Contamination dose from photoneutron processes in bodily tissues during therapeutic radiation delivery

NOVEL 'PHOTONEUTRON VOLUME DOSE EQUIVALENT' HYPOTHESIS AND METHODOLOGY FOR SECOND PRIMARY CANCER RISK ESTIMATION IN HIGH-ENERGY X-RAY MEDICAL ACCELERATORS

A novel 'photoneutron (PN) volume dose equivalent' methodology was hypothesized and applied for the first time for estimating PN second primary cancer (PN-SPC) risks in high-energy X-ray medical accelerators. Novel position-sensitive mega-size polycarbonate dosimeters with 10B converter (with or without cadmium covers) were applied for determining fast, epithermal and thermal PN dose equivalents at positions on phantom surface and depths. The methodology was applied to sites of tumors such as brain, stomach and prostate in 47 patients. The PN-SPC risks were estimated for specific organs/tissues using linear International Commission on Radiological Protection cancer risks and were compared with some available data. The corresponding PN-SPC risk estimates ranged from 1.450 × 10-3 to 1.901 cases per 10 000 persons per Gray. The method was applied to 47 patients for estimating PN-SPC risks in patients undergoing radiotherapy. The PN-SPC risk estimates well match those calculated by simulation but are comparatively different from those estimated by 'PN point dose equivalent' methods, as expected.

Simultaneous induction of dispersed and clustered DNA lesions compromises DNA damage response in human peripheral blood lymphocytes

Due to its ability to induce DNA damage in a space and time controlled manner, ionising radiation is a unique tool for studying the mechanisms of DNA repair. The biological effectiveness of ionising radiation is related to the ionisation density which is defined by the linear energy transfer (LET). Alpha particles are characterised by high LET, while X-rays by low LET values. An interesting question is how cells react when exposed to a mixed beam of high and low LET radiation. In an earlier study carried out with human peripheral blood lymphocytes (PBL) we could demonstrate that alpha radiation X-rays interact in producing more chromosomal aberrations than expected based on additivity. The aim of the present investigation was to look at the mechanism of the interaction, especially with respect to the question if it is due to an augmented level of initial damage or impaired DNA repair. PBL were exposed to various doses of alpha particles, X-rays and mixed beams. DNA damage and the kinetics of damage repair was quantified by the alkaline comet assay. The levels of phosphorylated, key DNA damage response (DDR) proteins ATM, p53 and DNA-PK were measured by Western blotting and mRNA levels of 6 damage-responsive genes were measured by qPCR. Alpha particles and X-rays interact in inducing DNA damage above the level predicted by assuming additivity and that the repair of damage occurs with a delay. The activation levels of DDR proteins and mRNA levels of the studied genes were highest in cells exposed to mixed beams. The results substantiate the idea that exposure to mixed beams presents a challenge for the cellular DDR system.

Calculation of Nuclear Particles Production at High-Energy Photon Beams from a Linac Operating at 6, 10 and 15 MV

Production of photonuclear particles in a tissue-equivalent medium has been calculated for linacs at 6, 10 and 15 MV from Varian TrueBeam. Based on the knowledge of bremsstrahlung fluence spectra and linac photon beam parameters, numerical integration was performed on the cross sections for photoparticle production of the constituent elements of tissue (2H,12C,13C,16O,17O,18O,14N,15N). At 15 MV, at the depth of photon maximum dose, the total absorbed dose due to neutrons, protons, alphas and residual nuclei from photon reactions in tissue (5.5E-05 Gy per Gy of photons) is comparable to that due to neutrons from accelerator head. Results reasonably agree with data reported in the literature using Monte Carlo models simulating linac head components. This work suggests a simple method to estimate the dose contributed by the photon-induced nuclear particles for high-energy photon beams produced by linacs in use, as it might be relevant for late stochastic effects.

