摘要:

The thesis of this report is that potentially useful treatment beams can be chosen based on geometric heuristics and that a genetic algorithm (GA) can be constructed to find an optimal combination of beams based on a formal objective function. The paper describes the basic principles of a GA and the particular implementation developed. The code represents each plan in the population as two paired lists comprised of beam identifiers and relative weights. Reproduction operators, which mimic sexual reproduction with crossover, mutation, cloning, spontaneous generation, and death, manipulate the lists to grow optimal plans. The necessary gene pool is created by software modules which generate beams, distribute calculation points, obtain clinical constraints, add wedges, and calculate doses. The code has been tested on a set of artificial patients and on four clinical cases: prostate, pancreas, esophagus, and glomus. All demonstrated consistent results, indicating that the code is a reliable optimizer. Additional experiments compared the results for a full set of open beams to the geometrically selected set and the GA code with simulated annealing. Geometric selection of beam directions did not significantly compromise optimization quality. Compared to simulated annealing, the genetic algorithm was equally able to optimize the objective function, and calculations suggest it may be the faster method when the number of beams to be considered exceeds approximately 70.

ISSN:0269-3127    

DOI:10.1118/1.597660

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of dynamic movements on linear accelerators during irradiation has found a revised interest lately due to the integration of computers to control the accelerator. In this paper, dynamic wedge fields that are produced by moving one of the collimator blocks during irradiation are studied. Since these wedge fields differ from those of mechanical wedges, certain requirements are to be met on the treatment planning system. A pencil‐beam‐based treatment planning system that uses the resultant energy fluence distribution from the dynamic collimator movement has been extensively reviewed. In calculations, the system treats the dynamic collimated field as a single, modulated field that yields calculation times close to those for open beams. Details are given on the theoretical model used for the calculation of dynamically generated dose distributions. Measurements of depth doses, profiles, and output factors in dynamic wedge fields indicate that calculations accurately predict the outcome from dynamic wedges without any additional measurements other than those used for characterization of static open beams.

ISSN:0269-3127    

DOI:10.1118/1.597797

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The optimization of intensity distributions and the delivery of intensity‐modulated treatments with dynamic multi‐leaf collimators (MLC) offer important improvements to three‐dimensional conformal radiotherapy. In this study, a nine‐beam intensity‐modulated prostate plan was generated using the inverse radiotherapy technique. The resulting fluence profiles were converted into dynamic MLC leaf motions as functions of monitor units. The leaf motion pattern data were then transferred to the MLC control computer and were used to guide the motions of the leaves during irradiation. To verify that the dose distribution predicted by the optimization and planning systems was actually delivered, a homogeneous polystyrene phantom was irradiated with each of the nine intensity‐modulated beams incident normally on the phantom. For each exposure, a radiographic film was placed normal to the beam in the phantom to record the deposited dose. The films were calibrated and scanned to generate 2‐D isodose distributions. The dose was also calculated by convolving the incident fluence pattern with pencil beams. The measured and calculated dose distributions were compared and found to have discrepancies in excess of 5% of the central axis dose. The source of discrepancies was suspected to be the rounded edges of the leaves and the scattered radiation from the various components of the collimation system. After approximate corrections were made for these effects, the agreement between the two dose distributions was within 2%. We also studied the impact of the “tongue‐and‐groove” effect on dynamic MLC treatments and showed that it is possible to render this effect inconsequential by appropriately synchronizing leaf motions. This study also demonstrated that accurate and rapid delivery of realistic intensity‐modulated plans is feasible using a dynamic multi‐leaf collimator.

ISSN:0269-3127    

DOI:10.1118/1.597661

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dosimetric stability, linearity, dose rate dependence, and flatness of both photon and electron beams have been evaluated for a racetrack microtron at low monitor unit settings. For photons, the variation in dosimetric output about the mean is<0.4% at 100 monitor units (MU),<1% at 10 MU, and<4% at 2 MU. The output dependence on the dose rate varied by<0.6% between 85–300 MU/min. Flatness and symmetry for the 25‐ and 50‐MV beams showed deviations of<3% at bothdmaxand 10‐cm depths, and only slightly ≳3% at 20 cm, even at only 3 MU, in contrast with other scanned beam accelerators. Broad electron beams on the microtron are created by the superposition of the elementary beam pulses either directly from the scan magnets, or after their broadening through a scattering foil. The dosimetric instability both with and without the foil was less than 0.6% for both the 25‐ and 50‐MeV electrons. Dose nonlinearity was<1% above 10 MU. Field flatness was determined for scan matrices designed to produce a flat field both with and without a scattering foil. Symmetry and flatness deviations were<3% for both electron energies when a scattering foil was used, even for a single scan. The variation of the electron dose per monitor unit between dose rates of 85–300 MU/min was<1% (25 MeV) and<4% (50 MeV) when a scattering foil was used, but as high as 22% (25 MeV) and 36% (50 MeV) for broad beams generated by elementary beam pulses directly from the scanning magnets. The microtron exhibits dosimetric properties which fulfill the recommendations of Task Groups 21 and 25. Based on the stability of the scanned beam at low monitor unit settings, the microtron can be used for 3‐D conformal therapy with both photons and electrons.

ISSN:0269-3127    

DOI:10.1118/1.597662

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method to produce wedged fields at any orientation relative to the collimator is described. The wedged field is generated by combining two appropriately weighted orthogonal wedged field segments at fixed collimator and gantry positions. The method requires only that wedged fields can be produced in orthogonal directions without rotating the collimator, such as is commonly provided on most radiation therapy accelerators by sets of standard and rotated wedges. Expressions are derived relating the effective wedge angle and orientation to the weighting and wedge angles of the orthogonal wedged field segments. This technique will be important when using multileaf collimator field shaping for which collimator rotation is dictated by target or critical structure shape and orientation. The term omni wedge is introduced to describe this technique.

