Influence of trace elements in human tissue in low-energy photon brachytherapy dosimetry

Heterogeneous breast phantom for computed tomography and magnetic resonance imaging

In this article, a heterogeneous multimodal anthropomorphic breast phantom with carcinoma is introduced to meet the response of the natural breast tissue when imaged using ionizing and non-ionizing machines. The skin, adipose, fibroglandular, pectoral muscle, and carcinoma tissue were mimicked. A T1-weighted breast magnetic resonance image with BI-RADS I tissue segmentation was used for molds creation. The tissue-mimicking materials (TMMs) were tailored in terms of their elemental composition weight fractions and their response to ionization radiation parameters. These are the mass attenuation coefficient (MAC), electron density (ne) and effective atomic number (Zeff). The behaviour of the TMMs, when exposed to a wide range of ionization radiation energy, was investigated analytically and numerically using X-COM. The achieved results showed an excellent agreement with the corresponding properties of the natural breast elemental compositions as reported by the International Commission on Radiation Units and Measurements (ICRU). The MAC of the TMMs and the ICRU-based breast tissue were found to be consistent. The maximum percentage of error in ne and Zeff amounts to only 2.93% and 5.76%, respectively. For non-ionizing imaging, the TMMs were characterized in term of T1 and T2 relaxation times. Using our preclinical MRI unit, the TMMs relaxation times were measured and compared to the natural tissue. The fabricated phantom was validated experimentally using CT, MRI, and Mammographic machines. The achieved images of the TMMs were in alignment with the real tissue in terms of CT HU values and grayscale colors. T1W and T2W images on MRI revealed the expected contrast between TMMs as in natural tissue.

Reference man for radiological protection: 71 chemical elements' content of the prostate gland (normal and cancerous)

Frequently knowledge of elemental content of human organs and tissues is required for a variety of applications. These can include brachytherapy and radiotherapy planning, radiation dosimetry and radiation protection. Revised reference values of chemical element mass fractions in normal and cancerous prostate tissues of the Reference (European Caucasian) Man are suggested as a result of this work. Autopsies of 37 apparently healthy males (mean age 55 ± 11 years, range 41-87 years) provided the prostatic tissues studied. The investigated individuals lived in a non-industrial, Central European region of Russia and had suffered sudden death. Also, tissues were studied from 62 subjects with prostate cancer (mean age 65 ± 10 years, range 40-79 years). Sixty-seven elemental mass fractions were determined in each of these 99 prostates. Analytical methods employed were inductively coupled plasma atomic emission spectrometry, neutron activation analysis with high-resolution spectrometry of short-lived and long-lived radionuclides, energy dispersive X-ray fluorescence analysis, and inductively coupled plasma mass spectrometry. Whichever method was employed, the necessary quality control measures were utilized. Results presented here include a systematic analysis of both the prostatic data presented here for 67 elements and also others' published findings, to make a total of 71 elemental mass fraction values.

The dosimetric impact of replacing the TG-43 algorithm by model based dose calculation for liver brachytherapy

PURPOSE

To compare treatment plans for interstitial high dose rate (HDR) liver brachytherapy with ¹⁹²Ir calculated according to current-standard TG-43U1 protocol with model-based dose calculation following TG-186 protocol.

METHODS

We retrospectively evaluated dose volume histogram (DVH) parameters for liver, organs at risk (OARs) and clinical target volumes (CTVs) of 20 patient cases diagnosed with hepatocellular carcinoma (HCC) or metastatic colorectal cancer (mCRC). Dose calculations on a homogeneous water geometry (TG-43U1 surrogate) and on a computed tomography (CT) based geometry (TG-186) were performed using Monte Carlo (MC) simulations. The CTs were segmented based on a combination of assigning TG-186 recommended tissues to fixed Hounsfield Unit (HU) ranges and using organ contours delineated by physicians. For the liver, V_5Gy and V_10Gy were analysed, and for OARs the dose to 1 cubic centimeter (D_1cc). Target coverage was assessed by calculating V₁₅₀, V₁₀₀, V₉₅ and V₉₀ as well as D₉₅ and D₉₀. For every DVH parameter, median, minimum and maximum values of the deviations of TG-186 from TG-43U1 were analysed.

RESULTS

TG-186-calculated dose was found to be on average lower than dose calculated with TG-43U1. The deviation of highest magnitude for liver parameters was -6.2% of the total liver volume. For OARs, the deviations were all smaller than or equal to -0.5 Gy. Target coverage deviations were as high as -1.5% of the total CTV volume and -3.5% of the prescribed dose.

CONCLUSIONS

In this study we found that TG-43U1 overestimates dose to liver tissue compared to TG-186. This finding may be of clinical importance for cases where dose to the whole liver is the limiting factor.

The influence of tissue composition uncertainty on dose distributions in brachytherapy

BACKGROUND AND PURPOSE

Model-based dose calculation algorithms (MBDCAs) have evolved from serving as a research tool into clinical practice in brachytherapy. This study investigates primary sources of tissue elemental compositions used as input to MBDCAs and the impact of their variability on MBDCA-based dosimetry.

MATERIALS AND METHODS

Relevant studies were retrieved through PubMed. Minimum dose delivered to 90% of the target (D₉₀), minimum dose delivered to the hottest specified volume for organs at risk (OAR) and mass energy-absorption coefficients (μ_en/ρ) generated by using EGSnrc "g" user-code were compared to assess the impact of compositional variability.

