Incorporating Treatment Time into Butterfly Optimization to Reduce Total Treatment Time for Vaginal Cylinder Brachytherapy.

Purpose For patient comfort and safety, irradiation times should be kept at a minimum while maintaining high treatment quality. In this study of high dose rate (HDR) therapy with a vaginal cylinder, we used the butterfly optimization algorithm (BOA) to simultaneously optimize individual dwell times for precise dose conformity and for the reduction of total dwell time. Material and methods BOA is a population-based, meta-heuristic algorithm that averts local minima by conducting intensive local and global searching based on switching probability. We constructed an objective function (a stimulus intensity function) that consisted of two components. The first one was the root-mean-squared dose error (RMSE) defined as the square root of the sum of squared differences between the prescribed and delivered dose at the constraint points. The second component was weighted total treatment time. Eight previously treated cases were retrospectively reviewed by re-optimizing the clinical treatment plans with BOA.  Results Compared to the eight original plans generated with the commercial adaptive volume optimization algorithm (AVOA), the BOA-optimized plans reduced treatment times by 5.4% to 8.9%, corresponding to a time-saving of 13.1 to 47.7 seconds with the activities on the treatment day and saving from 29.3 to 64.6 seconds if treated with an activity of 5 CI. Dose deviations from the prescription were smaller than in the original plans. Conclusion  Dose optimizations based on the BOA algorithm yield closer dose conformity in vaginal HDR treatment than AVOA. Incorporating total treatment time into the optimization algorithm reduces the delivery time while having only a small effect on dose conformity.

Stereotactic radiosurgery with MLC-defined arcs: Verification of dosimetry, spatial accuracy, and end-to-end tests.

PURPOSE: To measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator-(MLC) equipped linear accelerator without cones or other special attachments or modifications. METHODS: Dosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ-index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC-shaped fields ranging from 1.6 × 5 to 30 × 30 mm2 were planned and delivered to an in-house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple-arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm2 fields applied at six different couch angles. For end-to-end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine-arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy. RESULTS: In the single-arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ-index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm2 and rose to 4.4% as field size increased to 20 × 20 mm2 . Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine-arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end-to-end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained <1 at all pixels. To date, more than 50 functional SRS treatments using MLC-shaped static field arcs have been delivered. CONCLUSION: Stereotactic radiosurgery (SRS) can be planned and delivered on a standard linac without cones or other modifications with better than 0.5 mm spatial and 5% dosimetric accuracy.

The virtual cone: A novel technique to generate spherical dose distributions using a multileaf collimator and standardized control-point sequence for small target radiation surgery.

PURPOSE: The study aimed to develop and demonstrate a standardized linear accelerator multileaf collimator-based method of delivering small, spherical dose distributions suitable for radiosurgical treatment of small targets such as the trigeminal nerve. METHODS AND MATERIALS: The virtual cone is composed of a multileaf collimator-defined field with the central 2 leaves set to a small gap. For 5 table positions, clockwise and counter-clockwise arcs were used with collimator angles of 45 and 135 degrees, respectively. The dose per degree was proportional to the sine of the gantry angle. The dose distribution was calculated by the treatment planning system and measured using radiochromic film in a skull phantom for leaf gaps of 1.6, 2.1, and 2.6 mm. Cones with a diameter of 4 mm and 5 mm were measured for comparison. Output factor constancy was investigated using a parallel-plate chamber. RESULTS: The mean ratio of the measured-to-calculated dose was 0.99, 1.03, and 1.05 for 1.6, 2.1, and 2.6 mm leaf gaps, respectively. The diameter of the measured (calculated) 50% isodose line was 4.9 (4.6) mm, 5.2 (5.1) mm, and 5.5 (5.5) mm for the 1.6, 2.1, and 2.6 mm leaf gap, respectively. The measured diameter of the 50% isodose line was 4.5 and 5.7 mm for the 4 mm and 5 mm cones, respectively. The standard deviation of the parallel-plate chamber signal relative to a 10 cm × 10 cm field was less than 0.4%. The relative signal changed 32% per millimeter change in leaf gap, indicating that the parallel-plate chamber is sensitive to changes in gap width. CONCLUSIONS: The virtual cone is an efficient technique for treatment of small spherical targets. Patient-specific quality assurance measurements will not be necessary in routine clinical use. Integration directly into the treatment planning system will make planning using this technique extremely efficient.

A novel phantom and procedure providing submillimeter accuracy in daily QA tests of accelerators used for stereotactic radiosurgery*.

Stereotactic radiosurgery (SRS) places great demands on spatial accuracy. Steel BBs used as markers in quality assurance (QA) phantoms are clearly visible in MV and planar kV images, but artifacts compromise cone-beam CT (CBCT) isocenter localization. The purpose of this work was to develop a QA phantom for measuring with sub-mm accuracy isocenter congruence of planar kV, MV, and CBCT imaging systems and to design a practical QA procedure that includes daily Winston-Lutz (WL) tests and does not require computer aid. The salient feature of the phantom (Universal Alignment Ball (UAB)) is a novel marker for precisely localizing isocenters of CBCT, planar kV, and MV beams. It consists of a 25.4mm diameter sphere of polymethylmetacrylate (PMMA) containing a concentric 6.35mm diameter tungsten carbide ball. The large density difference between PMMA and the polystyrene foam in which the PMMA sphere is embedded yields a sharp image of the sphere for accurate CBCT registration. The tungsten carbide ball serves in finding isocenter in planar kV and MV images and in doing WL tests. With the aid of the UAB, CBCT isocenter was located within 0.10 ± 0.05 mm of its true positon, and MV isocenter was pinpointed in planar images to within 0.06 ± 0.04mm. In clinical morning QA tests extending over an 18 months period the UAB consistently yielded measurements with sub-mm accuracy. The average distance between isocenter defined by orthogonal kV images and CBCT measured 0.16 ± 0.12 mm. In WL tests the central ray of anterior beams defined by a 1.5 × 1.5 cm2 MLC field agreed with CBCT isocenter within 0.03 ± 0.14 mm in the lateral direction and within 0.10 ± 0.19 mm in the longitudinal direction. Lateral MV beams approached CBCT isocenter within 0.00 ± 0.11 mm in the vertical direction and within -0.14 ± 0.15 mm longitudinally. It took therapists about 10 min to do the tests. The novel QA phantom allows pinpointing CBCT and MV isocenter positions to better than 0.2 mm, using visual image registration. Under CBCT guidance, MLC-defined beams are deliverable with sub-mm spatial accuracy. The QA procedure is practical for daily tests by therapists.

End-to-end test of spatial accuracy in Gamma Knife treatments for trigeminal neuralgia.

PURPOSE: Spatial accuracy is most crucial when small targets like the trigeminal nerve are treated. Although current quality assurance procedures typically verify that individual apparatus, like the MRI scanner, CT scanner, Gamma Knife, etc., are meeting specifications, the cumulative error of all equipment and procedures combined may exceed safe margins. This study uses an end-to-end approach to assess the overall targeting errors that may have occurred in individual patients previously treated for trigeminal neuralgia. METHODS: The trigeminal nerve is simulated by a 3 mm long, 3.175 mm (1/8 in.) diameter MRI-contrast filled cavity embedded within a PMMA plastic capsule. The capsule is positioned within the head frame such that the location of the cavity matches the Gamma Knife coordinates of an arbitrarily chosen, previously treated patient. Gafchromic EBT2 film is placed at the center of the cavity in coronal and sagittal orientations. The films are marked with a pinprick to identify the cavity center. Treatments are planned for radiation delivery with 4 mm collimators according to MRI and CT scans using the clinical localizer boxes and acquisition protocols. Shots are planned so that the 50% isodose surface encompasses the cavity. Following irradiation, the films are scanned and analyzed. Targeting errors are defined as the distance between the pinprick, which represents the intended target, and the centroid of the 50% isodose line, which is the center of the radiation field that was actually delivered. RESULTS: Averaged over ten patient simulations, targeting errors along the x, y, and z coordinates (patient's left-to-right, posterior-to-anterior, and head-to-foot) were, respectively, -0.060 ± 0.363, -0.350 ± 0.253, and 0.348 ± 0.204 mm when MRI was used for treatment planning. Planning according to CT exhibited generally smaller errors, namely, 0.109 ± 0.167, -0.191 ± 0.144, and 0.211 ± 0.094 mm. The largest errors along individual axes in MRI- and CT-planned treatments were, respectively, -0.761 mm in the y-direction and 0.428 mm in the x-direction, well within safe limits. CONCLUSIONS: The highly accurate dose delivery was possible because the Gamma Knife, MRI scanner, and other equipment performed within tight limits and scans were acquired using the thinnest slices and smallest pixel sizes available. Had the individual devices performed only near the limits of their specifications, the cumulative error could have left parts of the trigeminal nerve undertreated. The presented end-to-end test gives assurance that patients had received the expected high quality treatment. End-to-end tests should become part of clinical practice.