Gene expression responses in human lung fibroblasts exposed to alpha particle radiation

This study examined alpha (α-) particle radiation effects on global changes in gene expression for the purposes of identifying potential signaling pathways that may be involved in Radon ((222)Rn) gas exposure and lung carcinogenesis. Human lung fibroblast cells were exposed to α-particle radiation at a dose range of 0-1.5Gy. Twenty-four hours post-exposure, transcript modulations were monitored using microarray technology. A total of 208 genes were shown to be dose-responsive (FDR adjusted p<0.05, Fold change>|2|) of which 32% were upregulated and 68% downregulated. Fourteen of the high expressing genes (>|4| fold) were further validated using alternate technology and among these genes, GDF15 and FGF2 were assessed at the protein level. GDF15, a known marker of lung injury, had expression levels 3-fold higher in exposed cell culture media, 24h post-irradiation as detected by ELISA. Further, pathway analysis of the dose-responsive transcripts showed them to be involved in biological processes related to cell cycle control/mitosis, chromosome instability and cell differentiation. This panel of genes with particular focus on GDF15 may merit further analysis to determine their specific role in mechanisms leading to α-particle induced lung carcinogenesis.

Gamma-H2AX foci in cells exposed to a mixed beam of X-rays and alpha particles

Background

Little is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation. So far, the effects of combined exposures have mainly been assessed with clonogenic survival or cytogenetic methods, and the results are contradictory. The gamma-H2AX assay has up to now not been applied in this context, and it is a promising tool for investigating the early cellular response to mixed beam irradiation.

Purpose

To determine the dose response and repair kinetics of gamma-H2AX ionizing radiation-induced foci in VH10 human fibroblasts exposed to mixed beams of ²⁴¹Am alpha particles and X-rays.

Results

VH10 human fibroblasts were irradiated with each radiation type individually or both in combination at 37°C. Foci were scored for repair kinetics 0.5, 1, 3 and 24 h after irradiation (one dose per irradiation type), and for dose response at the 1 h time point. The dose response effect of mixed beam was additive, and the relative biological effectiveness for alpha particles (as compared to X-rays) was of 0.76 ± 0.52 for the total number of foci, and 2.54 ± 1.11 for large foci. The repair kinetics for total number of foci in cells exposed to mixed beam irradiation was intermediate to that of cells exposed to alpha particles and X-rays. However, for mixed beam-irradiated cells the frequency and area of large foci were initially lower than predicted and increased during the first 3 hours of repair (while the predicted number and area did not).

Conclusions

The repair kinetics of large foci after mixed beam exposure was significantly different from predicted based on the effect of the single dose components. The formation of large foci was delayed and they did not reach their maximum area until 1 h after irradiation. We hypothesize that the presence of low X-ray-induced damage engages the DNA repair machinery leading to a delayed DNA damage response to the more complex DNA damage induced by alpha particles.

Characterisation of a setup for mixed beam exposures of cells to 241Am alpha particles and X-rays

Exposure of humans to mixed fields of high- and low-linear energy transfer (LET) radiation occurs in many situations-for example, in urban areas with high levels of indoor radon as well as background gamma radiation, during airplane flights or certain forms of radiation therapy. From the perspective of health risk associated with exposure to mixed fields, it is important to understand the interactions between different radiation types. In most cellular investigations on mixed beams, two types of irradiations have been applied sequentially. Simultaneous irradiation is the desirable scenario but requires a dedicated irradiation facility. The authors have constructed a facility where cells can be simultaneously exposed to (241)Am alpha particles and 190-kV X-rays at 37°C. This study presents the technical details and the dosimetry of the setup, as well as validates the performance of the setup for clonogenic survival in AA8 Chinese hamster ovary cells. No significant synergistic effect was observed. The relative biological effectiveness of the alpha particles was 2.56 for 37 % and 1.90 for 10 % clonogenic survival.

Micronuclei in human peripheral blood lymphocytes exposed to mixed beams of X-rays and alpha particles

The purpose of this study was to analyse the cytogenetic effect of exposing human peripheral blood lymphocytes (PBL) to a mixed beam of alpha particles and X-rays. Whole blood collected from one donor was exposed to different doses of alpha particles ((241)Am), X-rays and a combination of both. All exposures were carried out at 37 °C. Three independent experiments were performed. Micronuclei (MN) in binucleated PBL were scored as the endpoint. Moreover, the size of MN was measured. The results show that exposure of PBL to a mixed beam of high and low linear energy transfer radiation led to significantly higher than expected frequencies of MN. The measurement of MN size did not reveal any differences between the effect of alpha particles and mixed beam. In conclusion, a combined exposure of PBL to alpha particles and X-rays leads to a synergistic effect as measured by the frequency of MN. From the analysis of MN distributions, we conclude that the increase was due to an impaired repair of X-ray-induced DNA damage.

