Belal Moftah

Radiochromic film based dosimetry of image‐guidance procedures on different radiotherapy modalities

In this work we compare doses from imaging procedures performed on todays state‐of‐the‐art integrated imaging systems using a reference radiochromic film dosimetry system. Skin dose and dose profile measurements from different imaging systems were performed using radiochromic films at different anatomical sites on a humanoid RANDO phantom. EBT3 film was used to measure imaging doses from a TomoTherapy MVCT system, while XRQA2 film was used for dose measurements from kilovoltage imaging systems (CBCT on 21eX and TrueBeam Varian linear accelerators and CyberKnife stereoscopic orthogonal imagers). Maximum measured imaging doses in cGy at head, thorax, and pelvis regions were respectively 0.50, 1.01, and 4.91 for CBCT on 21eX, 0.38, 0.84, and 3.15 for CBCT on TrueBeam, 4.33, 3.86, and 6.50 for CyberKnife imagers, and 3.84, 1.90, and 2.09 for TomoTherapy MVCT. In addition, we have shown how an improved calibration system of XRQA2 film can achieve dose uncertainty level of better than 2% for doses above 0.25 cGy. In addition to simulation‐based studies in literature, this study provides the radiation oncology team with data necessary to aid in their decision about imaging frequency for image‐guided radiation therapy protocols. PACS number: 87.53.Bn, 87.55.Qr, 87.56.Fc

Influence of electron density spatial distribution and X-ray beam quality during CT simulation on dose calculation accuracy

Impact of the various kVp settings used during computed tomography (CT) simulation that provides data for heterogeneity corrected dose distribution calculations in patients undergoing external beam radiotherapy with either high‐energy photon or electron beams have been investigated. The change of the Hounsfield Unit (HU) values due to the influence of kVp settings and geometrical distribution of various tissue substitute materials has also been studied. The impact of various kVp settings and electron density (ED) distribution on the accuracy of dose calculation in high‐energy photon beams was found to be well within 2%. In the case of dose distributions obtained with a commercially available Monte Carlo dose calculation algorithm for electron beams, differences of more than 10% were observed for different geometrical setups and kVp settings. Dose differences for the electron beams are relatively small at shallow depths but increase with depth around lower isodose values. PACS numbers: 87.57.Q‐, 87.55.D‐

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.

Detecting electron beam energy shifts with a commercially available energy monitor.

Routine electron beam quality assurance requires an accurate, yet practical, method of energy characterization. Subtle shifts in beam energy may be produced by the linac bending magnet assembly, and the sensitivity of a commercially available electron beam energy-monitoring device for monitoring these small energy drifts has been evaluated. The device shows an 11% change in signal for a 2 mm change in the I50 energy parameter for low energy electron beams (in the vicinity of 6 MeV) and a 2.5% change in signal for a 2 mm change in the I50 energy parameter for high energy electron beams (in the vicinity of 22 MeV). Thus the device is capable of detecting small energy shifts resulting from bending magnet drift for all clinically relevant electron beams.

New normoxic N-(Hydroxymethyl)acrylamide based polymer gel for 3D dosimetry in radiation therapy

A novel composition of normoxic polymer gel dosimeters based on radiation-induced polymerization of N-(Hydroxymethyl)acrylamide (NHMA) is introduced in this study for 3D dosimetry for Quality Assurance (QA) in radiation therapy. Dosimeters were irradiated by 6, 10 and 18MV photon beams of a medical linear accelerator at various dose rates to doses of up to 20Gy. The dose response of polymer gel dosimeters was studied using nuclear magnetic resonance (NMR) spin-spin relaxation rate (R2) of hydrogen protons within the water molecule. Also, we measured gel response using absorption spectroscopy and found that this novel gel can be successfully utilized for both MRI- and OCT- (Optical Computed Tomography) based 3D dosimetry. We investigated dosimetric properties of six different compositions of the new NHMA-based gel in terms of dose rate, radiation beam quality and stability of dose-dependent polymerization after irradiation. We found no significant effects of these parameters on the novel gel dosimeter performance in both relaxation rate and absorbance measurements.