Beam geometry selection using sequential beam addition.

PURPOSE: The selection of optimal beam geometry has been of interest since the inception of conformal radiotherapy. The authors report on sequential beam addition, a simple beam geometry selection method, for intensity modulated radiation therapy. METHODS: The sequential beam addition algorithm (SBA) requires definition of an objective function (score) and a set of candidate beam geometries (pool). In the first iteration, the optimal score is determined for each beam in the pool and the beam with the best score selected. In the next iteration, the optimal score is calculated for each beam remaining in the pool combined with the beam selected in the first iteration, and the best scoring beam is selected. The process is repeated until the desired number of beams is reached. The authors selected three treatment sites, breast, lung, and brain, and determined beam arrangements for up to 11 beams from a pool comprised of 25 equiangular transverse beams. For the brain, arrangements were additionally selected from a pool of 22 noncoplanar beams. Scores were determined for geometries comprised equiangular transverse beams (EQA), as well as two tangential beams for the breast case. RESULTS: In all cases, SBA resulted in scores superior to EQA. The breast case had the strongest dependence on beam geometry, for which only the 7-beam EQA geometry had a score better than the two tangential beams, whereas all SBA geometries with more than two beams were superior. In the lung case, EQA and SBA scores monotonically improved with increasing number of beams; however, SBA required fewer beams to achieve scores equivalent to EQA. For the brain case, SBA with a coplanar pool was equivalent to EQA, while the noncoplanar pool resulted in slightly better scores; however, the dose-volume histograms demonstrated that the differences were not clinically significant. CONCLUSIONS: For situations in which beam geometry has a significant effect on the objective function, SBA can identify arrangements equivalent to equiangular geometries but using fewer beams. Furthermore, SBA provides the value of the objective function as the number of beams is increased, allowing the planner to select the minimal beam number that achieves the clinical goals. The method is simple to implement and could readily be incorporated into an existing optimization system.

Patient safety considerations concerning the scheduling of emergency-off system tests.

Emergency-off systems (EOS) are essential to the safe operation of medical accelerators and other high-risk equipment. To assure reliable functioning, some states require weekly tests; others permit monthly, tri-monthly or even six-monthly tests, while some do not specify test intervals. We investigate the relative safety of the various test schedules by computing the fraction of time during which a nonfunctional state of the EOS may remain undetected. Special attention is given to the effect of flexibility (i.e., to regulations that specify the number of tests that have to be done in any given time interval, but allow a range within the interval during which a test can be done). Compared to strict test intervals, a schedule that provides flexibility increases risk only marginally. Performing tests on any arbitrary day of the week when weekly tests are required increases the time span during which a nonfunctionality goes undetected by only 17%, compared to an exact one-week schedule. The same ratio applies for monthly tests. For a three-month schedule, the relative risk increases by only 2% if tests are done on an arbitrarily chosen day during each due-month, compared to tests done on an exact three-month schedule. The most irregular time intervals possible in a three-calendar month schedule increase the relative risk by 11%. For the six-month and the 12-month schedule the ratio of risks is even smaller. The relative risk is virtually independent of the mean time between failures of the EOS, but the absolute risk decreases in proportion the mean time between failures. Adherence to strict, resource-intensive test intervals provides little extra safety compared to flexible intervals that require the same number of tests per year. Regulations should be changed to provide the practicality offered by flexible test schedules. Any additional increase in patient safety could be achieved by strict regulations concerning reliability of emergency-stop (e-stop) systems.