ISSN:0269-3127    

DOI:10.1118/1.597663

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Parametrizing the collimator scatter factor,Sc(or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change inScwhen the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predictSc's of rectangular fields more accurately. The first method parametrizesScby the side of the equivalent square of the flattening filter region visible from the point of calculation. TheSc's of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computesScof rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattening filter. The radial distribution of the extended source at the level of the flattening filter is computed fromScof square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy.

ISSN:0269-3127    

DOI:10.1118/1.597798

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose planning programs originally intended for use with symmetric fields have been adapted for use with asymmetric fields. An accurate representation of the change in primary beam quality with off‐axis distance and depth is essential for accurate dose calculation and is usually represented in the computer as a primary radiation profile or primary off‐center ratio (POCR). The original field edge correction (FEC) method described by Cadman [Med. Phys.22, 457 (1995)] to determine POCRs has been extended to allow accurate POCR values to be obtained to an off‐axis distance defined by the corners of the largest field, typically at an off‐axis distance of 28.3 cm. This technique requires only routine symmetric field measurements including beam profiles, TMRs, and collimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off‐center ratios (OCRs) using the boundary factor technique of Chuietal. [Med. Phys.15, 92 (1988)]. Excellent agreement with measured values was obtained for cross‐beam OCRs using a 10×10‐cm2field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20×20‐cm2field with each pair of jaws in a half‐blocked configuration.

ISSN:0269-3127    

DOI:10.1118/1.597664

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using megavoltage radiography and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the optical film density and the corresponding equivalent thickness of overlying phantom material. Results for Co‐60, 4‐MV, and 10‐MV photons show that the relationship depends on the sensitometric properties of the radiographic film and the photon beam quality. For an actual patient undergoing total body irradiation (TBI), a chest radiograph in treatment geometry provides the optical density information that is used to calculate the tissue deficit in the lung region. The compensators are made of lead whose thickness is chosen to replace the tissue deficit over the lung region. The validity of the method is established both by comparing its results to that obtained from multiple‐slice computed tomography (CT) data for 4‐ and 10‐MV photons, and by in‐phantom thermoluminescent dosimetry (TLD) for Co‐60, 4‐MV, and 10‐MV beams.

ISSN:0269-3127    

DOI:10.1118/1.597799

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using Monte Carlo simulations we have studied the electron mean energy, Ēo, and the most probable energy,Eo,p, at the phantom surface and their relationships with half‐value depth, R50, and the practical range, Rp, for a variety of beams from five commercial medical accelerators with an energy range of 5–50 MeV. It is difficult to obtain a relation between R50and Ēofor all electrons at the surface because the number of scattered lower‐energy electrons varies with the machine design. However, using only direct electrons to calculate Ēo, there is a relationship which is in close agreement with that calculated using monoenergetic beams by Rogers and Bielajew [Med. Phys.13, 687–694 (1986)]. We show that the empirical formulaEo,p=0.22+1.98Rp+0.0025Rp2describes accurately the relationship between RpandEo,pfor clinical beams of energies from 5 to 50 MeV with an accuracy of 3%. The electron mean energy, Ēd, is calculated as a function of depth in water as well as plastic phantoms and is compared both with the relation, Ēd=Ēo(1−d/Rp), employed in AAPM protocols and with values in the IAEA Code of Practice. The conventional relations generally overestimate Ēdover the entire therapeutic depth, e.g., the AAPM and IAEA overestimate Ēdat dmaxby up to 20% for an 18 MeV beam from a Clinac 2100C. It is also found that at all depths mean energies are 1%–3% higher near the field edges than at the central axis. We calculated depth‐scaling factors for plastic phantoms by scaling the depth in plastics to the water‐equivalent depth where the mean energies are equal. The depth‐scaling factor is constant with depth in a given beam but there is a small variation (<1.5%) depending on the incident beam energies. Depth‐scaling factors as a function of R50in plastic or water are presented for clear polystyrene, white polystyrene and PMMA phantom materials. The calculated depth‐scaling factor is found to be equal toR50water/R50plastic. This is just the AAPM definition of effective density but there are up to 2% discrepancies between our calculated values and those recommended by the AAPM and the IAEA protocols. We find that the depth‐scaling factors obtained by using the ratio of continuous‐slowing‐down ranges are inaccurate and overestimate our calculated values by 1%–2% in all cases. We also find that for accurate work, it is incorrect to use a simple 1/r2correction to convert from parallel beam depth‐dose curves to point source depth‐dose curves, especially for high‐energy beams.

ISSN:0269-3127    

DOI:10.1118/1.597788

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The absorbed dose values determined with the Exradin and PTW‐Markus plane‐parallel chambers were compared to the values obtained with the ferrous sulphate dosimetry for a number of the Philips SL25 and the Therac 20 electron beams. For the plane‐parallel chambers, the cavity‐gas calibration factorNgaspp, was derived by a direct comparison with a calibrated cylindrical chamber using the three different calibration methods as proposed by the newly published AAPM TG 39 protocol. For the ferrous sulphate dosimetry, anεmGvalue of 352×10−6m−2kg−1Gy−1was adopted from ICRU Report No. 35. The average ratio of the dose values determined with the plane‐parallel chambers and the dose values determined with the Fricke dosimetry system was 1.001±1.4%. These measurements are consistent with the AAPM TG 39 protocol.

ISSN:0269-3127    

DOI:10.1118/1.597800

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY