Narinder Sidhu

Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors

PURPOSEnThe goal of this work was to implement a recently proposed small field dosimetry formalism [Alfonso et al., Med. Phys. 35(12), 5179-5186 (2008)] for a comprehensive set of diode detectors and provide the required Monte Carlo generated factors to correct measurement.nnnMETHODSnJaw collimated square small field sizes of side 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 3.0 cm normalized to a reference field of 5.0 cm ×u20095.0 cm were used throughout this study. Initial linac modeling was performed with electron source parameters at 6.0, 6.1, and 6.2 MeV with the Gaussian FWHM decreased in steps of 0.010 cm from 0.150 to 0.100 cm. DOSRZnrc was used to develop models of the IBA stereotactic field diode (SFD) as well as the PTW T60008, T60012, T60016, and T60017 field diodes. Simulations were run and isocentric, detector specific, output ratios (OR(det)) calculated at depths of 1.5, 5.0, and 10.0 cm. This was performed using the following source parameter subset: 6.1 and 6.2 MeV with a FWHMu2009=u20090.100, 0.110, and 0.120 cm. The source parameters were finalized by comparing experimental detector specific output ratios with simulation. Simulations were then run with the active volume and surrounding materials set to water and the replacement correction factors calculated according to the newly proposed formalism.nnnRESULTSnIn all cases, the experimental field size widths (at the 50% level) were found to be smaller than the nominal, and therefore, the simulated field sizes were adjusted accordingly. At a FWHM =u20090.150 cm simulation produced penumbral widths that were too broad. The fit improved as the FWHM was decreased, yet for all but the smallest field size worsened again at a FWHMu2009=u20090.100 cm. The simulated OR(det) were found to be greater than, equivalent to and less than experiment for spot size FWHMu2009=u20090.100, 0.110, and 0.120 cm, respectively. This is due to the change in source occlusion as a function of FWHM and field size. The corrections required for the 0.5 cm field size were 0.95 (± 1.0%) for the SFD, T60012 and T60017 diodes and 0.90 (± 1.0%) for the T60008 and T60016 diodes-indicating measured output ratios to be 5% and 10% high, respectively. Our results also revealed the correction factors to be the same within statistical variation at all depths considered.nnnCONCLUSIONSnA number of general conclusions are evident: (1) small field OR(det) are very sensitive to the simulated source parameters, and therefore, rigorous Monte Carlo linac model commissioning, with respect to measurement, must be pursued prior to use, (2) backscattered dose to the monitor chamber should be included in simulated OR(det) calculations, (3) the corrections required for diode detectors are design dependent and therefore detailed detector modeling is required, and (4) the reported detector specific correction factors may be applied to experimental small field OR(det) consistent with those presented here.

Monte Carlo modelling of diode detectors for small field MV photon dosimetry: detector model simplification and the sensitivity of correction factors to source parameterization

The goal of this work was to examine the use of simplified diode detector models within a recently proposed Monte Carlo (MC) based small field dosimetry formalism and to investigate the influence of electron source parameterization has on MC calculated correction factors. BEAMnrc was used to model Varian 6u2007MV jaw-collimated square field sizes down to 0.5 cm. The IBA stereotactic field diode (SFD), PTW T60016 (shielded) and PTW T60017 (un-shielded) diodes were modelled in DOSRZnrc and isocentric output ratios (OR(fclin)(detMC)) calculated at depths of d = 1.5, 5.0 and 10.0 cm. Simplified detector models were then tested by evaluating the percent difference in (OR(fclin)(detMC)) between the simplified and complete detector models. The influence of active volume dimension on simulated output ratio and response factor was also investigated. The sensitivity of each MC calculated replacement correction factor (k(fclin,fmsr)(Qclin,Qmsr)), as a function of electron FWHM between 0.100 and 0.150 cm and energy between 5.5 and 6.5 MeV, was investigated for the same set of small field sizes using the simplified detector models. The SFD diode can be approximated simply as a silicon chip in water, the T60016 shielded diode can be modelled as a chip in water plus the entire shielding geometry and the T60017 unshielded diode as a chip in water plus the filter plate located upstream. The detector-specific (k(fclin,fmsr)(Qclin,Qmsr)), required to correct measured output ratios using the SFD, T60016 and T60017 diode detectors are insensitive to incident electron energy between 5.5 and 6.5 MeV and spot size variation between FWHM = 0.100 and 0.150 cm. Three general conclusions come out of this work: (1) detector models can be simplified to produce OR(fclin)(detMC) to within 1.0% of those calculated using the complete geometry, where typically not only the silicon chip, but also any high density components close to the chip, such as scattering plates or shielding material is necessary to be included in the model, (2) diode detectors of smaller active radius require less of a correction and (3) (k(fclin,fmsr)(Qclin,Qmsr)) is insensitive to the incident the electron energy and spot size variations investigated. Therefore, simplified detector models can be used with acceptable accuracy within the recently proposed small field dosimetry formalism.

Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations

BACKGROUND AND PURPOSEnThe goal of this work was to measure 6MV small field, detector specific, output ratios (OR(det)) using the IBA stereotactic field diode (SFD) and the PTW T60008, T60012, T60016 and T60017 field diodes on both Varian iX and Elekta Synergy accelerators, to establish estimates for the experimental uncertainty and characterize the measurement precision under various conditions.nnnMATERIALS AND METHODSnData were acquired at depths of 1.5, 5.0 and 10.0 cm for square field sizes of 3.0, 1.0, 0.9, 0.8, 0.7, 0.6 and 0.5 cm. Three isocentric measurements comprised of five readings were made to calculate an experimental output ratio OR(det) with respect to a field size of 5.0 cm. The coefficient of variation (CV) was calculated to characterize the precision associated with each detector-linac combination. Another measurement set was made to investigate the influence of jaw position accuracy.nnnRESULTSnAs expected for field sizes smaller than 3.0 cm, the measured OR(det) were not consistent across all detectors. The standard percent uncertainty in measured OR(det) was found to be nearly consistent across all detector-linac combinations: less than ±0.25% for the 3.0 cm field size, increasing to approximately ±1.25% for the smallest field sizes. As the field size was reduced to 0.5 cm the CV increased to 0.10% and 0.15% on the Varian and Elekta linacs, respectively.nnnCONCLUSIONnExperimental small field OR(det) measured with the diode detectors used in this study are reproducible to within ±1.25% (standard uncertainty), with the precision of any one set of measurements can be characterized with a CV between 0.10% and 0.15%.

Combined radiation therapy and dendritic cell vaccine for treating solid tumors with liver micro‐metastasis

Tumor metastasis and relapse are major obstacles in combating human malignant diseases. Neither radiotherapy alone nor injection of dendritic cells (DCs) can successfully overcome this problem. Radiation induces tumor cell apoptosis and necrosis, resulting in the release of tumor antigen and danger signals, which are favorable for DC capturing antigens and maturation. Hence, the strategy of combined irradiation and DC vaccine may be a novel approach for treating human malignancies and early metastasis.

Relative biological effectiveness (RBE) of 210 Po alpha-particles versus X-rays on lethality in bovine endothelial cells

Purpose : Alpha-radiation from polonium-210 (210 Po) can elevate background radiation dose by an order of magnitude in people consuming large quantities of meat and seafood, particularly caribou and reindeer. Because up to 50% of the ingested 210 Po body burden is initially found in the blood, a primary target for the short range alpha-particles is the endothelial cells lining the blood vessels. This study examined the relative biological effectiveness (RBE) of 210 Po alpha-particles versus 250 kVp X-rays in producing injury to cultured bovine aortic endothelial cells. Materials and methods : Radiation effects on cells were measured in four different ways: the percentage viable cells by trypan blue dye exclusion, the number of live cells, the lactate dehydrogenase (LDH) release to medium and the ability to form colonies (clonogenic survival). Results : Comparison of dose-response curves yielded RBE values of 13.1 ±2.5 (SEM) for cell viability, 10.3 ±1.0 for live cell number and 11.1 ±3.0 for LDH activity. The RBE values for clonogenic survival were 14.0 ±1.0 based on the ratio of the initial slopes of the dose-response curves and 13.1, 9.9 and 7.7 for 50, 10 and 1% survival rate, respectively. At X-ray doses <0.25 Gy, a pronounced stimulatory effect on proliferation was noted. Conclusions : Exposure to 210 Po alpha-particles was seven to 14 times more effective than X-ray exposure in causing endothelial cell damage.

