Medical linear accelerator mounted mini-beam collimator: design, fabrication and dosimetric characterization.

The goal of this work was to design, build and experimentally characterize a linear accelerator mounted mini-beam collimator for use at a nominal 6 MV beam energy. Monte Carlo simulation was used in the design and dosimetric characterization of a compact mini-beam collimator assembly mounted to a medical linear accelerator. After fabrication, experimental mini-beam dose profiles and central axis relative output were measured and the results used to validate the simulation data. The simulation data was then used to establish traceability back to an established dosimetric code of practice. The Monte Carlo simulation work revealed that changes in collimator blade width have a greater influence on the valley-to-peak dose ratio than do changes in blade height. There was good agreement between the modeled and measured profile data, with the exception of small differences on either side of the central peak dose. These differences were found to be systematic across all depths and result from limitations associated with the collimator fabrication. Experimental mini-beam relative output and simulation data agreed to better than ± 2.0%, which is well within the level of uncertainty required for dosimetric traceability of non-standard field geometries. A mini-beam collimator has now been designed, built and experimentally characterized for use with a commercial linear accelerator operated at a nominal 6 MV beam energy.

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 6 MV 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.

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

PURPOSE: The 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. METHODS: Jaw 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 × 5.0 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 FWHM = 0.100, 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. RESULTS: In 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 = 0.150 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 FWHM = 0.100 cm. The simulated OR(det) were found to be greater than, equivalent to and less than experiment for spot size FWHM = 0.100, 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. CONCLUSIONS: A 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.

Dosimetric considerations for validation of a sequential IMRT process with a commercial treatment planning system.

Commercial multileaf collimator (MLC) systems can employ leaves with rounded ends. Treatment planning beam modelling should consider the effects of transmission through rounded leaf ends to provide accurate dosimetry for IMRT treatments delivered with segmented MLC. We determined that an MLC leaf gap reduction of 1.4 mm is required to obtain an agreement between calculated and measured profile 50% dose points. A head and neck dosimetry phantom, supplied by the Radiological Physics Center (RPC), was planned and irradiated as a necessary credentialing requirement for the RTOG H-0022 protocol. The agreement between the RPC TLD measurements and treatment planning calculations was within experimental error for the primary and secondary planning target volumes (PTVs); however, the calculated mean dose for the critical structure was approximately 9% lower than the RPC TLD measurements. RPC radiochromic film profile measurements also indicated significant discrepancies (>5%) with calculated values especially in the high dose gradient region in the vicinity of the critical structure. These results substantiate our own in-house phantom measurements, performed with the same IMRT fields as for the RPC phantom experiment, using Kodak EDR2 film to measure absolute dose. Our results indicate a maximum underestimate of calculated dose of 12% with no leaf gap reduction. The discrepancy between measured and calculated phantom values is reduced to +/- 5% when a leaf gap reduction of 1.4 mm is used. A further improvement in the accuracy of dose calculation is not possible without a more accurate modelling of the leaf end transmission by the planning system. In the absence of published dosimetric criteria for IMRT our results stress the need for stringent in-house dosimetric QA and validation for IMRT treatments. We found the dosimetric validation service provided by the RPC to be a valuable component of our IMRT validation efforts.

Feasibility of penumbra compensating filters for conformal prostate radiotherapy.

Radiation therapy beams demonstrate a gradual dose fall off at the field edges, due to factors affecting the physical penumbra and transport of radiation. Adequate target coverage requires an increase in field size larger than the target volume itself for a uniform dose to be delivered to that target volume. A method is presented for the design and fabrication of penumbra compensating filters (PCFs) which essentially sharpen the penumbra on a field-by-field basis and are used in conjunction with custom shielding blocks. We have explored the feasibility of using PCFs to reduce the field margins required for our four-field conformal prostate treatments. The penumbra compensation is designed based on a profile measured along the direction perpendicular to the blocked field edge that shows the greatest 50% to 95% isodose distance for a typical conformal prostate patient. Rigid foam material is milled and filled with a low melting point alloy material to create a filter which provides dose compensation in the field periphery of the custom shielding block. The accuracy of our methodology has been established using film dosimetry. By employing PCFs, the reduction in the rectal margin ranges from approximately 4 mm in the posterior region to 13 mm in the superior-posterior region, as compared with the shielding blocks alone. The reduction in bladder margin ranges from approximately 4 mm in the superior-anterior region to 10 mm in the superior region. Dose-volume histograms for an idealized cylindrical rectum indicate a substantial reduction in the volume treated to high doses. The calculated normal tissue complication probability values were 8.7% and 10.5% with and without PCFs included in the blocked fields respectively. The advantages of using PCFs, compared with multileaf collimator based techniques, are discussed.

Use of a radioactive check device for redundancy check of ionization chambers.

The use of a radioactive check device containing Strontium 90 was investigated to carry out the redundancy checks of Farmer-type ionization chambers. It was not possible to meet the recommended tolerance limits of the redundancy checks without taking into account the angular response of the ionization chambers. The ionization chambers exhibited a maximum variation of 1% in the angular response in this check device. After accounting for the angular response the maximum variation of the short-term repeatability was 0.14% with a standard deviation of +/-0.05%. The long-term constancy checked over a period of nine months was less than +/-0.6% for measurements, taking into account the angular response of the ionization chambers. No significant effect of the background radiation levels was observed on the measurements.

Comparison of superheated drop detector with phosphorous pentoxide powder for the detection of neutrons in 18 MV x rays.

