Kilovoltage X-ray Research Articles

Evaluating TOPAS for the calculation of backscatter factors for low energy x-ray beams

For low x-ray energies, backscatter is an important parameter for determining the absorbed dose, making accurate knowledge of backscatter factors (BSFs) essential. BSFs can be difficult to measure experimentally so published values are often derived using Monte Carlo methods. This study evaluated the Monte Carlo code TOPAS as a tool for dose calculations for kilovoltage x-rays, and for calculating BSFs for energies ranging from 50 to 280 kVp. BSFs were also measured experimentally for comparison using Gafchromic EBT3 film and optically stimulated luminescence dosimeters (OSLs). The BSFs calculated using TOPAS were found to be consistent within 2% of the values published in the AAPM TG-61 protocol. The TOPAS BSF calculations were also found to be consistent with film measurements, typically within 2%. The largest discrepancy measured was 5% for the 3 cm field size and 180 kVp beam. OSLs were found to overestimate BSFs for large field sizes and high energies so were found to be unsuitable for BSF measurements with differences of up to 18%.

Ionization chamber dosimetry based on 60Co absorbed dose to water calibration for diagnostic kilovoltage x-ray beams

The purpose of this study was to develop a new ionization chamber dosimetry formalism for diagnostic kilovoltage x-ray beams based on a 60Co absorbed dose-to-water calibration coefficient . To validate the new chamber dosimetry, 39 fluence spectra of kilovoltage x-rays were calculated, using a SpekCalc program, for Al half-value-layer (Al-HVL) values of 1.4–8.5 mm and tube voltages of 50–137.6 kVp. We used these spectra to calculate the beam quality conversion factor for kilovoltage x-ray beams Qx to a 60Co beam for a PTW 30013 Farmer® ionization chamber in a reference geometrical setup (depth = 1 cm in water, field size = 20 × 20 cm2). The absorbed dose to water for kilovoltage x-rays was obtained using and . Meanwhile, the water surface dose based on the air kerma calibration coefficient NK was obtained using a Monte Carlo-calculated correction factor CF (corresponding to in AAPM TG-61) for the 39 x-ray fluence spectra. The absorbed dose based on was validated by comparing the water surface dose for the 39 x-ray fluence spectra with the water surface dose based on NK. The water surface dose based on was obtained using Monte Carlo-calculated percentage depth doses. For Al-HVL ranging from 1.4 to 8.5 mm, ranged from 0.884 to 0.956 and CF ranged from 1.264 to 1.663. The water surface dose based on for the x-ray fluence spectra was 1.0% higher on average than that based on NK, except for Al-HVLs of 1.4 and 1.5 mm and 1% was within their uncertainty ranges. Therefore, it is possible to use the absorbed dose-to-water value based on instead of on NK. With the new chamber dosimetry, the chamber reading can be easily converted to the absorbed dose using and the calculated .

Iterative Material Decomposition Method Eliminating Photoelectric Effect for Dual-MeV Energy Computed Tomography

Photons in kilovolt (kV) X-ray dual-energy computed tomography (DECT) interact with matter mainly by photoelectric effect and Compton scatter. Any material’s mass attenuation coefficient (MAC) function is composed of these two effects. Thus, from a function space perspective, any MAC function can be written as a linear combination of the MAC functions of two basis materials. The projection-domain method based on this decomposition model is fit for two data sets of kV DECT. However, photons in megavolt (MV) X-ray DECT interact with matter through another way, pair production. Accordingly, three basis functions are needed to delineate this 3-D function space. Direct implementation of the projection-domain method with the decomposition model of two basis materials is feasible in spite of the unsatisfactory results. However, the projection-domain method based on the decomposition model of three basis materials becomes unsolvable, because the number of unknowns exceeds the number of data sets. Several methods add mass or volume conservation constraint to make the problem solvable, but they are not appropriate for MV DECT. We propose a new method in which the influence of photoelectric effect is numerically eliminated to regain the consistency among the decomposition model, physics model, and data sets of MV DECT. An iteration framework based on the projection-domain method with the decomposition model of two basis materials is designed to achieve this goal. The results from the numerical experiments show that the proposed method provides better decomposition coefficients and material information. Although our method is theoretically designed for element objects, water as a common compound can also be accurately decomposed. Moreover, beside the projection-domain method, the proposed one here can also be applied with other processing methods.