Relative biological effectiveness (RBE) of alpha radiation in cultured porcine aortic endothelial cells.

Purpose: Northern peoples can receive elevated radiation doses (1 – 10 mSv/y) from transfer of polonium-210 (210Po) through the lichen-caribou-human food chain. Ingested 210Po is primarily blood-borne and thus many of its short range alpha particles irradiate the endothelial cells lining the blood vessels. The relative biological effectiveness (RBE) of alpha particles vs. x-rays was examined in porcine aortic endothelial cells as a surrogate for understanding what might happen to human endothelial cells in northern populations consuming traditional foods. Materials and methods: Cultured porcine aortic endothelial cells were exposed to x-ray and 210Po alpha particle radiation. Alpha irradiation was applied to the cell cultures internally via the culture medium and externally, using thin-bottomed culture dishes. The results given here are based on the external irradiation method, which was found to be more reliable. Dose-response curves were compared for four lethal endpoints (cell viability, live cell fraction, release of lactate dehydrogenase [LDH] and clonogenic survival) to determine the relative biological effectiveness (RBE) of alpha radiation. Results: The alpha RBE for porcine cells varied from 1.6 – 21, depending on the endpoint: 21.2 ± 4.5 for cell viability, 12.9 ± 2.7 for decrease in live cell number, 5.3 ± 0.4 for LDH release to the medium but only 1.6 ± 0.1 for clonogenic survival. The low RBE of 1.6 was due to x-ray hypersensitivity of endothelial cells at low doses.

Increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries

This article presents a method for increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries defined in any coordinate system. Nonvoxelated geometries are used to isolate regions of uniform density and composition from the scoring grid. Particle transport within these geometric regions is not restricted by the boundary constraints of the scoring grid. This allows for larger particle steps, which in turn reduces the calculation time. A water tank phantom, water-lung interface phantom, cylindrical calibration phantom, and CT phantom were each used to test the application of the nonvoxelated approach. Each phantom was simulated using both the original DOSXYZnrc code and the new nonvoxelated code. The equivalence between the original and nonvoxelated simulations were quantified using a chi2 analysis. To within the statistical uncertainty, the voxelated and nonvoxelated simulations were found to give nearly identical results, regardless of boundary crossing algorithm. The speed increase was found to be a function of both voxel dimension and field size. Using nonvoxelated geometries and the EXACT boundary crossing algorithm, the speed increase was as high as 9.0, 5.1, 5.7, and 1.3 times faster for the water tank, water-lung interface, cylindrical calibration, and CT phantoms, respectively. If the PRESTA-I boundary crossing algorithm was used, the calculation speed increase was up to 6.0, 2.7, 3.3, and 1.2 times faster. These results clearly show that the nonvoxelated technique greatly increases simulation speed without any loss in dose accuracy.

Planning Target Volume Margin Evaluation and Critical Structure Sparing for Rectal Cancer Patients Treated Prone on a Bellyboard