Dynamic MLC leaf sequencing for integrated linear accelerator control systems.

PURPOSE: Leaf positions for dynamic multileaf collimator (DMLC) intensity modulated radiation therapy must be closely synchronized with MU delivery. For the Varian C3 series MLC controller, if the planned trajectory (leaf position vs. MU) requires velocities exceeding the capability of the MLC, the leaves fall behind the planned positions, causing the controller to momentarily hold the beam and thereby introduce dosimetric errors. We investigated the merits of a new commercial linear accelerator, TrueBeam™, that integrates MLC control with prospective dose rate modulation. If treatment is delivered at dose rates so high that leaves would fall behind, the controller reduces the dose rate such that harmony between MU and leaf position is preserved. METHODS: For three sets of DMLC leaf trajectories, point doses and two-dimensional dose distributions were measured in phantom using an ionization chamber and film, respectively. The first set, delivered using both a TrueBeam™ and a conventional C3 controller, comprised a single leaf bank closing at planned velocities of 2.4, 7.1, and 14 cm/s. The maximum achievable leaf velocity for both systems was 3 cm/s. The remaining two sets were derived from clinical fluence maps using a commercial treatment planning system for a range of planned dose rates and were delivered using TrueBeam™ set to the maximum dose rate, 600 MU/min. Generating trajectories using a planned dose rate that is lower than the delivery dose rate effectively increased the leaf velocity constraint used by the planning system for trajectory calculation. The second set of leaf trajectories was derived from two fluence maps containing regions of zero fluence obtained from representative beams of two different patient treatment plans. The third set was obtained from all nine fields of a head and neck treatment plan. For the head and neck plan, dose-volume histograms of the spinal cord and target for each planned dose rate were obtained. RESULTS: For the single closing leaf bank trajectories, the TrueBeam™ control system reduced the dose rate such that the leaf velocity was less than the maximum. Dose deviations relative to the 2.4 cm/s trajectory were less than 3%. For the conventional controller, the leaves repeatedly fell behind the planned positions until the beam hold threshold was reached, resulting in deviations of up to 19% relative to the 2.4 cm/s trajectory. For the two clinical fluence maps, reducing the planned dose rate reduced the dose in the zero fluence regions by 15% and 24% and increased the delivery time by 5 s and 14 s. No significant differences were noted in the high and intermediate dose regions measured using film. The DVHs for the head and neck plan showed a 10% reduction in cord dose for 20 MU/min relative to 600 MU/min sequencing dose rate, which was confirmed by measurement. No difference in target DVHs were observed. The reduction in cord dose increased total treatment time by 1.8 min. CONCLUSIONS: Leaf sequencing algorithms for integrated control systems should be modified to reflect the reduced importance of maximum leaf velocity for accurate dose delivery.

RapidArc radiation therapy: first year experience at the University of Alabama at Birmingham.

