência , Células Tumorais Cultivadas/efeitos da radiação , Ensaio Tumoral de Célula-TroncoRESUMO
BACKGROUND AND PURPOSE: The causes of tumor response variation to radiation remain obscure, thus hampering the development of predictive assays and strategies to decrease resistance. The present study evaluates the impact of host tumor stromal elements and the in vivo environment on tumor cell kill, and relationship between tumor cell radiosensitivity and the tumor control dose. MATERIAL AND METHODS: Five endpoints were evaluated and compared in a radiosensitive DNA double-strand break repair-defective (DNA-PKcs(-/-)) tumor line, and its DNA-PKcs repair competent transfected counterpart. In vitro colony formation assays were performed on in vitro cultured cells, on cells obtained directly from tumors, and on cells irradiated in situ. Permanent local control was assessed by the TCD50 assay. Vascular effects were evaluated by functional vascular density assays. RESULTS: The fraction of repair competent and repair deficient tumor cells surviving radiation did not substantially differ whether irradiated in vitro, i.e., in the absence of host stromal elements and factors, from the fraction of cells killed following in vivo irradiation. Additionally, the altered tumor cell sensitivity resulted in a proportional change in the dose required to achieve permanent local control. The estimated number of tumor cells per tumor, their cloning efficiency and radiosensitivity, all assessed by in vitro assays, were used to predict successfully, the measured tumor control doses. CONCLUSION: The number of clonogens per tumor and their radiosensitivity govern the permanent local control dose.
Assuntos
Quebras de DNA de Cadeia Dupla/efeitos da radiação , Proteína Quinase Ativada por DNA/metabolismo , Neoplasias/radioterapia , Tolerância a Radiação/efeitos da radiação , Animais , Apoptose/efeitos da radiação , Linhagem Celular Tumoral , Dose Letal Mediana , Camundongos Nus , Camundongos SCID , Transplante de Neoplasias , Neoplasias/irrigação sanguínea , Neoplasias/genética , Tolerância a Radiação/genética , Dosagem Radioterapêutica , Células Estromais/efeitos da radiação , Transfecção , Células Tumorais CultivadasRESUMO
PURPOSE: To evaluate the life span and risk of cancer following whole-body exposure of mice to neutrons generated by a passively scattered clinical spread-out Bragg peak (SOBP) proton beam. METHODS AND MATERIALS: Three hundred young adult female FVB/N mice, 152 test and 148 control, were entered into the experiment. Mice were placed in an annular cassette around a cylindrical phantom, which was positioned lateral to the mid-SOBP of a 165-MeV, clinical proton beam. The average distance from the edge of the mid-SOBP to the conscious active mice was 21.5 cm. The phantom was irradiated with once-daily fractions of 25 Gy, 4 days per week, for 6 weeks. The age at death and cause of death (ie, cancer and type vs noncancer causes) were assessed over the life span of the mice. RESULTS: Exposure of mice to a dose of 600 Gy of proton beam-generated neutrons, reduced the median life span of the mice by 4.2% (Kaplan-Meier cumulative survival, P=.053). The relative risk of death from cancer in neutron exposed versus control mice was 1.40 for cancer of all types (P=.0006) and 1.22 for solid cancers (P=.09). For a typical 60 Gy dose of clinical protons, the observed 22% increased risk of solid cancer would be expected to decrease by a factor of 10. CONCLUSIONS: Exposure of mice to neutrons generated by a proton dose that exceeds a typical course of radiation therapy by a factor of 10, resulted in a statistically significant increase in the background incidence of leukemia and a marginally significant increase in solid cancer. The results indicate that the risk of out-of-field second solid cancers from SOBP proton-generated neutrons and typical treatment schedules, is 6 to 10 times less than is suggested by current neutron risk estimates.
Assuntos
Longevidade/efeitos da radiação , Neoplasias Experimentais/etiologia , Neoplasias Induzidas por Radiação/etiologia , Nêutrons/efeitos adversos , Terapia com Prótons/efeitos adversos , Espalhamento de Radiação , Irradiação Corporal Total/efeitos adversos , Fatores Etários , Animais , Causas de Morte , Fracionamento da Dose de Radiação , Feminino , Camundongos , Método de Monte Carlo , Neoplasias Experimentais/mortalidade , Neoplasias Induzidas por Radiação/mortalidade , Irradiação Corporal Total/métodos , Irradiação Corporal Total/mortalidadeRESUMO
Recent developments in external beam radiotherapy, both in technical advances and in clinical approaches, have prompted renewed discussions on the potential influence of dose-rate on radio-response in certain treatment scenarios. We consider the multiple factors that influence the dose-rate effect, e.g. radical recombination, the kinetics of sublethal damage repair for tumors and normal tissues, the difference in alpha/beta ratio for early and late reacting tissues, and perform a comprehensive literature review. Based on radiobiological considerations and the linear-quadratic (LQ) model we estimate the influence of overall treatment time on radio-response for specific clinical situations. As the influence of dose-rate applies to both the tumor and normal tissues, in oligo-fractionated treatment using large doses per fraction, the influence of delivery prolongation is likely important, with late reacting normal tissues being generally more sensitive to the dose-rate effect than tumors and early reacting tissues. In conventional fractionated treatment using 1.8-2Gy per fraction and treatment times of 2-1 min, the influence of dose-rate is relatively small. Lastly, the dose-rate effect in external beam radiotherapy is governed by the overall beam-on-time, not by the average linac dose-rate, nor by the instantaneous dose-rate within individual linac pulses which could be as high as 3 x 10(6)MU/min.