Dosimetry of biological irradiations using radiochromic films

Dosimetric evaluation of a novel modular cell irradiation platform for multi-modality in vitro studies including high dose rate brachytherapy

PURPOSE

High dose-rate (HDR) brachytherapy is integral for the treatment of numerous cancers. Preclinical studies involving HDR brachytherapy are limited. We aimed to describe a novel platform allowing multi-modality studies with clinical HDR brachytherapy and external beam irradiators, establish baseline dosimetry standard of a preclinical orthovoltage irradiator, to determine accurate dosimetric methods.

METHODS

A dosimetric assessment of a commercial preclinical irradiator was performed establishing the baseline dosimetry goals for clinical irradiators. A 3D printed platform was then constructed with 14 brachytherapy channels at 1cm spacing to accommodate a standard tissue culture plate at a source-to-cell distance (SCD) of 1 cm or 0.4 cm. 4-Gy CT-based treatment plans were created in clinical treatment planning software and delivered to 96-well tissue culture plates using an Ir192 source or a clinical linear accelerator. Standard calculation models for HDR brachytherapy and external beam were compared to corresponding deterministic model-based dose calculation algorithms (MBDCAs). Agreement between predicted and measured dose was assessed with 2D-gamma passing rates to determine the best planning methodology.

RESULTS

Mean (±standard deviation) and median dose measured across the plate for the preclinical irradiator was 423.7 ± 8.5 cGy and 430.0 cGy. Mean percentage differences between standard and MBDCA dose calculations were 9.4% (HDR, 1 cm SCD), 0.43% (HDR, 0.4 cm SCD), and 2.4% (EBRT). Predicted and measured dose agreement was highest for MBDCAs for all modalities.

CONCLUSION

A 3D-printed tissue culture platform can be used for multi-modality irradiation studies with great accuracy. This tool will facilitate preclinical studies to reveal biologic differences between clinically relevant radiation modalities.

Radiochromic Films for the Two-Dimensional Dose Distribution Assessment

Radiochromic films are mainly used for two-dimensional dose verification in photon, electron, and proton therapy treatments. Moreover, the radiochromic film types available today allow their use in a wide dose range, corresponding to applications from low-medical diagnostics to high-dose beam profile measurements in charged particle medical accelerators. An in-depth knowledge of the characteristics of radiochromic films, of their operating principles, and of the dose reading techniques is of paramount importance to exploit all the features of this interesting and versatile radiation detection system. This short review focuses on these main aspects by considering the most recent works on the subject.

Establishing a Reference Dose-Response Calibration Curve for Dicentric Chromosome Aberrations to Assess Accidental Radiation Exposure in Saudi Arabia

In cases of nuclear and radiological accidents, public health and emergency response need to assess the magnitude of radiation exposure regardless of whether they arise from disaster, negligence, or deliberate act. Here we report the establishment of a national reference dose–response calibration curve (DRCC) for dicentric chromosome (DC), prerequisite to assess radiation doses received in accidental exposures. Peripheral blood samples were collected from 10 volunteers (aged 20–40 years, median = 29 years) of both sexes (three females and seven males). Blood samples, cytogenetic preparation, and analysis followed the International Atomic Energy Agency EPR-Biodosimetry 2011 report. Irradiations were performed using 320 kVp X-rays. Metafer system was used for automated and assisted (elimination of false-positives and inclusion of true-positives) metaphases findings and DC scoring. DC yields were fit to a linear–quadratic model. Results of the assisted DRCC showed some variations among individuals that were not statistically significant (homogeneity test, P = 0.66). There was no effect of age or sex (P > 0.05). To obtain representative national DRCC, data of all volunteers were pooled together and analyzed. The fitted parameters of the radiation-induced DC curve were as follows: Y = 0.0020 (±0.0002) + 0.0369 (±0.0019) ^*D + 0.0689 (±0.0009) ^*D². The high significance of the fitted coefficients (z-test, P < 0.0001), along with the close to 1.0 p-value of the Poisson-based goodness of fit (χ² = 3.51, degrees of freedom = 7, P = 0.83), indicated excellent fitting with no trend toward lack of fit. The curve was in the middle range of DRCCs published in other populations. The automated DRCC over and under estimated DCs at low (<1 Gy) and high (>2 Gy) doses, respectively, with a significant lack of goodness of fit (P < 0.0001). In conclusion, we have established the reference DRCC for DCs induced by 320 kVp X-rays. There was no effect of age or sex in this cohort of 10 young adults. Although the calibration curve obtained by the automated (unsupervised) scoring misrepresented dicentric yields at low and high doses, it can potentially be useful for triage mode to segregate between false-positive and near 2-Gy exposures from seriously irradiated individuals who require hospitalization.

Calibration of MTT assay in proton beams using radiochromic films

PURPOSE

This study provides methodology of calibrating as well as controlling the output for an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay irradiated in a low energy proton beam using EBT3-model GAFCHROMIC^TM film, without correcting for quenching effect.

METHODS

A calibrated Markus ionization chamber was used to measure the depth dose and beam output for 26.5 MeV protons produced by a CS30 cyclotron. A time-controlled aluminum cylinder was added in front of the horizontal beam-exit serving as a radiation shutter. Following the TRS-398 reference dosimetry protocol for proton beams, the output was calibrated in water at a reference depth of 3 mm. EBT3 film was calibrated for doses up to 8 Gy at the same depth. To verify the dose distribution for each 96-well MTT assay plate, EBT3 film was placed at the reference depth during irradiation and cell doses were scaled by measured percent depth dose (PDD) data.