PURPOSE: To evaluate treatment planning and delivery for patients treated during our initial year of experience with RapidArc radiation therapy. METHODS AND MATERIALS: RapidArc was used to treat 52 patients at The University of Alabama at Birmingham between May 2008 and April 2009. A single ionization chamber phantom with film and a two-dimensional ionization chamber array were used for quality assurance measurements. Of the 52 patients, 44 had a static gantry dynamic multileaf collimated (SG-DMLC) IMRT treatment plan, seven of which had quality assurance (QA) measurements. RESULTS: The mean difference between ionization chamber measurement and calculation was 1.2% +/- 0.9% (1 standard deviation). For film, the mean fraction of pixels with gamma > 1 (3%/3 mm criterion) was 4.6% and for the two-dimensional chamber array was 1.4%. For the seven corresponding SG-DMLC plans, the results were similar. Differences in important dosimetric indicators were typically within 1% relative to SG-DMLC. The volume of nontarget tissue that received >20 Gy was less for RapidArc compared with SG-DMLC, whereas the volume that received more than 10 Gy was larger. The mean difference between the measured and planned leaf positions and the monitor units obtained from machine log files was 0.0 +/- 0.5 mm and 0.4 +/- 0.3 MU, respectively. Mean delivery times were 1.5 +/- 0.2 and 3.3 +/- 0.4 min for one- and two-arc plans, respectively. On average, SG-DMLC delivery took 4.4 min longer. CONCLUSIONS: RapidArc plans have quality comparable to our standard SG-DMLC IMRT technique, and are delivered with similar accuracy in shorter time.

Performance of a commercial Macro Monte Carlo dose calculation algorithm for determining output factors of clinical electron fields.

We compare measured output factors of clinical electron fields to those calculated by a commercial treatment planning system based on an electron Monte Carlo algorithm. The measured data is comprised of 195 fields with energies 6 to 18 MeV, applicator sizes 6 x 6 cm(2) to 25 x 25 cm(2), and source to surface distances (SSDs) of 97 to 107 cm. Due to a scarcity of clinical fields for the highest energies and the largest applicator sizes, additional measurements were made at arbitrarily chosen large field sizes at previously not used energies, for a total of 223 output factors. The difference between calculation and measurement ranged from -2.9% to 3.9%, with a mean difference of -0.2%. Half of the field shapes had a difference with magnitude less than 0.8%. Only 7 (3%) of the field shapes were outliers, having differences greater than 2%. All outliers had field widths at the normalization point < 3.5 cm, were applied at SSDs > 100 cm, were inserts for the 25 _ 25 cm(2) applicator, or had more than one of these characteristics. For narrow and elongated fields the TPS slightly overestimated output factors, whereas for field shapes with aspect ratio close to 1 the TPS slightly underestimated the output factors. No strong dependence of the difference on energy was observed.

Development of an accelerated GVF semi-automatic contouring algorithm for radiotherapy treatment planning.

Fast contouring is important in image-guided radiation therapy (IGRT) and adaptive radiation therapy (ART) where large computed tomography (CT) volumes have to be segmented. In this study, a modified active contour (also called snake) segmentation method based on a faster gradient-vector-flow (GVF) calculation algorithm is proposed. The accelerated method was tested on multiple organs, including lung, right ventricle, kidney and prostate. Compared to the original algorithm, the improved one reduced GVF calculation times to one-half or less without compromising contour accuracy.

Bi- and tri-exponential fitting to TG-43 radial dose functions of brachytherapy sources based on a genetic algorithm.

PURPOSE: To find the coefficients for bi- and tri-exponential fitting functions to represent the radial dose functions of 16 commercially available brachytherapy sources. METHODS AND MATERIALS: The search for the coefficients was done using a genetic algorithm. Coefficients were encoded into chromosomes, which were subjected to crossover and mutation. After each operation, chromosomes were evaluated according to their fitness and the better ones were chosen with higher probability for the next generation. The best chromosomes obtained after 2000 operations were used for the coefficients. RESULTS: For all brachytherapy sources, tri-exponential dose functions agreed with the respective input data within 1.4%. The mean deviation, obtained by averaging absolute deviations of all sources and input data, was <1.0%. For 8 of the 16 sources, the fit offered by bi-exponential functions was virtually identical to that of tri-exponential ones. CONCLUSION: Tri-exponential functions can accurately represent the radial dose functions of all commercially available brachytherapy sources. For the eight sources where bi-exponential functions provide nearly equally accurate fits, their continued usage is recommended.

