Nathan H. Wells

Potential and limitations of invariant kernel conformal therapy.

A treatment planning methodology was developed to investigate the invariant kernel form of conformal therapy proposed by Brahme. Three-dimensional dose distributions were calculated by convolving a rotationally symmetric, invariant kernel with weighting distributions. Fourier transform convolution techniques implemented on an array processor were used to achieve high calculation speeds, thereby allowing iterative techniques in the spatial and frequency domains for computing dose distributions that asymptotically approach a desired dose distribution. To use rotationally symmetric kernels, the generality of the solution is traded for a fast, deterministic, inverse planning approach. The limitations imposed on the dose distributions by this loss of generality are characterized and tentative conclusions are drawn about the potentials and limits of clinical application of this form of the methodology. Further developments of the concept are suggested.

Use of a multileaf collimator as a dynamic missing-tissue compensator

A dynamic multileaf collimator (D-MLC) was used to investigate the feasibility of producing missing-tissue compensators. The modulation of the x-ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D-MLC-shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moiré camera. The tissue deficit data were used to generate a series of D-MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D-MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters.

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

A new calibration protocol, developed by the AAPM Task Group 51 (TG‐51) to replace the TG‐21 protocol, is based on an absorbed‐dose to water standard and calibration factor (ND,w), while the TG‐21 protocol is based on an exposure (or air‐kerma) standard and calibration factor (Nx). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG‐51 may be somewhat different from those based on TG‐21. The Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG‐51 report, covering the majority of current manufacturers. Measured ratios between absorbed‐dose and air‐kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG‐21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed‐dose to water determined by TG‐51 and that determined by TG‐21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG‐51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG‐51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy. PACS number(s): 87.66.–a, 87.53.–j

On the need for monitor unit calculations as part of a beam commissioning methodology for a radiation treatment planning system.

This paper illustrates the need for validating the calculation of monitor units as part of the process of commissioning a photon beam model in a radiation treatment planning system. Examples are provided in which this validation identified subtle errors, either in the dose model or in the implementation of the dose algorithm. These errors would not have been detected if the commissioning process only compared relative dose distributions. A set of beam configurations, with varying field sizes, source‐to‐skin distances, wedges, and blocking, were established to validate monitor unit calculations for two different beam models in two different radiation treatment planning systems. Monitor units calculated using the treatment planning systems were compared with monitor units calculated from point dose calculations from tissue‐maximum ratio (TMR) tables. When discrepancies occurred, the dose models and the code were analyzed to identify the causes of the discrepancies. Discrepancies in monitor unit calculations were both significant (up to 5%) and systematic. Analysis of the dose computation software found: (1) a coordinate system transformation error, (2) mishandling of dose‐spread arrays, (3) differences between dose calculations in the commissioning software and the planning software, and (4) shortcomings in modeling of head scatter. Corrections were made in the beam calculation software or in the data sets to overcome these discrepancies. Consequently, we recommend incorporating validation of monitor unit calculations as part of a photon beam commissioning process. PACS number(s): 87.53.–j, 87.66.–a

Rotational kernels for conformal therapy

Conformal therapy treatment planning involves determining an irradiation strategy to deliver a dose distribution which is optimized for a given tumor volume. A technique proposed by Brahme restricts the search for the optimal treatment strategy to x-ray distributions in which points within the target volume are irradiated uniformly under rotation of the beam source around the patient. The dose distribution in this case may be calculated as a convolution of an irradiation weighting distribution with an invariant kernel. A procedure is described for calculating three-dimensional kernels to be used for clinical treatment planning with x rays produced by an electron accelerator. The convolution kernel is calculated as the sum of pencil beams irradiating the center of a cylindrical phantom uniformly from all angles. The shape of the kernel at points off the center of the phantom is investigated by means of numerical calculations which support the assumption that the kernel is invariant with respect to position within the phantom. The calculated kernels are verified by comparison with experimentally measured rotational arc dose distributions.

Dosimetry of effective wedge fields produced by an internal wedge

A procedure is described to calculate the monitor unit ratios required to produce effective wedge fields having a desired wedge angle by combining an internal 60 degrees wedge with an open field. Complementary procedures are derived and demonstrated for calculating the effective wedge dose distributions with wedge angles of 15 degrees, 30 degrees, and 45 degrees using the central axis depth dose data and off-axis ratios of the open field and the 60 degrees wedged field. Measurements at five points on and off the central axis within each field and measurements of the effective wedge factor demonstrated that the calculated wedge distributions were correctly delivered to within 2% in all cases.

Consistency of absorbed dose to water measurements using 21 ion-chamber models following the AAPM TG51 and TG21 calibration protocols