Effective spatially fractionated GRID radiation treatment planning for a passive grid block

OBJECTIVE To commission a grid block for spatially fractionated grid radiation therapy (SFGRT) treatments and describe its clinical implementation and verification through the record and verify (R&V) system. METHODS SFGRT was developed as a treatment modality for bulky tumours that cannot be easily controlled with conventionally fractionated radiation. Treatment is delivered in the form of open-closed areas. Currently, SFGRT is performed by either using a commercially available grid block or a multileaf collimator (MLC) of a linear accelerator. In this work, 6-MV photon beam was used to study dosimetric characteristics of the grid block. We inserted the grid block into a commercially available treatment planning system (TPS), and the feasibility of delivering such treatment plans on a linear accelerator using a R&V system was verified. Dose measurements were performed using a miniature PinPoint(TM) ion chamber (PTW, Freiburg, Germany) in a water phantom and radiochromic film within solid water slabs. PinPoint ion chamber was used to measure the output factors, percentage depth dose (PDD) curves and beam profiles at two depths, depth of maximum dose (zmax) and 10 cm. Film sheets were used to measure dose profiles at zmax and 10-cm depth. RESULTS The largest observed percentage difference between output factors for the grid block technique calculated by the TPS and measured with the PinPoint ion chamber was 3.6% for the 5 × 5-cm(2) field size. Relatively significant discrepancies between measured and calculated PDD values appear only in the build-up region, which was found to amount to <4%, while a good agreement (differences <2%) at depths beyond zmax was observed. Dose verification comparisons performed between calculated and measured dose distributions were in clinically acceptable agreements. When comparing the MLC-based with the grid block technique, the advantage of treating large tumours with a single field reduces treatment time by at least 3-5 times, having significant impact on patient throughput. CONCLUSION The proposed method supports and helps to standardize the clinical implementation of the grid block in a safer and more accurate way. ADVANCES IN KNOWLEDGE This work describes the method to implement treatment planning for the grid block technique in radiotherapy departments.

Appraisal of mechanisms of radioprotection and therapeutic approaches of radiation countermeasures

Radiation countermeasures are radioprotective agents that reduce the harmful effects of ionizing radiation. They have wide range of applications extending from protection of normal tissues of cancer patients during radiotherapy to safeguard people aftermath of radiologic or nuclear accidents. Despite the screening of thousands of natural and synthetic compounds, only few found place in clinic with limited tolerance. Therefore, mechanistic understanding is essential in the development of more suitable and customized radiation countermeasure agents. This review focuses on the mechanisms of radioprotection imparted by these agents. Radioprotectors are diverse and act through widely varying mechanisms that can be classified in 10 categories: 1) scavenging of free radicals; 2) enhancing DNA repair; 3) synchronizing of cells; 4) modulating redox sensitive genes; 5) modulating growth factors and cytokines; 6) inhibiting apoptosis; 7) repurposing of drug; 8) interacting and chelating of radionuclides; and therapeutic methods of tissue regeneration such as 9) gene therapy; and 10) stem cell therapy. The most common mechanism of radioprotection is the scavenging of free radicals whereas, modulation of growth factors, cytokines and redox genes emerge as effective strategies. Gene and stem cell therapies as therapeutic radiation countermeasures are being developed and can be applied in the near future to minimize the side effects of radiation exposure through tissues regenerations. Thus, the management of radiation exposure may require a holistic multi-mechanistic approaches to achieve optimal radiation protection during radiotherapy of cancer patients and in cases of nuclear eventualities.

PD-0444: Dose calibration and monitoring for radiobiological experiments with low energy proton beams

withdrawn. PD-0446 In vivo EPID dosimetry: 3D analysis applied to prostate VMAT treatments E. Villaggi AUSL Piacenza, Medical Physics, Piacenza, Italy Purpose/Objective: To investigate the feasibility of backprojection portal dosimetry for accurate in vivo 3D dosimetric verification of Volumetric Modulated Arc Therapy (VMAT) prostate treatments by an EPID gantry angle-resolved data acquisition, throught the calculation of patient transmission. The novel approach is analysing data by dose volume histograms (DVH), that provide information on actual delivered dose to the tumor volume and surrounding critical structures.

