Radiation dose escalation based on FDG-PET driven dose painting by numbers in oropharyngeal squamous cell carcinoma: a dosimetric comparison between TomoTherapy-HA and RapidArc.

PURPOSE: Validation of dose escalation through FDG-PET dose painting (DP) for oropharyngeal squamous cell carcinoma (SCC) requires randomized clinical trials with large sample size, potentially involving different treatment planning and delivery systems. As a first step of a joint clinical study of DP, a planning comparison was performed between Tomotherapy HiArt® (HT) and Varian RapidArc® (RA). METHODS: The planning study was conducted on five patients with oropharyngeal SCC. Elective and therapeutic CTVs were delineated based on anatomic information, and the respective PTVs (CTVs + 4 mm) were prescribed a dose of 56 (PTV56) and 70 Gy (PTV70). A gradient-based method was used to delineate automatically the external contours of the FDG-PET volume (GTVPET). Variation of the FDG uptake within the GTVPET was linearly converted into a prescription between 70 and 86 Gy. A dilation of the voxel-by-voxel prescription of 2.5 mm was applied to account for geometric errors in dose delivery (PTVPET). The study was divided in two planning phases aiming at maximizing target coverage (phase I) and lowering doses to OAR (phase II). A Quality-Volume Histogram (QVH) assessed conformity with the DP prescription inside the PTVPET. RESULTS: In phase I, for both HT and RA, all plans achieved comparable target coverage for PTV56 and PTV70, respecting the planning objectives. A median value of 99.9 and 97.2% of all voxels in the PTVPET received at least 95% of the prescribed dose for RA and HT, respectively. A median value of 0.0% and 3.7% of the voxels in the PTVPET received 105% or more of prescribed dose for RA and HT, respectively. In phase II, no significant differences were found in OAR sparing. Median treatment times were 13.7 min for HT and 5 min for RA. CONCLUSIONS: Both HT and RA can generate similar dose distributions for FDG-PET based dose escalation and dose painting in oropharyngeal SCC patients.

Towards 3D printed multifunctional immobilization for proton therapy: Initial materials characterization.

PURPOSE: 3D printing technology is investigated for the purpose of patient immobilization during proton therapy. It potentially enables a merge of patient immobilization, bolus range shifting, and other functions into one single patient-specific structure. In this first step, a set of 3D printed materials is characterized in detail, in terms of structural and radiological properties, elemental composition, directional dependence, and structural changes induced by radiation damage. These data will serve as inputs for the design of 3D printed immobilization structure prototypes. METHODS: Using four different 3D printing techniques, in total eight materials were subjected to testing. Samples with a nominal dimension of 20 × 20 × 80 mm3 were 3D printed. The geometrical printing accuracy of each test sample was measured with a dial gage. To assess the mechanical response of the samples, standardized compression tests were performed to determine the Young's modulus. To investigate the effect of radiation on the mechanical response, the mechanical tests were performed both prior and after the administration of clinically relevant dose levels (70 Gy), multiplied with a safety factor of 1.4. Dual energy computed tomography (DECT) methods were used to calculate the relative electron density to water ρe, the effective atomic number Zeff, and the proton stopping power ratio (SPR) to water SPR. In order to validate the DECT based calculation of radiological properties, beam measurements were performed on the 3D printed samples as well. Photon irradiations were performed to measure the photon linear attenuation coefficients, while proton irradiations were performed to measure the proton range shift of the samples. The directional dependence of these properties was investigated by performing the irradiations for different orientations of the samples. RESULTS: The printed test objects showed reduced geometric printing accuracy for 2 materials (deviation > 0.25 mm). Compression tests yielded Young's moduli ranging from 0.6 to 2940 MPa. No deterioration in the mechanical response was observed after exposure of the samples to 100 Gy in a therapeutic MV photon beam. The DECT-based characterization yielded Zeff ranging from 5.91 to 10.43. The SPR and ρe both ranged from 0.6 to 1.22. The measured photon attenuation coefficients at clinical energies scaled linearly with ρe. Good agreement was seen between the DECT estimated SPR and the measured range shift, except for the higher Zeff. As opposed to the photon attenuation, the proton range shifting appeared to be printing orientation dependent for certain materials. CONCLUSIONS: In this study, the first step toward 3D printed, multifunctional immobilization was performed, by going through a candidate clinical workflow for the first time: from the material printing to DECT characterization with a verification through beam measurements. Besides a proof of concept for beam modification, the mechanical response of printed materials was also investigated to assess their capabilities for positioning functionality. For the studied set of printing techniques and materials, a wide variety of mechanical and radiological properties can be selected from for the intended purpose. Moreover the elaborated hybrid DECT methods aid in performing in-house quality assurance of 3D printed components, as these methods enable the estimation of the radiological properties relevant for use in radiation therapy.