In 1999, the AAPM introduced a reference dosimetry protocol, known as TG51, based on an absorbed dose standard. This replaced the previous protocol, known as TG21, which was based on an air kerma standard. A significant body of literature has emerged discussing the improved accuracy and robustness of the absorbed dose standard, and quantifying the changes in baseline dosimetry with the introduction of the absorbed dose protocol. A significant component playing a role in the overall accuracy of beam output determination is the variability due to the use of different dosimeters. This issue, not adequately addressed in the past, is the focus of the present study. This work provides a comparison of absorbed dose determinations using 21 different makes and models of ion chambers for low- and high-energy photon and electron beams. The study included 13 models of cylindrical ion chambers and eight models of plane-parallel chambers. A high degree of precision (<0.25%) resulted from measurements with all chambers in a single setting, a sufficient number of repeat readings, and the use of high quality ion chambers as external monitors. Cylindrical chambers in photon beams show an improvement in chamber-to-chamber consistency with TG51. For electron dosimetry with plane-parallel chambers, the parameters Ngas and the product ND,w x k(ecal) were each determined in two ways, based on (i) an ADCL calibration, and (ii) a cross comparison with an ADCL-calibrated cylindrical chamber in a high-energy electron beam. Plane-parallel chamber results, therefore, are presented for both methods of chamber calibration. Our electron results with technique (i) show that plane-parallel chambers, as a group, overestimate the beam output relative to cylindrical chambers by 1%-2% with either protocol. Technique (ii), by definition, normalizes the plane-parallel results to the cylindrical results. In all cases, the maximum spread in output from the various cylindrical chambers is <2% implying a standard deviation of less than 0.5%. For plane-parallel chambers, the maximum spread is somewhat larger, up to 3%. A few chambers have been identified as outliers.

Monitor unit calculations as part of beam commissioning for a radiation treatment planning system

The purpose of this paper is to illustrate the need for validating the calculation of monitor units as part of the process of commissioning a photon beam model in a treatment planning system. Three examples are provided in which this validation identified subtle errors, either in the dose model or in the implementation of the dose algorithm. These errors would not have been detectable if the commissioning process only compared relative dose distributions. A set of beam configurations, with varying rectangular field sizes, secondary blocking, wedges, and SSDs, was established to validate monitor unit calculations for two beam models in two treatment planning systems. Monitor units calculated using the treatment planning systems were compared with monitor units calculated from point dose calculations (TMR tables). When discrepancies occurred, the dose models and the code were analyzed to identify the causes of the discrepancies. Discrepancies in monitor unit calculations were both significant (up to 5%) and systematic. Analysis of the dose computation software found (1) a coordinate system transformation error, (2) mishandling of dose-spread arrays, (3) differences between dose calculations in the commissioning software and the planning software, and (4) shortcomings in modeling of head scatter. Corrections were made in the beam calculation software or in the data sets to overcome these discrepancies. Consequently, we recommend incorporating a comprehensive validation of monitor unit calculations as part of any beam commissioning process.

SU‐FF‐T‐302: Response of OneDoseTM and TLD100 Dosimeters Over 6MV to Pd‐103 Equivalent Energies

Purpose: Two widely available in vivo dosimeters are TLD and MOSFETdetectors. Both detectors are significantly energy dependent, especially in the keV energy region. The purpose of this work is to measure the energy dependence of LiF:Mg,Ti TLD‐100 and OneDose™ (Sicel) MOSFETdetectors with particular emphasis on low energy x‐rays and gamma‐rays. Methods: Eleven beams, covering an energy range 6MV down to equivalent of 103Pd x‐rays have been employed for this work. All detectors were irradiated in air to ∼50 cGy water‐equivalent dose. Other than the 60Co, beams were calibrated with Farmer ionization chambers just prior to detector irradiations. Dose rate for 60Co was based on clinical calibration data. For the 60Co and 137Cs beams, detectors were “sandwiched” between 5mm and 1.2mm buildups respectively. Thicknesses of the detectors were adequate to establish electronic equilibrium for the lower energy beams. To reduce uncertainty in the mean response, twelve samples were irradiated with each beam. The OneDose™ detectors have dimensions 6×33×1mm. TLD samples in the form of flat packs (10×10×0.3mm) were made by sealing ∼22mg powder in thin (0.06mm) poyethylene. Results: Since OneDose™ detectors were factory‐calibrated with 60Co, both OneDose™ and TLD responses were normalized to 60Co. For OneDose™, over‐response relative to 60Co is observed to be largest (3.50±0.04) at 100 kVp beam (HVL 4.14 mmAl), and negligible at high energies (137Cs to 6MV). For TLD, over‐ response at 75kVp beam (HVL 2.09 mmAl) is observed to be 1.53. More data at lower energies and at 192Ir energy are in progress. Conclusion: Response of TLD and OneDose™ relative to 60Co has been measured over a wide energy range (6MV down to equivalent of 103Pd x‐rays). For TLD, our measured results agree with the recently published values fairly closely. OneDose™ results appear reasonable when compared with other type of MOSFETdetectors.

SU‐GG‐T‐308: Polymer Cross‐Linking and Sensitivity Enhancement in Normoxic Polymer Gels Used for Three‐Dimensional Dosimetry

Purpose: To discuss the relation between cross‐linking and sensitivity enhancement in normoxic polymergels in a dose range suitable for intensity modulated radiotherapy, based on considerations related to actual sensor preparation methods and a theoretical description of the optical behavior of irradiated polymergel when dose maps are obtained through optical CT.Method and Materials: Two set of samples of MAGIC gel (9% in weight of methacrylic acid) were prepared at 37°C and 45°C and irradiated with a 60 Co and 6 MV photon beams to doses in the range from 0.1 to 5 Gy. The samples were scanned in an optical CT. Non‐irradiated samples were also scan. A theory based on the number of sites for water solvation, which takes into account polymer cross‐linking, is introduced in order to explain the observed changes in the optical density. The slope of the polymergel response to dose is a measurement of sensitivity and it is analyzed for the actual experimental conditions as well as those for the optimum in connection to cross‐linking phenomena. Conclusion: The analysis showed a proper monotonic behavior for the polymergel response and maximum sensitivity when cross‐linking occurs in an important fraction in the preparation process.