Development and implementation of EPID-based quality assurance tests for the small animal radiation research platform (SARRP).

Although small animal image-guided radiotherapy (SA-IGRT) systems are used increasingly in preclinical research, tools for performing routine quality assurance (QA) have not been optimized and are not readily available. Robust, efficient, and reliable QA tools are needed to ensure the accuracy and reproducibility of SA-IGRT systems. Several investigators have reported custom-made phantoms and protocols for SA-IGRT systems QA. These are typically time and resource intensive and are therefore not well suited to the preclinical radiotherapy environment, in which physics support is limited and routine QA is performed by technical staff. We investigated the use of the inbuilt electronic portal imaging device (EPID) to develop and validate routine QA tests and procedures. In this work, we focus on the Xstrahl Small Animal Radiation Research Platform (SARRP) EPID. However, the methodology and tests developed here are applicable to any SA-IGRT system that incorporates an EPID. We performed a comprehensive characterization of the dosimetric properties of the camera-based EPID at kilovoltage energies over a 11-month period, including detector warm-up time, radiation dose history effect, stability and short- and long-term reproducibility, gantry angle dependency, output factor, and linearity of the EPID response. We developed a test to measure the constancy of beam quality in terms of half-value layer and tube peak potential using the EPID. We verified the SARRP daily output and beam profile constancy using the imager. We investigated the use of the imager to monitor beam-targeting accuracy at various gantry and couch angles. The EPID response was stable and reproducible, exhibiting maximum variations of ≤0.3% and ≤1.9% for short and long terms, respectively. The detector showed no dependence on response at different gantry angles, with a maximum variation ≤0.5%. We found close agreement in output factor measurement between the portal imager and reference dosimeters, with maximum differences ≤3% for ionization chamber and ≤1.7% for Gafchromic EBT3 dosimetry film, respectively. We have shown that the EPID response is linear with tube current (mA) for the entire range of tube kilovoltage peak. Notably, a close relationship was seen between the detector response vs mA slope, and the kilovoltage peak, allowing an independent verification of kilovoltage peak stability based solely on EPID response. In addition to dosimetry tests, according to the beam-targeting measurement using portal images, maximum displacement of the central axis of the x-ray beam (due to sag) was 0.76±0.09mm at gantry 135°/couch 0° and 0.89±0.06mm at gantry 0°/couch -135°. We performed the first comprehensive analysis on the dosimetric properties of an EPID operating at kilovoltage x-ray energies. We characterized the detector performance over a 11-month period. Our results indicate that the imager is a stable and convenient tool for SARRP routine QA tests. We then developed EPID-based tests to perform routine SA-IGRT systems QA tasks, such as verifying constancy of beam quality, energy, output, and profile measurements, relative output factors, and beam targeting.

Automatic online and real-time tumour motion monitoring during stereotactic liver treatments on a conventional linac by combined optical and sparse monoscopic imaging with kilovoltage x-rays (COSMIK)

The purpose of this study was to develop, validate and clinically demonstrate fully automatic tumour motion monitoring on a conventional linear accelerator by combined optical and sparse monoscopic imaging with kilovoltage x-rays (COSMIK).COSMIK combines auto-segmentation of implanted fiducial markers in cone-beam computed tomography (CBCT) projections and intra-treatment kV images with simultaneous streaming of an external motion signal. A pre-treatment CBCT is acquired with simultaneous recording of the motion of an external marker block on the abdomen. The 3-dimensional (3D) marker motion during the CBCT is estimated from the auto-segmented positions in the projections and used to optimize an external correlation model (ECM) of internal motion as a function of external motion. During treatment, the ECM estimates the internal motion from the external motion at 20 Hz. KV images are acquired every 3 s, auto-segmented, and used to update the ECM for baseline shifts between internal and external motion. The COSMIK method was validated using Calypso-recorded internal tumour motion with simultaneous camera-recorded external motion for 15 liver stereotactic body radiotherapy (SBRT) patients. The validation included phantom experiments and simulations hereof for 12 fractions and further simulations for 42 fractions. The simulations compared the accuracy of COSMIK with ECM-based monitoring without model updates and with model updates based on stereoscopic imaging as well as continuous kilovoltage intrafraction monitoring (KIM) at 10 Hz without an external signal. Clinical real-time tumour motion monitoring with COSMIK was performed offline for 14 liver SBRT patients (41 fractions) and online for one patient (two fractions).The mean 3D root-mean-square error for the four monitoring methods was 1.61 mm (COSMIK), 2.31 mm (ECM without updates), 1.49 mm (ECM with stereoscopic updates) and 0.75 mm (KIM).COSMIK is the first combined kV/optical real-time motion monitoring method used clinically online on a conventional accelerator. COSMIK gives less imaging dose than KIM and is in addition applicable when the kV imager cannot be deployed such as during non-coplanar fields.