Different dose rate-dependent responses of human melanoma cells and fibroblasts to low dose fast neutrons.

PURPOSE: To analyze the dose rate influence in hyper-radiosensitivity (HRS) of human melanoma cells to very low doses of fast neutrons and to compare to the behaviour of normal human skin fibroblasts. MATERIALS AND METHODS: We explored different neutron dose rates as well as possible implication of DNA double-strand breaks (DSB), apoptosis, and energy-provider adenosine-triphosphate (ATP) levels during HRS. RESULTS: HRS in melanoma cells appears only at a very low dose rate (VLDR), while a high dose rate (HDR) induces an initial cell-radioresistance (ICRR). HRS does not seem to be due either to DSB or to apoptosis. Both phenomena (HRS and ICRR) appear to be related to ATP availability for triggering cell repair. Fibroblast survival after neutron irradiation is also dose rate-dependent but without HRS. CONCLUSIONS: Melanoma cells or fibroblasts exert their own survival behaviour at very low doses of neutrons, suggesting that in some cases there is a differential between cancer and normal cells radiation responses. Only the survival of fibroblasts at HDR fits the linear no-threshold model. This new insight into human cell responses to very low doses of neutrons, concerns natural radiations, surroundings of accelerators, proton-therapy devices, flights at high altitude. Furthermore, ATP inhibitors could increase HRS during high-linear energy transfer (high-LET) irradiation.

Combination of the mTOR inhibitor RAD001 with temozolomide and radiation effectively inhibits the growth of glioblastoma cells in culture.

The present in vitro study aimed to assess the effects of combining the mTOR inhibitor RAD001 and temozolomide (TMZ) together with irradiation by either low-linear energy transfer (LET) radiation (γ-rays) or high-LET radiation (fast neutrons) on the growth and cell survival of the human glioblastoma cell line U-87. We observed a strong decrease in cell proliferation along with a concomitant increase in cell death as a function of the radiation dose. As expected, high-LET radiation was more effective and induced more sustained damage to DNA than low-LET radiation. While RAD001 in association with TMZ induced autophagic cell death, additional combination with either type of radiation did not further increase its occurrence. On the contrary, apoptosis remained at a low level in all experimental groups.

The mTOR inhibitor RAD001 augments radiation-induced growth inhibition in a hepatocellular carcinoma cell line by increasing autophagy.

Treatment of hepatocellular carcinoma (HCC) is a major concern for physicians as its response to chemotherapy and radiotherapy remains generally poor, due, in part, to intrinsic resistance to either form of treatment. We previously reported that an irradiation with fast neutrons, which are high-linear energy transfer (LET) particles, massively induced autophagic cell death in the human HCC SK-Hep1 cell line. In the present study, we tested the capacity of the mammalian target of rapamycin (mTOR) inhibitor RAD001 to augment the cytotoxicity of low and high-LET radiation in these cells. As mTOR is a key component in a series of pathways involved in tumor growth and development, it represents a potential molecular target for cancer treatment. Results indicate that RAD001, at clinically relevant nanomolar concentrations, enhances the efficacy of both high- and low-LET radiation in SK-Hep1 cells, and that the induction of autophagy may account for this effect. However, fast neutrons were found to be more efficient at reducing tumor cell growth than low-LET radiation.