RESULTS

The radiochromic film dosimetry system in this study provides dose measurements with an uncertainty better than 3.3% for doses higher than 1 Gy. From a single exposure and utilizing the Gaussian shape of the beam, multiple dose points can be obtained within different wells of the same plate ranging from 6.9 Gy (sigma ∼4%) in the central well, and 2 Gy (sigma ∼8%) for wells positioned closer to the periphery.

CONCLUSIONS

We described a methodology for radiochromic film-based dose monitoring system, using low-energy protons, which can be used for the MTT assay in any proton beam, except within Bragg peak region.

Gender bias in individual radiosensitivity and the association with genetic polymorphic variations

PURPOSE

To assess the extent of variation in radiosensitivity between individuals, gender-related dissimilarity and impact on the association with single nucleotide polymorphisms (SNPs).

MATERIALS AND METHODS

Survival curves of 152 fibroblast cell strains derived from both gender were generated. Individual radiosensitivity was characterized by the surviving fraction at 2Gy (SF2). SNPs in 10 radiation responsive genes were genotyped by direct sequencing.

RESULTS

The wide variation in SF2 (0.12-0.50; mean=0.33) was significantly associated with 3 SNPs: TP53 G72C (P=0.007), XRCC1 G399A (P=0.002) and ATM G1853A (P=0.01). Females and males differed significantly in radiosensitivity (P=0.004) that impacted genetic association where only XRCC1 remained significant in both gender (P<0.05). Meanwhile, discordant association was observed for TP53 that was significant in females (P=0.012) and ATM that was significant in males (P=0.0006). When gender-specific SF2-mean (0.31 and 0.35 for females and males; respectively) was considered, further discordance was observed where XRCC1 turned out not to be associated with radiosensitivity in males (P>0.05).

CONCLUSIONS

Although the variation in individual radiosensitivity was associated with certain SNPs, gender bias for both endpoints was evident. Therefore, assessing the risk of radiation exposure in females and males should be considered separately in order to achieve the ultimate goal of personalized radiation medicine.

Characterization of an orthovoltage biological irradiator used for radiobiological research

Orthovoltage irradiators are routinely used to irradiate specimens and small animals in biological research. There are several reports on the characteristics of these units for small field irradiations. However, there is limited knowledge about use of these units for large fields, which are essential for emerging large-field irregular shape irradiations, namely total marrow irradiation used as a conditioning regimen for hematological malignancies. This work describes characterization of a self-contained Orthovoltage biological irradiator for large fields using measurements and Monte Carlo simulations that could be used to compute the dose for in vivo or in vitro studies for large-field irradiation using this or a similar unit. Percentage depth dose, profiles, scatter factors, and half-value layers were measured and analyzed. A Monte Carlo model of the unit was created and used to generate depth dose and profiles, as well as scatter factors. An ion chamber array was also used for profile measurements of flatness and symmetry. The output was determined according to AAPM Task Group 61 guidelines. The depth dose measurements compare well with published data for similar beams. The Monte Carlo-generated depth dose and profiles match our measured doses to within 2%. Scatter factor measurements indicate gradual variation of these factors with field size. Dose rate measured by placing the ion chamber atop the unit's steel plate or solid water indicate enhanced readings of 5 to 28% compared with those measured in air. The stability of output over a 5-year period is within 2% of the 5-year average.

Establishing cytogenetic biodosimetry laboratory in Saudi Arabia and producing preliminary calibration curve of dicentric chromosomes as biomarker for medical dose estimation in response to radiation emergencies

In cases of public or occupational radiation overexposure and eventual radiological accidents, it is important to provide dose assessment, medical triage, diagnoses and treatment to victims. Cytogenetic bio-dosimetry based on scoring of dicentric chromosomal aberrations assay (DCA) is the “gold standard” biotechnology technique for estimating medically relevant radiation doses. Under the auspices of the National Science, Technology and Innovation Plan in Saudi Arabia, we have set up a biodosimetry laboratory and produced a national standard dose–response calibration curve for DCA, pre-required to estimate the doses received. For this, the basic cytogenetic DCA technique needed to be established. Peripheral blood lymphocytes were collected from four healthy volunteers and irradiated with radiation doses between 0 and 5 Gy of 320 keV X-rays. Then, lymphocytes were PHA stimulated, Colcemid division arrested and stained cytogenetic slides were prepared. The Metafer4 system (MetaSystem) was used for automatic and manually assisted metaphase finding and scoring of dicentric chromosomes. Results were fit to the linear-quadratic dose–effect model according to the IAEA EPR-Biodosimetry-2011 report. The resulting manually assisted dose–response calibration curve (Y = 0.0017 + 0.026 × D + 0.081 × D²) was in the range of those described in other populations. Although the automated scoring over-and-under estimates DCA at low (<1 Gy) and high (>2 Gy) doses, respectively, it showed potential for use in triage mode to segregate between victims with potential risk to develop acute radiotoxicity syndromes. In conclusion, we have successfully established the first biodosimetry laboratory in the region and have produced a preliminary national dose–response calibration curve. The laboratory can now contribute to the national preparedness plan in response to eventual radiation emergencies in addition to providing information for decision makers and public health officials who assess the magnitude of public, medical, occupational and accidental radiation exposures.