Synthesis of gold nanomaterials and their cancer-related biomedical applications: an update.

Recently, the advances in the synthesis of new types of nanomaterials have created several opportunities in drug delivery and targeted therapy applications. Among the various nanostructures, gold nanostructures with controllable physical and chemical properties have received attention for various biomedical uses, including sensing of biomolecules, in vitro and in vivo bioimaging (as advanced contrast agents for photothermal and bioimaging techniques), photothermolysis of cancer cells, and targeted drug delivery. The attractive properties of gold nanomaterials, particularly, anti-angiogenic properties, are highly useful in a variety of cancers studies. In addition, they can bind many proteins and drugs and can be actively targeted to cancer cells over-expressing cell surface receptors and they are biocompatible in nature with a high atomic number, which directs to greater absorption of kilovoltage X-rays and provides greater contrast than standard agents. In this review, we have summarized the synthesis, structure and functionalization of gold nanostructures, and their biomedical applications with special reference to cancer studies.

Evaluation of the positional differences between bone matching and fiducial marker matching in target localization of the expiratory breath-hold radiation therapy for pancreatic cancer.

517 Background: In radiation therapy, the accuracy of target localization is very important to increase the agreement between the planned treatment and the actual dose delivered to the patient. Therefore, image-guided techniques, such as bone matching, soft-tissue matching, fiducial marker (FM) matching and so on, are used to improve the precision and effectiveness of radiation therapy. In this study, we evaluate the positional differences of the target localization between bone matching and FM matching in the expiratory breath-hold radiation therapy (BH-RT) for pancreatic cancer. Methods: We analyzed 15 patients who underwent BH-RT for pancreatic cancer using FM matching between July 2016 and August 2017 in our institution. The FMs were implanted in the pancreatic cancer using endoscopic ultrasound. Prior to each treatment session, bone matching using orthogonal kilovolt x-ray images and FM matching using cone beam computed tomography at end-exhalation (BH-CBCT) was performed. A total of 226 matching data from 15 patients was analyzed. The positional differences of these matching were evaluated in the left - right (LR), anterior - posterior (AP) and superior - inferior (SI) directions. Results: The systematic and random errors between bone matching and FM matching were 3.6 and 3.0 mm, 3.3 and 2.0 mm, and 3.7 and 3.5 mm in the LR, AP and SI directions, respectively. Absolute positional differences of > 5 mm were observed in 78 fractions (34.5%), 40 fractions (17.7%), and 69 fractions (30.5%), and those of even > 10 mm were observed in 14 fractions (6.2%), 2fractions (0.6%), and 11 fractions (4.9%) in each direction, respectively. There were no significant correlations between positional differences in SI and both LR and AP. Conclusions: In pancreatic irradiation using BH-RT, it was considered that bone matching alone was insufficient for accurate target localization and other target matching techniques, such as FM matching, were required to achieve effective treatment.