Cell death after high-LET irradiation in orthotopic human hepatocellular carcinoma in vivo.

Hepatocellular carcinoma (HCC) represents the sixth most common cancer worldwide and a major health problem since the choice of treatment is limited due to chemo- and radio-resistance. It was previously reported that high linear energy transfer (LET) radiation induced massive autophagic cell death in the human HCC SK-Hep1 cell line in vitro. This study analyzed the effects of high-LET radiation on the same HCC tumor model, orthotopically transplanted into nude mice. For this purpose, after surgical xenograft in the liver, animals were irradiated with fast neutrons and cell death occurring in the tumors was assessed with various techniques, including electron microscopy and probe-based confocal laser endomicroscopy. Results indicate that considerable autophagy and only limited apoptosis took place in the tumor xenografts after high-LET irradiation. These data confirm the previous in vitro results, suggesting that autophagy may act as a predominant mode of cell death in the efficacy of high-LET radiation.

Synthesis and radioprotective properties of pulvinic acid derivatives.

A high-throughput screening method has highlighted the marked antioxidant activity of some pulvinic acid derivatives (PADs) towards oxidation of thymidine, under γ and UV irradiation, and Fenton-like conditions. Here, we report the synthesis of a series of new hydrophilic PADs and the evaluation of their radioprotective efficacy in cell culture. Using a cell-based fluorescent assay, we show that some of these compounds have a pronounced ability to prevent cell death caused by radiation and to allow the subsequent resumption of proliferation. Thus, PADs may be considered as a novel class of radioprotective agents.

Pharmacological enhancement of autophagy induced in a hepatocellular carcinoma cell line by high-LET radiation.

The aim of the present study was to determine the cytotoxic consequences of high-linear energy transfer (LET) irradiation in the presence of oxaliplatin on hepatocellular carcinoma (HCC) cells in vitro. We attempted to correlate the induction of apoptosis and autophagy with the formation of DNA double-strand breaks (DSBs). SK-Hep1 cells were irradiated by 65 MeV neutrons in the presence of oxaliplatin and/or the poly(ADP-ribose) polymerase (PARP) inhibitor PJ34. DSBs were measured by the formation of gammaH2AX foci. Results show that in SK-Hep1 cells exposed to fast neutrons in the presence of oxaliplatin, DSBs occurred and persisted with time after irradiation. While apoptosis remained low in co-treated cells, autophagy was considerably increased after irradiation and augmented by the addition of oxaliplatin. Thus, autophagic cell death appears to play a prominent role in the cytotoxicity of the combined treatment and may be linked to the generation of heavy damage to DNA.

Evaluation of the dose distribution gradient in the close vicinity of brachytherapy seeds using electron paramagnetic resonance imaging.

Electron paramagnetic resonance (EPR) spectroscopy has been successfully employed to determine radiation dose using alanine. The EPR signal intensity reflects the number of stable free radicals produced, and provides a quantitative measurement of the absorbed dose. The aim of the present study was to explore whether this principle can be extended to provide information on spatial dose distribution using EPR imaging (EPRI). Lithium formate was selected because irradiation induces a single EPR line, a characteristic that is particularly convenient for imaging purposes. (125)I-brachytherapy seeds were inserted in tablets made of lithium formate. Images were acquired at 1.1 GHz. Monte Carlo (MC) calculations were used for comparison. The dose gradient can be determined using two-dimensional (2D) EPR images. Quantitative data correlated with the dose estimated by the MC simulations, although differences were observed. This study provides a first proof-of-concept that EPRI can be used to estimate the gradient dose distribution in phantoms after irradiation.

High-LET radiation combined with oxaliplatin induce autophagy in U-87 glioblastoma cells.

Modern protocols of concomitant chemo/radiotherapy provide a very effective strategy to treat certain types of tumors. High-linear energy transfer (LET) radiations, on the other hand, have an increased efficacy against cancer with low radiosensibility and critical localization. We previously reported that oxaliplatin, a third generation platinum drug, was able to reinforce the cytotoxicity of an irradiation by fast neutrons towards human glioblastoma U-87 cells in culture. We show here that such a combination has the capacity to enhance the number of double strand breaks in DNA and to induce autophagy in these cells. Xenografts experiments were further performed in nude mice subcutaneously transplanted with U-87 cells. When injected shortly before a single irradiation by fast neutrons, oxaliplatin causes a marked reduction of tumor growth compared with the irradiation alone. Overall, our data indicate the unique cytotoxic mechanism of a combined high-LET irradiation and oxaliplatin treatment modality and suggest its potential application in anticancer therapy.

The cytotoxicity of high-linear energy transfer radiation is reinforced by oxaliplatin in human glioblastoma cells.

The combination of high-linear energy transfer (LET) radiation with chemotherapeutic agents may offer new perspectives in cancer treatment. We therefore assessed the consequences of a treatment in which U-87 human glioblastoma cells were irradiated with p(65)+Be neutrons in the presence of oxaliplatin, a third generation platinum anticancer drug having higher apoptosis-inducing activity than cisplatin. Cell survival, apoptosis, cell cycle progression as well as p21 and p53 protein expressions were analyzed. Results show that an enhanced cytotoxic effect was obtained when the two treatments were combined and that, unlike what we previously observed with cisplatin, this was not due to a reinforcement of apoptosis. Altogether, our results also indicate the potential of oxaliplatin for use in association with high-LET radiation against tumors refractory to conventional photon radiotherapy.

Cell death induced in a human glioblastoma cell line by p(65)+Be neutrons combined with cisplatin.

High linear energy transfer (LET) radiation have the ability to kill cancer cells resistant to conventional radiotherapy. On the other hand, protocols combining radiotherapy and chemotherapy are effective in eradicating certain inoperable cancers. In this study, we investigated the cytotoxicity of a co-treatment with fast neutrons and cisplatin in a human glioblastoma cell line, U-87. Cells cultured in vitro were irradiated with p(65)+Be neutrons in the presence of cisplatin. Cell survival and the induction of apoptosis and premature senescence were assessed at different time intervals thereafter, using a variety of methods. A marked reinforcement of the cytotoxicity was obtained when irradiation and cisplatin were associated. This reflected both an amplification of the apoptotic process and the induction of premature cell senescence. The efficiency of a combination between fast neutrons and cisplatin in inducing cell death in U-87 is more than additive. The present data concur with those we previously reported in a mouse lymphoma and suggest the potential utility of platinum compounds as adjuncts to future cancer therapy protocols using high-LET radiation.

Fast neutrons-induced apoptosis is Fas-independent in lymphoblastoid cells.

We have previously shown that ionizing radiation-induced apoptosis in human lymphoblastoid cells differs according to their p53 status, and that caspase 8-mediated cleavage of BID is involved in the p53-dependent pathway. In the present study, we investigated the role of Fas signaling in caspase 8 activation induced by fast neutrons irradiation in these cells. Fas and FasL expression was assessed by flow cytometry and by immunoblot. We also measured Fas aggregation after irradiation by fluorescence microscopy. We found a decrease of Fas expression after irradiation, but no change in Fas ligand expression. We also showed that, in contrast to the stimulation of Fas by an agonistic antibody, Fas aggregation did not occur after irradiation. Altogether, our data strongly suggest that fast neutrons induced-apoptosis is Fas-independent, even in p53-dependent apoptosis.

Caspase 8-mediated cleavage of the pro-apoptotic BCL-2 family member BID in p53-dependent apoptosis.

The objective of this study was to characterize the apoptotic pathways activated by fast neutrons in the human lymphoblastoid cell line TK6 and in its p53 -/- derivative. Our results demonstrate that while p53 is not required for neutron-induced apoptosis, as previously shown, it does affect the kinetics of apoptosis and the molecular pathways leading to the activation of effector caspases. Indeed, rapid p53-dependent apoptosis was associated with the activation of caspase 9, 8, 3, and 7 and the cleavage of BID by caspase 8. In contrast, the slow-occurring p53-independent apoptotic process, mediated by caspase 7, took place without BID cleavage and loss of transmembrane mitochondrial potential. Altogether, our findings highlight an essential role for caspase 8-mediated BID cleavage, in the course of p53-dependent apoptosis triggered by fast neutrons in lymphoid cells. They also demonstrate that this mechanism is not involved in p53-independent apoptosis.

A dosimetry study comparing NCS report-5, IAEA TRS-381, AAPM TG-51 and IAEA TRS-398 in three clinical electron beam energies.

New codes of practice for reference dosimetry in clinical high-energy photon and electron beams have been published recently, to replace the air kerma based codes of practice that have determined the dosimetry of these beams for the past twenty years. In the present work, we compared dosimetry based on the two most widespread absorbed dose based recommendations (AAPM TG-51 and IAEA TRS-398) with two air kerma based recommendations (NCS report-5 and IAEA TRS-381). Measurements were performed in three clinical electron beam energies using two NE2571-type cylindrical chambers, two Markus-type plane-parallel chambers and two NACP-02-type plane-parallel chambers. Dosimetry based on direct calibrations of all chambers in 60Co was investigated, as well as dosimetry based on cross-calibrations of plane-parallel chambers against a cylindrical chamber in a high-energy electron beam. Furthermore, 60Co perturbation factors for plane-parallel chambers were derived. It is shown that the use of 60Co calibration factors could result in deviations of more than 2% for plane-parallel chambers between the old and new codes of practice, whereas the use of cross-calibration factors, which is the first recommendation in the new codes, reduces the differences to less than 0.8% for all situations investigated here. The results thus show that neither the chamber-to-chamber variations, nor the obtained absolute dose values are significantly altered by changing from air kerma based dosimetry to absorbed dose based dosimetry when using calibration factors obtained from the Laboratory for Standard Dosimetry, Ghent, Belgium. The values of the 60Co perturbation factor for plane-parallel chambers (k(att) x k(m) for the air kerma based and p(wall) for the absorbed based codes of practice) that are obtained from comparing the results based on 60Co calibrations and cross-calibrations are within the experimental uncertainties in agreement with the results from other investigators.

B(C6F5)3-catalyzed formation of B-P bonds by dehydrocoupling of phosphine-boranes.

Tris(pentafluorophenyl)borane was used as a new catalyst in the formation of P-B bonds by dehydrocoupling of phosphine-boranes.

Carbon ions-induced apoptosis in hematopoietic tumor cell lines.

BACKGROUND: The purpose of this study was to assess apoptosis in hematopoietic tumor cells irradiated with carbon ions, in order to define its contribution to the cytotoxicity of these high-LET radiations. MATERIALS AND METHODS: RDM4 (murine T lymphoma), MOLT-4, TK6 and WTK1 (human lymphoblastoid) cells were irradiated with 12C or 13C. Apoptosis was assessed by flow cytometry. Cell growth and activities of caspases were determined during the same periods. RESULTS: The ability of carbon ions to induce apoptosis markedly varied according to the cell line. MOLT-4 and TK6 cells underwent apoptosis at 1 Gy within 12 hours post-irradiation, whereas RDM4 and WTK1 showed little apoptosis under the same conditions. CONCLUSION: These results indicate that apoptosis contributes to the overall cytotoxicity of carbon ions towards hematopoietic tumor cells although other death mechanisms must also account for this cytotoxicity.

Sensitivity of whole human teeth to fast neutrons and gamma-rays estimated by L-band EPR spectroscopy.

This paper reports the first attempt to use L-band spectroscopy for estimating the sensitivity of whole teeth to fast neutrons and gamma-rays. Three teeth were successively irradiated first with fast neutrons with a wide energy spectrum (mean energy around 30 MeV) up to approximately 160 Gy and then with gamma-rays up to approximately 14 Gy. After each irradiation, L-band (approximately 1 GHz) EPR spectra of each whole tooth surrounded by the surface-coil resonator were recorded, yielding a single composite line principally due to CO2- and native radicals. The sensitivities are estimated by the slopes of the linear dose response curves of the dosimetric CO2- radicals. The ratios of the gamma/neutron sensitivities were found to be in the range 8-9 (+/- 2) for the three teeth.

Fluence correction factors in plastic phantoms for clinical proton beams.

In recent codes of practice for reference dosimetry in clinical proton beams using ionization chambers, it is recommended to perform the measurement in a water phantom. However, in situations where the positioning accuracy is very critical, it could be more convenient to perform the measurement in a plastic phantom. In proton beams, a similar approach as in electron beams could be applied by introducing fluence correction factors in order to account for the differences in particle fluence distributions at equivalent depths in plastic and water. In this work, fluence correction factors as a function of depth were determined for proton beams with different energies using the Monte Carlo code PTRAN for PMMA and polystyrene with reference to water. The influence of non-elastic nuclear interaction cross sections was investigated. It was found that differences in proton fluence distributions are almost entirely due to differences in non-elastic nuclear interaction cross sections between the plastic materials and water. For proton beams with energies lower than 100 MeV, for which the contributions from non-elastic interactions become small compared to the total dose, the fluence corrections are smaller than 1%. For beams with energies above 200 MeV, depending on the cross sections dataset for non-elastic nuclear interactions, fluence corrections of 2-5% were found at the largest depths. The results could, with an acceptable accuracy, be represented as a correction per cm penetration of the beam, yielding values between 0.06% and 0.15% per cm for PMMA and 0.06% to 0.20% per cm for polystyrene. Experimental information on these correction factors was obtained from depth dose measurements in PMMA and water. The experiments were performed in 75 MeV and 191 MeV non-modulated and range-modulated proton beams. From the experiments, values ranging from 0.03% to 0.15% per cm were obtained. A decisive answer about which dataset for non-elastic nuclear interactions would result in a better representation of the measurements could not be given. We conclude that below 100 MeV, dosimetry could be performed in plastic phantoms without a dramatic loss of accuracy. On the other hand, in clinical high-energy proton beams, where accurate positioning in water is in general not an issue, substantial correction factors would be required for converting dose measurements in a plastic phantom to absorbed dose to water. It is therefore not advisable to perform absorbed dose measurements nor to measure depth dose distributions in a plastic phantom in high-energy proton beams.

Dosimetry using plane-parallel ionization chambers in a 75 MeV clinical proton beam.

Reference ionization chamber dosimetry in clinical proton beams is generally performed with cylindrical ionization chambers. However, when the measurement is performed in the presence of a large depth dose gradient or in a narrow spread out Bragg peak (SOBP), it could be advisable to use a plane-parallel chamber. Few recommendations and studies have been devoted to this subject. In this paper, experimental information on perturbation correction factors for four plane-parallel ionization chamber types in proton beams is presented. The experiments were performed in 75 MeV modulated and non-modulated proton beams. Monte Carlo calculations have been performed to support the conclusions of the experimental work. Overall, we were not able to find experimental evidence for significant differences between the secondary electron perturbation correction factors for plane-parallel chambers and those for a cylindrical NE2571. We found experimental ratios of perturbation correction factors that did not differ by more than 0.6% from unity for a Roos and two NACP02 chambers, and by not more than 1.2% for a Calcam-2 and two Markus chambers. Monte Carlo simulations result in corrections that are limited to 0.6% in absolute value, but given the overall uncertainties of the measurements, the deviations of the correction factors from unity could not be resolved from the experimental results. The results of the simulations thus support the experimental conclusion that perturbation correction factors for the set of plane-parallel chambers in both proton beams (relative to NE2571) do not deviate from unity by more than 1.2%. This confirms, within the experimental uncertainties, the assumption that the overall perturbation correction factor for a plane-parallel chamber in a low-energy proton beam is unity, made in IAEA TRS-398 and other dosimetry protocols. Given the large uncertainties of the gradient correction factors to be applied when using a cylindrical ionization chamber in a narrow SOBP or in the presence of a strong depth dose gradient, the level of agreement between plane-parallel and cylindrical ionization chambers observed in this study shows that plane-parallel chambers are a reliable alternative for reference dosimetry in low-energy proton beams.