PD-0382: Radiation response of an improved optical CT based 3D gel sdosimeter for radiation therapy quality assurance

All images were acquired at 100 cm SID using the integrating dosimetry mode at the maximum available dose rate: 600 MU/min at 6 and 10 MV for both EPIDs, and 1400 MU/min at 6 MV FFF and 2400 MU/min at 10 MV FFF for the aS1200. Different fields were delivered from 3×3 cm up to the maximum accommodated by the panel to verify the CAX signal increased with field size. Dose linearity was investigated by irradiating 10×10 cm fields from 1 to 100 MU and considering the signal on the CAX. Ghosting was assessed by delivering three 100 MU fields: a 10×10 cm, followed immediately by a 20×20 cm then a second 10×10 cm and measuring residual signal ±7.5 cm from the CAX. The ratio of signals ±3 cm vertically from the CAX was calculated for fields > 8×8 cm to determine the impact of backscatter. The Modulation Transfer Function (MTF) was determined at each energy from images of an opaque angled edge. All image analysis was performed using IQWorks. A number of IMRT and VMAT plans were verified pretreatment using Portal Dosimetry (PD, Varian Medical Systems) and in vivo using Dosimetry Check (DC, Math Resolutions LLC). Results: Saturation was not evident and dose linearity was within ±1.0% at all energies / dose-rates for both detectors. Signal lag (Fig 1a) was <1.5% for the aS1000 and <0.6% for the aS1200. Differential backscatter (Fig 1b) from the aS1000 support arm increases with field size up to a maximum of 2.5%. On the aS1200 it is negligible (<0.5%) and independent of field size due to additional shielding material attached to the back of the panel. However, the increased backscatter overall results in marginally poorer MTF at lower spatial frequencies (Fig 1c). All patient plans evaluated using PD yielded gamma pass rates (gamma < 1) of 100% when using criteria of 3%/3mm and >99.5% using criteria of 2%/2mm. Test plans with 2.5 mm MLC leaf errors required 2%/2mm to be detected on the aS1200 compared with 3%/3mm on the aS1000. This may be due to the higher bit depth and lower signal lag. IVD was successfully performed for large field, multi arc VMAT plans, with gamma pass rates > 90% at 5%/3mm for irradiated areas of side up to 26 cm at 150 cm SID.

SU-E-T-665: Radiochromic Film Quenching Effect Reduction for Proton Beam Dosimetry

Purpose: Depending on the useful dose range in which radiochromic films operate, number of different radiochromic film models have been designed. The impact of different film models on quenching effect for percent depth dose (PDD) measurements in proton beams has been investigated. Methods: Calibrated PTW Markus ionization chamber was used to measure PDD and beam output for 26.5 MeV protons produced by CS30 cyclotron. An aluminum cylinder was added in front of the beam exit serving as a radiation shutter. The measured signal was normalized to a monitor chamber reading and subsequently scaled by ratio of water-to-air stopping powers at given depth, while the effective depth of measurements was scaled by ratios of material-to-water physical densities and CSDA ranges. Output was measured in water at 2.1 mm reference-depth in the plateau upstream from the Bragg peak. Following the TRS-398 reference dosimetry protocol for proton beams, the output was calibrated in water. Three radiochromic film models (EBT, EBT3 and HD-V2) were calibrated within Lexan phantom positioned at the same water-equivalent depth. Thicknesses of films sensitive layers were 34 µm, 30 µm and 8 µm, respectively. Small film pieces (1 x 2 cm 2 ) were positioned within polyethylene phantom along the beam central axis with an angulation of 5° for PDD measurements. Results: While the output of the proton beam was found to be around 7 Gy/sec, the actual value of the output per monitor chamber reading (2.32 Gy/nC) was used for reference-dose irradiations during film calibration. Dose ratios at the Bragg peak relative to the reference-depth were 3.88, 2.52, 2.19, and 2.02 for the Markus chamber, HD-V2, EBT3, and EBT film models, respectively. Conclusion: Results at hand suggest that quenching effect is reduced when a radiochromic film model with smaller sensitive layer thickness is used for PDD measurements in proton beams. David Lewis is the owner of RCF Consulting, LLC