A study on the suitability of the PTW microDiamond detector for kilovoltage x-ray beam dosimetry

Kilovoltage x-ray beams are widely used in treating skin cancers and in biological irradiators. In this work, we have evaluated four dosimeters (ionization chambers and solid state detectors) in their suitability for relative dosimetry of kilovoltage x-ray beams in the energy range of 50 – 280kVp. The solid state detectors, which have not been investigated with low energy x-rays, were the PTW 60019 microDiamond synthetic diamond detector and the PTW 60012 diode. The two ionization chambers used were the PTW Advanced Markus parallel plate chamber and the PTW PinPoint small volume chamber. For each of the dosimeters, percentage depth doses were measured in water over the full range of x-ray beams and for field sizes ranging from 2cm diameter to 12 × 12cm. In addition, depth doses were measured for a narrow aperture (7mm diameter) using the PTW microDiamond detector. For comparison, the measured data was compared with Monte Carlo calculated doses using the EGSnrc Monte Carlo package. The depth dose results indicate that the Advanced Markus parallel plate and PinPoint ionization chambers were suitable for depth dose measurements in the beam quality range with an uncertainty of less than 3%, including in the regions closer to the surface of the water as compared with Monte Carlo depth dose data for all six energy beams. The response of the PTW Diode E detector was accurate to within 4% for all field sizes in the energy range of 50–125kVp but showed larger variations for higher energies of up to 12% with the 12 × 12cm field size. In comparison, the microDiamond detector had good agreement over all energies for both smaller and larger field sizes generally within 1% as compared to the Advanced Markus chamber field and Monte Carlo calculations. The only exceptions were in measuring the dose at the surface of the water phantom where larger differences were found. For the 7mm diameter field, the agreement between the microDiamond detector and Monte Carlo calculations was good being better than 1% except at the surface. Based on these results, the PTW microDiamond detector has shown to be a suitable detector for relative dosimetry of low energy x-ray beams over a wide range of x-ray beam energies.

Improvement of kilovoltage beam output with a transmission x-ray target: radiological optimization and cooling system design

The purpose of this work is to introduce the design of a transmission x-ray target suitable to be used in kilovoltage x-ray tubes such as those employed in orthovoltage x-ray therapy. The target consists of a 20 μm thick tungsten slab deposited on top of a 1 mm copper plate. Monte Carlo simulation is used to determine both the energy absorbed in the tungsten and copper slabs, and also to calculate the characteristics of the resultant x-ray production. The absorbed energy distribution in the target is then fed into a finite element software to determine the heat distribution and its evolution in time as a function of both the electron current incident on the target and the focal spot size. A cooling system is proposed and modeled based on these results to prevent the target assembly from melting. Absorbed dose rates in water are also calculated for both the proposed transmission and a standard reflection target as a function of the incident electron current. We show that, compared to the reflection target, our design increases by more than 100% the emission of x-rays, and that the proposed cooling system is capable of operating at a sustained power rating above 4 kW.

Kilovoltage radiotherapy for companion animals: dosimetric comparison of 300 kV, 450 kV, and 6 MV X-ray beams.

Radiotherapy for the treatment of cancer in companion animals is currently administered by using megavoltage X-ray machines. Because these machines are expensive, most animal hospitals do not perform radiotherapy. This study evaluated the ability of relatively inexpensive kilovoltage X-ray machines to treat companion animals. A simulation study based on a commercial treatment-planning system was performed for tumors of the brain (non-infectious meningoencephalitis), nasal cavity (malignant nasal tumors), forefoot (malignant muscular tumors), and abdomen (malignant intestinal tumors). The results of kilovoltage (300 kV and 450 kV) and megavoltage (6 MV) X-ray beams were compared. Whereas the 300 kV and 6 MV X-ray beams provided optimal radiation dose homogeneity and conformity, respectively, for brain tumors, the 6 MV X-rays provided optimal homogeneity and radiation conformity for nasal cavity, forefoot, and abdominal tumors. Although megavoltage X-ray beams provided better radiation dose distribution in most treated animals, the differences between megavoltage and kilovoltage X-ray beams were relatively small. The similar therapeutic effects of the kilovoltage and 6 MV X-ray beams suggest that kilovoltage X-ray beams may be effective alternatives to megavoltage X-ray beams in treating cancers in companion animals.