Patient-specific internal radionuclide dosimetry

The development of patient-specific treatment planning systems is of outmost importance in the development of radionuclide dosimetry, taking into account that quantitative three-dimensional nuclear medical imaging can be used in this regard. At present, the established method for dosimetry is based on the measurement of the biokinetics by serial gamma-camera scans, followed by calculations of the administered activity and the residence times, resulting in the radiation-absorbed doses of critical organs. However, the quantification of the activity in different organs from planar data is hampered by inaccurate attenuation and scatter correction as well as because of background and organ overlay. In contrast, dosimetry based on quantitative three-dimensional data can be more accurate and allows an individualized approach, provided that all effects that degrade the quantitative content of the images have been corrected for. In addition, inhomogeneous organ accumulation of the radionuclide can be detected and possibly taken into account. The aim of this work is to provide adequate information on internal emitter dosimetry and a state-of-the-art review of the current methodology and future trends.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.

Monte Carlo simulation estimates of neutron doses to critical organs of a patient undergoing 18MV x-ray LINAC-based radiotherapy

Absorbed photoneutron dose to patients undergoing 18 MV x-ray therapy was studied using Monte Carlo simulations based on the MCNPX code. Two separate transport simulations were conducted, one for the photoneutron contribution and another for neutron capture gamma rays. The phantom model used was of a female patient receiving a four-field pelvic box treatment. Photoneutron doses were determinate to be higher for organs and tissues located inside the treatment field, especially those closest to the patient's skin. The maximum organ equivalent dose per x-ray treatment dose achieved within each treatment port was 719 microSv/Gy to the rectum (180 degrees field), 190 microSv/Gy to the intestine wall (0 degrees field), 51 microSv/Gy to the colon wall (90 degrees field), and 45 microSv/Gy to the skin (270 degrees field). The maximum neutron equivalent dose per x-ray treatment dose received by organs outside the treatment field was 65 microSv/Gy to the skin in the antero-posterior field. A mean value of 5 +/- 2 microSv/Gy was obtained for organs distant from the treatment field. Distant organ neutron equivalent doses are all of the same order of magnitude and constitute a good estimate of deep organ neutron equivalent doses. Using the risk assessment method of the ICRP-60 report, the greatest likelihood of fatal secondary cancer for a 70 Gy dose is estimated to be 0.02% for the pelvic postero-anterior field, the rectum being the organ representing the maximum contribution of 0.011%.

Effects of field parameters on IMRT plan quality for gynecological cancer: a case study

Traditional external beam radiotherapy of gynecological cancer consists of a 3D, four‐field‐box technique. The radiation treatment area is a large region of normal tissue, with greater inhomogeneity over the treatment volume, which could benefit more with intensity‐modulated radiation therapy (IMRT). This is a case report of IMRT planning for a patient with endometrial cancer. The planning target volume (PTV) spanned the intrapelvic and periaortic lymph nodes to a 33‐cm length. Planning and treatment were accomplished using double isocenters. The IMRT plan was compared with a 3D plan, and the effects of field parameters were studied. Delineated anatomical contours included the intrapelvic nodes (PTV), bone marrow, small bowel, bladder, rectum, sigmoid colon, periaortic nodes (PTV), spinal cord, left kidney, right kidney, large bowel, liver, and tissue (excluding the PTVs). Comparisons were made between IMRT and 3D plans, 23‐MV and 6‐MV energies, zero and rotated collimator angles, different numbers of segments, and opposite gantry angle configurations. The plans were evaluated based on dose‐volume histograms (DVHs). Compared with the 3D plan, the IMRT plan had superior dose conformity and spared the bladder and sigmoid colon embedded in the intrapelvic nodes. The higher energy (23 MV) reduced the dose to most critical organs and delivered less integral dose. Zero collimator angles resulted in a better plan than “optimized” collimator angles, with lower dose to most of the normal structures. The number of segments did not have much effect on isodose distribution, but a reasonable number of segments was necessary to keep treatment time from being prohibitively long. Gantry angles, when evenly spaced, had no noticeable effect on the plan. The patient tolerated the treatment well, and the initial complete blood count was favorable. Our results indicated that large‐volume tumor sites may also benefit from precise conformal delivery of IMRT.

PACS numbers: 87.53.Kn, 87.53.Tf