RESULTS

Elemental composition for hydrogen, carbon, oxygen and nitrogen are derived from the gross contents of fats, proteins and carbohydrates for any given tissue, the compositions of which are taken from literature dating back to 1940-1950. Heavier elements are derived from studies performed in the 1950-1960. Variability in elemental composition impacts greatly D₉₀ for target tissues and doses to OAR for brachytherapy with low energy sources and less for ¹⁹²Ir-based brachytherapy. Discrepancies in μ_en/ρ are also indicative of dose differences.

CONCLUSIONS

Updated elemental compositions are needed to optimize MBDCA-based dosimetry. Until then, tissue compositions based on gross simplifications in early studies will dominate the uncertainties in tissue heterogeneity.

Dose heterogeneity correction for low-energy brachytherapy sources using dual-energy CT images

Permanent seed implant brachytherapy is currently used for adjuvant radiotherapy of early stage prostate and breast cancer patients. The current standard for calculation of dose around brachytherapy sources is based on the AAPM TG-43 formalism, which generates the dose in a homogeneous water medium. Recently, AAPM TG-186 emphasized the importance of accounting for tissue heterogeneities. We have previously reported on a methodology where the absorbed dose in tissue can be obtained by multiplying the dose, calculated by the TG-43 formalism, by an inhomogeneity correction factor (ICF). In this work we make use of dual energy CT (DECT) images to extract ICF parameters. The advantage of DECT over conventional CT is that it eliminates the need for tissue segmentation as well as assignment of population based atomic compositions. DECT images of a heterogeneous phantom were acquired and the dose was calculated using both TG-43 and TG-43 [Formula: see text] formalisms. The results were compared to experimental measurements using Gafchromic films in the mid-plane of the phantom. For a seed implant configuration of 8 seeds spaced 1.5 cm apart in a cubic structure, the gamma passing score for 2%/2 mm criteria improved from 40.8% to 90.5% when ICF was applied to TG-43 dose distributions.

Effect of tissue composition on dose distribution in brachytherapy with various photon emitting sources

Purpose

The aim of this study is to compare the dose in various soft tissues in brachytherapy with photon emitting sources.

Material and methods

¹⁰³Pd, ¹²⁵I, ¹⁶⁹Yb, ¹⁹²Ir brachytherapy sources were simulated with MCNPX Monte Carlo code, and their dose rate constant and radial dose function were compared with the published data. A spherical phantom with 50 cm radius was simulated and the dose at various radial distances in adipose tissue, breast tissue, 4-component soft tissue, brain (grey/white matter), muscle (skeletal), lung tissue, blood (whole), 9-component soft tissue, and water were calculated. The absolute dose and relative dose difference with respect to 9-component soft tissue was obtained for various materials, sources, and distances.

Results

There was good agreement between the dosimetric parameters of the sources and the published data. Adipose tissue, breast tissue, 4-component soft tissue, and water showed the greatest difference in dose relative to the dose to the 9-component soft tissue. The other soft tissues showed lower dose differences. The dose difference was also higher for ¹⁰³Pd source than for ¹²⁵I, ¹⁶⁹Yb, and ¹⁹²Ir sources. Furthermore, greater distances from the source had higher relative dose differences and the effect can be justified due to the change in photon spectrum (softening or hardening) as photons traverse the phantom material.

Conclusions

The ignorance of soft tissue characteristics (density, composition, etc.) by treatment planning systems incorporates a significant error in dose delivery to the patient in brachytherapy with photon sources. The error depends on the type of soft tissue, brachytherapy source, as well as the distance from the source.

INAA application in the assessment of chemical element mass fractions in adult and geriatric prostate glands

The variation with age of the mass fraction of 37 chemical elements in intact nonhyperplastic prostate of 65 healthy 21-87 year old males was investigated by instrumental neutron activation analysis with high resolution spectrometry of short- and long-lived radionuclides. Mean values (M±SΕΜ) for mass fractions (mg kg(-1), dry mass basis) of the chemical elements studied were: Ag-0.055±0.007, Br-33.2±3.3, Ca-2150±118, Cl-13014±703, Co-0.038±0.003, Cr-0.47±0.05, Fe-99.3±6.1, Hg-0.044±0.006, K-11896±356, Mg-1149±68, Mn-1.41±0.07, Na-10886±339, Rb-12.3±0.6, Sb-0.049±0.005, Sc-0.021±0.003, Se-0.65±0.03, and Zn-795±71. The mass fraction of other chemical elements measured in this study were lower than the corresponding detection limits (mg kg(-1), dry mass basis): As<0.1, Au<0.01, Ba<100, Cd<2, Ce<0.1, Cs<0.05, Eu<0.001, Gd<0.02, Hf<0.2, La<0.5, Lu<0.003, Nd<0.1, Sm<0.01, Sr<3, Ta<0.01, Tb<0.03, Th<0.05, U<0.07, Yb<0.03, and Zr<0.3. This work revealed that there is a significant trend for increase with age in mass fractions of Co (p<0.0085), Fe (p<0.037), Hg (p<0.035), Sc (p<0.015), and Zn (p<0.0014) and for a decrease in the mass fraction of Mn (p<0.018) in prostates, obtained from young adult up to about 60 years, with age. In the nonhyperplastic prostates of males in the sixth to ninth decades, the magnitude of mass fractions of all chemical element were maintained at near constant levels. Our finding of correlation between the prostatic chemical element mass fractions indicates that there is a great variation of chemical element relationships with age.