Detection of Spherical Gold Fiducials in kV X-Ray Images Using Intensity-Estimation-Based Method

Fiducial marker detection algorithms in kilovoltage x-ray images using physical characteristics of transmission x-ray have been proposed. It, however, has been suggested recently that factors besides transmission x-ray affect x-ray images. The purpose of this study was to develop a new fiducial detection algorithm using fiducial intensity estimation based on physical characteristics of x-ray images with gold fiducials. First, x-ray images of a fiducial on a water-equivalent phantom were acquired. It was observed that the ratio of background to fiducial intensity in the images decreased as phantom thickness increased. Based on the negative correlation, we identified a function for estimating fiducial intensity that consists of background intensity and the amount of scattered radiation by the other x-ray source of an orthogonal imaging system and a treatment beam. Then, we developed an algorithm that extracts fiducial candidates using the estimation function. Its performance was measured using x-ray images which had 3824 fiducials altogether. The average number of false-positive detection of the proposed algorithm in single image was one-tenth of an algorithm considering only transmission x-ray. The proposed algorithm detected 99.5% of all fiducials under an error of 1.0 mm, while the other algorithm detected 94.7% or less (Clinical trial number: UMIN000005324).

Sequential delivery of bismuth nanoparticles and doxorubicin by injectable macroporous hydrogels for combined anticancer kilovoltage X-ray radio- and chemo-therapy.

Sequential delivery systems are required to maximize synergistic anticancer therapeutic effects in combined X-ray radio- and chemo-therapy. Here, we described an injectable macroporous hydrogel as a sequential delivery platform for combined kilovoltage X-ray radio- and chemo-therapy of 4T1 breast cancer. The macroporous hydrogel offered two sequentially distinct delivery profiles for co-loaded radiosensitizers (bismuth nanoparticles, Bi NPs) and an anticancer drug (doxorubicin, DOX). Bi NPs were released preferentially and completely over 24 h; however, DOX was released slowly in the first 24 h and was sustainedly released over one week. This sequential release behavior of Bi NPs and DOX in macroporous hydrogels achieved significantly synergistic antitumor effects in vitro. In vivo studies further indicated that this macroporous hydrogel in interaction with kilovoltage X-ray radiation could effectively inhibit tumor growth and dramatically improve the survival ratio in mice to 100%. Furthermore, the released Bi NPs from macroporous hydrogels could be dissolved and discharged from the body as soluble bismuth ions after treatment. These results suggested that the injectable macroporous hydrogel may serve as a combinational therapeutic platform for clinical superficial cancer therapy.

Measurement of backscatter factor for kilovoltage x-ray beam using ionization chamber and Gafchromic XR-QA2 film

Backscatter factor (BSF) is an important parameter in the determination of surface dose for kilovoltage X-ray beam. The purpose of this study was to measure the BSF for kilovoltage diagnostic X-ray beam, and compare the measured BSF between Gafchromic XR-QA2 film and shadow free (SFD) ionization chamber (IC). The parameters that may affect the BSF, such as tube voltage (kVp) and field size, were also studied. The results were in good agreement with the TRS 457, with deviation of less than 12 %. Based on the film study, the BSF obtained from the film measurement were found to be lower than that of the IC, with average difference of 26 %. It was also found that smaller field size resulted in lower effective energy, and the amount of photons which scattered back onto the surface were also smaller. This study demonstrated that the Gafchromic XR-QA2 film was not suitable for the application of small field size.

Characterization of nanoDot optically stimulated luminescence detectors and high-sensitivity MCP-N thermoluminescent detectors in the 40-300kVp energy range.