Neutron dose equivalent measurements were performed in and around an 18 MV x-ray beam using superheated drop detectors (SDD) and phosphorous pentoxide (P2O5) powder. The neutron dose equivalent profiles for various field sizes of 10 X 10 cm2, 20 X 20 cm2, and 30 X 30 cm2 were measured. The results measured with the P2O5 were checked for any gross systematic errors by comparing with the published results computed by using Monte Carlo calculations. A comparison was then made between the neutron dose equivalent profiles measured with the P2O5 and the SDD. The results of this comparison show that the neutron dose equivalents measured with the two types of detectors agree with each other for measurements about 20 cm away from the beam edges. However, in and near the beam edges the SDD measurements are upto 50% less than the neutron dose equivalents measured using P2O5 for the 18 MV x-ray beam.

Interfacing a linear diode array to a conventional water scanner for the measurement of dynamic dose distributions and comparison with a linear ion chamber array.

A linear diode array was interfaced to a conventional water scanner for the measurement of dose distribution of dynamically wedged treatment fields. The system was validated by comparing the dose distribution of an open field measured with the system and with a scanning ionization chamber. The system was also compared with a commercial ion chamber array scanner for the measurement of dose distributions of dynamic wedges. The dose distributions of enhanced dynamic wedge fields measured with the two dosimetry systems agree well beyond the depth of maximum dose. However, in the buildup region, the measurements made with the linear diode array system differ by more than 10% from that of the ion chamber array scanner.

Simulation of irregularly shaped electron treatment fields using a digital camera.

The use of a digital camera to simulate irregularly shaped electron treatment fields was investigated. The simulation of electron treatment fields, drawn on patient's skin or shell, is required to fabricate cerrobend inserts for defining the beam apertures. The use of the digital camera made the procedure very easy and time efficient, which was essential for patient comfort as well. The mathematical formulism and accessories required to use the camera for simulating the electron treatment fields are described in detail.

Dosimetric effects of matching electron fields with cobalt 60 fields in the management of head and neck cancer.

The dose distributions in a blocked Co 60 beam with the area under the block irradiated with a 9-MeV electron beam at a nominal source to surface distance (SSD) of 100 cm and at an extended SSD of 115 cm were determined. The photon block was simulating the spinal cord shielding used when treating the posterior neck. A hot spot of 10% appeared close to the junction on the side of the photon field when electron treatment was given at a nominal SSD of 100 cm. At 115 cm SSD, the magnitude of the hot spot on the side of photon field increased to 20% and a cold spot on the order of 10% appeared in the distal area of the irradiated volume treated with electrons. A gap of 0.5 cm between the light field edges of the photon field and the electron field reduced the hot spot from 20% to 10% on the side of photon field. However, the magnitude of the cold spot on the side of electron field increased to 20%, and it appeared across the irradiated volume. Considering the limitation of the design of the electron applicators, which precludes the use of a nominal SSD of 100 cm for head and neck treatments, it is recommended that no gap should be allowed between the photon fields and the extended SSD electron fields. Clinically a limited hot spot of 20% is considered more acceptable than a cold spot of 20% extending from proximal to distal areas of the treatment volume.

Dosimetric characteristics of wedged fields.

The beam characteristics of the wedged fields in the nonwedged planes (planes normal to the wedged planes) were studied for 6 MV and 15 MV x-ray beams. A method was proposed for determining the maximum field length of a wedged field that can be used in the nonwedged plane without introducing undesirable alterations in the dose distributions of these fields. The method requires very few measurements. The relative wedge factors of 6 MV and 15 MV X-rays were determined for wedge filters of nominal wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees as a function of depth and field size. For a 6 MV beam the relative wedge factors determined for a field size of 10 x 10 cm2 for 30 degrees, 45 degrees, and 60 degrees wedge filters can be used for various field sizes ranging from 4 cm2 to 20 cm2 (except for the 60 degrees wedge for which the maximum field size that can be used is 15 x 20 cm2) without introducing errors in the dosimetric calculations of more than 0.5% for depths up to 20 cm and 1% for depths up to 30 cm. For the 15 degrees wedge filter the relative wedge factor for a field size of 10 x 10 cm2 can be used over the same range of field sizes by introducing slightly higher error, 0.5% for depths up to 10 cm and 1% for depths up to 30 cm. For a 15 MV beam the maximum magnitude of the relative wedge factors for 45 degrees and 60 degrees lead wedges is of the order of 1%, and it is not important clinically to apply a correction of that magnitude. For a 15 MV beam the relative wedge factors determined for a field size of 6 x 6 cm2 for the 15 degrees and 30 degrees steel wedges can be used over a range of field sizes from 4 cm2 to 20 cm2 without causing dosimetric errors greater than 0.5% for depths up to 10 cm.

The suitability of 160Tb as a PAC probe for studies of biomolecules.

The gamma-ray perturbed angular correlation (PAC) studies were made with 160Tb as a probe in MES (2[N-morpholino]-ethane sulphonic acid) solutions of various concentrations. The measurements indicate no influence of concentration of the environment on its interaction with Tb3+ ion and as such reveal the fact that 160Tb is not a good probe for PAC studies.

Neutron activation analysis of trace elements in human hair: effect of dietary and environmental factors.

Effect of environment and dietary habits on trace element contents in human scalp hair has been investigated in the present work. Trace elements were detected with neutron activation technique. It was found that diet and environment contribute largely towards the trace elements in human body. Further trace elements Te, Lu, Ba, Cs, Yb, Re, Hf, In and Ir were detected for the first time in human scalp hair.