AIMSnTo calculate a planning target volume (PTV) margin that would account for inter-fractional systematic and random clinical target volume positional errors for patients treated prone on a recently available couch top bellyboard and to evaluate potential critical structure dose reduction using intensity-modulated radiotherapy (IMRT) techniques.nnnMATERIALS AND METHODSnTwenty-four patients (12 men and 12 women) were included in this study, all treated on a commercial bellyboard. Cone beam computed tomography (CBCT) data were acquired once every five fractions for a total of five images per patient. A three-dimensional-three-dimensional bony anatomy auto-match was carried out off-line and the residual difference in position used as a surrogate for clinical target volume inter-fractional positional errors. Systematic (Σ) and random (σ) variations were evaluated and used in PTV(margin)=1.96Σ+0.7σ. The influence of intra-fractional positional errors was evaluated in the margin analysis by introducing published values. Critical structure sparing, as a function of PTV(margin) size, was investigated through the evaluation of three-dimensional conformal radiation therapy (3DCRT) and IMRT treatment plans developed using the margin derived from this work, the American Society for Radiation Oncology Contouring Atlas and the Radiation Therapy Oncology Group 0822 trial specifications.nnnRESULTSnThe PTV(margin) that accounts for only the inter-fractional positional errors was calculated to be (anterior-posterior (AP), superior-inferior (SI), left-right (LR))=(5.2mm, 3.1mm, 2.8mm). If we assumed a combined intra-fractional motion up to 3.0mm then the required PTV(margin) increased to (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm). Treatment plan evaluation showed that the bellyboard provides excellent small bowel sparing regardless of planning technique. In most cases, IMRT reduced the average femoral head, bladder and small bowel dose by 20, 15 and 40% with respect to 3DCRT planning.nnnCONCLUSIONnA PTV(margin) expansion of (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm) is required to account for all positional uncertainties. The use of a bellyboard with IMRT provides better critical structure sparing when compared with a bellyboard with 3DCRT.

Using kV-kV and CBCT imaging to evaluate rectal cancer patient position when treated prone on a newly available belly board

The goal of this work was to use daily kV-kV imaging and weekly cone-beam CT (CBCT) to evaluate rectal cancer patient position when treated on a new couch top belly board (BB). Quality assurance (QA) of the imaging system was conducted weekly to ensure proper performance. The positional uncertainty of the combined kV-kV image match and subsequent couch move was found to be no more than ± 1.0 mm. The average (1 SD) CBCT QA phantom match was anterior-posterior (AP) = -0.8 ± 0.2 mm, superior-inferior (SI) = 0.9 ± 0.2 mm, and left-right (LR) = -0.1 ± 0.1 mm. For treatment, a set of orthogonal kV-kV images were taken and a bony anatomy match performed online. Moves were made along each axis (AP, SI, and LR) and recorded for analysis. CBCT data were acquired once every 5 fractions for a total of 5 images per patient. The images were all taken after the couch move but before treatment. A 3-dimensional (3D-3D) bony anatomy auto-match was performed offline and the residual difference in position recorded for analysis. The average (± 1 SD) move required from skin marks, calculated over all 375 fractions (15 patients × 25 fractions/patient), were AP = -2.6 ± 3.7 mm, SI = -0.3 ± 4.9 mm, and LR = 1.8 ± 4.5 mm. The average residual difference in patient position calculated from the weekly CBCT data (75 total) were AP = -1.7 ± 0.4 mm, SI = 1.1 ± 0.6 mm, and LR = -0.5 ± 0.2 mm. These results show that the BB does provide simple patient positioning that is accurate to within ± 2.0 mm when using online orthogonal kV-kV image matching of the pelvic bony anatomy.

Collimator design for experimental minibeam radiation therapy

PURPOSEnTo design and optimize a minibeam collimator for minibeam radiation therapy studies using a 250 kVp x-ray machine as a simulated synchrotron source.nnnMETHODSnA Philips RT250 orthovoltage x-ray machine was modeled using the EGSnrc/BEAMnrc Monte Carlo software. The resulting machine model was coupled to a model of a minibeam collimator with a beam aperture of 1 mm. Interaperture spacing and collimator thickness were varied to produce a minibeam with the desired peak-to-valley ratio.nnnRESULTSnProper design of a minibeam collimator with Monte Carlo methods requires detailed knowledge of the x-ray source setup. For a cathode-ray tube source, the beam spot size, target angle, and source shielding all determine the final valley-to-peak dose ratio.nnnCONCLUSIONSnA minibeam collimator setup was created, which can deliver a 30 Gy peak dose minibeam radiation therapy treatment at depths less than 1 cm with a valley-to-peak dose ratio on the order of 23%.