To investigate empirically the energy dependence of the detector response of two invivo luminescence detectors, LiF:Mg,Cu,P (MCP-N) high-sensitivity TLDs and Al2 O3 :C OSLDs, in the 40-300-kVp energy range in the context of invivo surface dose measurement. As these detectors become more prevalent in clinical and preclinical invivo measurements, knowledge of the variation in the empirical dependence of the measured response of these detectors across a wide spectrum of beam qualities is important. We characterized a large range of beam qualities of three different kilovoltage x-ray units: an Xstrahl 300 Orthovoltage unit, a Precision x-Ray X-RAD 320ix biological irradiator, and a Varian On-Board Imaging x-ray unit. The dose to water was measured in air according to the AAPM's Task Group 61 protocol. The OSLDs and TLDs were irradiated under reference conditions on the surface of a water phantom to provide full backscatter conditions. To assess the change in sensitivity in the long term, we separated the invivo dosimeters of each type into an experimental and a reference group. The experimental dosimeters were irradiated using the kilovoltage x-ray units at each beam quality used in this investigation, while the reference group received a constant 10cGy irradiation at 6MV from a Varian clinical linear accelerator. The individual calibration of each detector was verified in cycles where both groups received a 10cGy irradiation at 6MV. The nanoDot OSLDs were highly reproducible, with ±1.5% variation in response following >40 measurement cycles. The TLDs lost ~20% of their signal sensitivity over the course of the study. The relative light output per unit dose to water of the MCP-N TLDs did not vary with beam quality for beam qualities with effective energies <50keV (~150kVp/6mmAl). At higher energies, they showed a reduced (~75-85%) light output per unit dose relative to 6MV x rays. The nanoDot OSLDs exhibited a very strong (120-408%) dependency of the light output relative to 6 MV x rays. Variations up to 15% between different x-ray units with equivalent effective energies were also observed. While convenient for clinical use, nanoDot OSLDs exhibit a strong variation in their measured light output per unit dose relative to 6MV in the 40-300kV x-ray range. This variability differs unit-to-unit, limiting their effective use for invivo dosimetry applications in the kilovoltage x-ray energy range. MCP-N TLDs offer a much more stable response, but suffer from variations in sensitivity over time dependent on radiation history, which requires careful experimental handling.

Experimental investigation of GafChromic® EBT3 intrinsic energy dependence with kilovoltage x rays, 137 Cs, and 60 Co.

To determine experimentally the intrinsic energy response, kbq , of EBT3 GafChromic® radiochromic film with kilovoltage x rays, 137 Cs, and 60 Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved. The EBT3 film was irradiated with known doses using 60 Co, 137 Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2×1.2) cm2 , were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, kbq , for each dose level was determined relative to 60 Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, β, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of β for each beam quality relative to 60 Co was also determined. kbq was determined for a wide range of doses and energies. The results show a unique variation of kbq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for kbq within the 500-7002mGy range outside of the expanded measurement uncertainty of 3.65% (k=2). For doses less than 500mGy, the signal-to-noise ratio was too low to determine kbq accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with kbq that suggests that β, at least in part, characterizes the microdosimetric interactions that determine kbq . For the beam qualities investigated, a single energy-dependent kbq correction can be used for doses between 500 and 7002mGy. Using the single-hit model with the single-free-parameter fit to solve for β shows promise in the determination of the intrinsic energy response of film, with β being the mathematical analog of the measured kbq .

Using 3D printing techniques to create an anthropomorphic thorax phantom for medical imaging purposes.

Imaging phantoms are widely used for testing and optimization of imaging devices without the need to expose humans to irradiation. However, commercially available phantoms are commonly manufactured in simple, generic forms and sizes and therefore do not resemble the clinical situation for many patients. Using 3D printing techniques, we created a life-size phantom based on a clinical CT scan of the thorax from a patient with lung cancer. It was assembled from bony structures printed in gypsum, lung structures consisting of airways, blood vessels >1mm, and outer lung surface, three lung tumors printed in nylon, and soft tissues represented by silicone (poured into a 3D-printed mold). Kilovoltage x-ray and CT images of the phantom closely resemble those of the real patient in terms of size, shapes, and structures. Surface comparison using 3D models obtained from the phantom and the 3D models used for printing showed mean differences <1mm for all structures. Tensile tests of the materials used for the phantom show that the phantom is able to endure radiation doses over 24,000Gy. It is feasible to create an anthropomorphic thorax phantom using 3D printing and molding techniques. The phantom closely resembles a real patient in terms of spatial accuracy and is currently being used to evaluate x-ray-based imaging quality and positional verification techniques for radiotherapy.

Feasibility study on the use of gold nanoparticles in fractionated kilovoltage X-ray treatment of melanoma

Purpose: Despite the high radioresistance of melanoma, unresectable lesions can be subjected to radiation treatment with the use of gold nanoparticles (AuNPs) as a dose-enhancing agent preferentially loaded on these lesions. The modality of single high-dose treatment has been investigated to confirm its therapeutic efficiency for AuNP-treated melanoma cells. This study explores the feasibility of utilizing AuNPs in fractionated radiation therapy of melanoma for further therapeutic gain.Materials and methods: The responses of human skin melanoma cells to 150-kVp X-ray exposure at 2 and 4 Gy were assessed by quantify gamma-H2AX expression and clonogenic survival, with or without 320 μM of 50 nm AuNP treatment in a culture medium. The influence of AuNPs on cell cycle distribution was observed before irradiation and during 3 d period after irradiation.Results: The AuNP treatment of melanoma cells influenced the cellular response to kilovoltage X-rays to similar extents in terms of the percentage of gamma-H2AX-positive cells and the fractional loss of clonogenicity. Without radiation exposure, AuNPs reduced the portion of melanoma cells at the G2/M phase from 11 to 7%. After irradiation, the progression of the melanoma cells treated with AuNPs toward the G2/M phase was more rapid than that of the AuNP-free cells, and the release of the former from the G2/M phase was slower than that of the latter. At 24 h after irradiation with AuNPs, the cell cycle was rearranged in a pattern that increased the vulnerability of the cells to radiation damage.Conclusions: In addition to the benefit of AuNP treatment to the control of melanoma in single high-dose treatment, further therapeutic gain is expected through fractionated X-ray treatment that involves daily exposure. The AuNP-treated melanoma cells of an increased portion in the radiosensitive G2/M phase following a fractionated dose delivery would respond to the next treatment with an enhanced chance of clonogenic death.

Monte Carlo simulations of a kilovoltage external beam radiotherapy system on phantoms and breast patients.

To determine the most suitable lesion size and depth for radiotherapy treatments with a prototype kilovoltage x-ray arc therapy (KVAT) system through Monte Carlo simulations of the dose delivered to lesion, dose homogeneity, and lesion-to-skin ratio. Monte Carlo simulations were used to calculate dose distributions generated by a novel low-energy kilovoltage x-ray system to a variety of clinically relevant lesion sizes and depths in phantoms and for hypothetical partial breast irradiations of patients in supine and prone positions. The treatments by 200 kV KVAT system were modeled for four sizes of tumor (1-4 cm diameter) at three depths (superficial, middle, and deep) in two sizes of cylindrical water phantoms (16.2-cm and 32.2-cm diameter). In addition, treatments of 3-cm and 4-cm diameter lesions were modeled for two breast patients in prone and supine positions. Dose distributions were calculated using the EGSnrc/DOSXYZnrc code package. Phantom study metrics included lesion-to-skin ratio, dose delivered to isocenter (cGy/min), dose homogeneity, dose profiles, and cumulative dose volume histograms. Lesion-to-skin ratio, lesion-to-rib ratio, dose profiles, and cumulative dose volume histograms were used to evaluate simulated breast patient treatments. Supine breast irradiations were compared to 6-MV VMAT plans. The criterion applied to evaluate the dose distributions was derived from NSABP-B39/RTOG 0413 for accelerated partial breast irradiation. Skin dose was limited to a maximum of 250 cGy for a prescribed lesion dose of 385 cGy per fraction (with the whole treatment being delivered in 10 fractions). This produced the minimum lesion-to-skin dose ratio of 1.5 that served as the main guideline, along with other metrics, for evaluation of future clinical viability of treatments. Phantom dose distributions in the centrally located lesions treated with 360-degree KVAT were found to be superior to dose distributions in off-center lesions with the exception of isocenter dose, which was highest for lesions located closer to the phantom surface. Dose metrics were more favorable for smaller lesions, suggesting that KVAT might be most suitable for treatment of lesions of 1-2 cm in diameter down to depths of 8.1 cm along with 3 cm lesions at depths from 3 cm to 8.1 cm. In addition, treatments of 4-cm lesions were found to be acceptable down to the depths of 4.1 cm (in the 16.2-cm phantom) and 8.1 cm (in the 32.2-cm phantom). At depths from 8.1-cm to 16.1-cm, treatments of 1-cm to 4-cm lesions are possible at the cost of decreased dose rate. KVAT breast treatments in the supine patient position demonstrated that increasing the arc angle and decreasing lesion size improved lesion-to-skin ratio and lesion-to-rib ratio. Supine breast data indicate that 3-cm lesions are treatable at a minimum depth of 3 cm. The 6-MV VMAT plan resulted in lower doses to the ipsilateral lung and the body, but a higher heart dose compared to the KVAT plans. Dose distributions for the prone breast phantoms were superior to the supine cases due to the increased treatment angle of 360-degrees. Although nonoptimized KVAT dose distributions presented here were of inferior quality to VMAT plans, this work has demonstrated the feasibility of delivering low-energy kilovoltage x-rays to lesions up to 4 cm in diameter to depths of 8.1 cm while sparing surrounding tissue.

Stability assessment of radiation isocenter with the gimbaled linac system

Purpose: We report the results of our year-long radiation isocenter accuracy verification for daily quality assurance (QA) implementation on a Vero4DRT system. Methods: The radiation isocenter was calculated using a cube phantom with a steel ball of diameter 10 mm fixed to the center of the phantom. A single photon beam was set with a field size of 100 × 100 mm 2 . Coincidence of the centroid of the steel ball at kiloVolt X-ray imaging isocenter and megaVolt beam radiation isocenter at each gantry and ring angle was tested. This procedure was performed for gantry angles of 0°, 90°, 180°, and 270°, and ring angles of 0°, 20°, and 340°. The centroid of the steel ball and the center of the radiation field were calculated to analyze the radiation isocenter error. This analysis was automatically calculated using the Daily Check tool in the Vero4DRT system. This QA was implemented between 24 August 2015 and 23 August 2016. Results: The average and standard deviation for pan and tilt directions were 0.12 ± 0.10 mm and -0.20 ± 0.13 mm, respectively. The maximum radiation isocenter accuracy error was 0.50 mm in both directions. Conclusion: The radiation isocenter alignment for the one year duration of the experiment was performed with high accuracy.

Revision of orthovoltage chest wall treatment using Monte Carlo simulations.

Given the high local control rates observed in breast cancer patients undergoing chest wall irradiation by kilovoltage x-rays, we aimed to revisit this treatment modality by accurate calculation of dose distributions using Monte Carlo simulation. The machine components were simulated using the MCNPX code. This model was used to assess the dose distribution of chest wall kilovoltage treatment in different chest wall thicknesses and larger contour or fat patients in standard and mid sternum treatment plans. Assessments were performed at 50 and 100 cm focus surface distance (FSD) and different irradiation angles. In order to evaluate different plans, indices like homogeneity index, conformity index, the average dose of heart, lung, left anterior descending artery (LAD) and percentage target coverage (PTC) were used. Finally, the results were compared with the indices provided by electron therapy which is a more routine treatment of chest wall. These indices in a medium chest wall thickness in standard treatment plan at 50 cm FSD and 15 degrees tube angle was as follows: homogeneity index 2.57, conformity index 7.31, average target dose 27.43 Gy, average dose of heart, lung and LAD, 1.03, 2.08 and 1.60 Gy respectively and PTC 11.19%. Assessments revealed that dose homogeneity in planning target volume (PTV) and conformity between the high dose region and PTV was poor. To improve the treatment indices, the reference point was transferred from the chest wall skin surface to the center of PTV. The indices changed as follows: conformity index 7.31, average target dose 60.19 Gy, the average dose of heart, lung and LAD, 3.57, 6.38 and 5.05 Gy respectively and PTC 55.24%. Coverage index of electron therapy was 89% while it was 22.74% in the old orthovoltage method and also the average dose of the target was about 50 Gy but in the given method it was almost 30 Gy. The results of the treatment study show that the optimized standard and mid sternum treatment for different chest wall thicknesses is with 50 cm FSD and zero (vertical) tube angle, while in large contour patients, it is with 100 cm FSD and zero tube angle. Finally, chest wall kilovoltage and electron therapies were compared, which revealed that electron therapy produces a better dose distribution than kilovoltage therapy.