Dose optimization of breast balloon brachytherapy using a stepping 192Ir HDR source.

There is a considerable underdosage (11%-13%) of PTV due to anisotropy of a stationary source in breast balloon brachytherapy. We improved the PTV coverage by varying multiple dwell positions and weights. We assumed that the diameter of spherical balloons varied from 4.0 cm to 5.0 cm, that the PTV was a 1-cm thick spherical shell over the balloon (reduced by the small portion occupied by the catheter path), and that the number of dwell positions varied from 2 to 13 with 0.25-cm steps, oriented symmetrically with respect to the balloon center. By assuming that the perfect PTV coverage can be achieved by spherical dose distributions from an isotropic source, we developed an optimization program to minimize two objective functions defined as: (1) the number of PTV-voxels having more than 10% difference between optimized doses and spherical doses, and (2) the difference between optimized doses and spherical doses per PTV-voxel. The optimal PTV coverage occurred when applying 8-11 dwell positions with weights determined by the optimization scheme. Since the optimization yields ellipsoidal isodose distributions along the catheter, there is relative skin sparing for cases with source movement approximately tangent to the skin. We also verified the optimization in CT-based treatment planning systems. Our volumetric dose optimization for PTV coverage showed close agreement to linear or multiple-points optimization results from the literature. The optimization scheme provides a simple and practical solution applicable to the clinic.

Effect of beam number on organ-at-risk sparing in dynamic multileaf collimator delivery of intensity modulated radiation therapy.

Due to practical limitations such as inter- and intraleaf transmission, nondivergent leaf end design, and leaf scatter, multileaf collimators (MLCs) are unable to accurately produce the ideal fluence patterns generated by inverse planning systems. Consequently, low dose regions receive substantially more radiation than they would with an ideal MLC that could generate the desired fluence pattern. Previous work by others has found that the discrepancy between desired and actual fluence patterns produced by an MLC increases rapidly with increasing complexity of the desired fluence map. In addition to the complexity of individual fluence maps, other parameters can contribute to the overall complexity of a treatment plan, most notably the number of beams. In this work, we investigate the effect of beam number on critical structure sparing for dynamic MLC delivered intensity modulated radiation therapy. Six cases from each of two challenging clinical sites, previously irradiated head and neck and paraspinal metastasis, were planned with the goal of minimizing the spinal cord dose. Plans were developed for five to 27 beams. All plans were renormalized such that the target volume receiving the prescription dose was the same for all plans of each site. For each case, we calculated the spinal cord D0.5 cm3 (the dose such that 0.5 cm3 of normal tissue receives greater than or equal to D0.5 cm3), normal tissue D1 cm3, the normal tissue mean dose, and the standard deviation of dose in the planning target volume (PTV). For the head and neck cases, the mean increase in spinal cord D0.5 cm3 between seven and 27 beam plans was 10% of the prescription dose, whereas for the paraspinal case, the increase was 2.6%. For the head and neck cases, the mean decrease in normal tissue D1 cm3 between seven and 11 beam plans was 2.6% and was constant for more than 11 beams. For the paraspinal cases, the mean decrease in normal tissue D1 cm3 between seven and 27 beam plans was 3.7%. The mean normal tissue dose was approximately independent of the number of beams for both sites. For the head and neck cases, the PTV standard deviation was independent of the number of beams, while for the paraspinal cases it decreased by an average of 1.8% from seven to 27 beams. Calculations for seven and 27 beams in which the MLC transmission was varied from 0% to 2% demonstrated that the increase in spinal cord D0.5 cm3 with increasing number of beams is largely due to MLC transmission, which is not included in the optimization. Increasing the number of beams increased the critical structure dose, although decreasing beam number results in increasing normal tissue D1 cm3 and target dose heterogeneity. The optimal tradeoff is dependent on the clinical situation, but seems to be seven to nine beams. Beam geometry optimization may reduce the number of beams required to provide adequate target coverage, thus limiting critical structure dose.

Analytic solution for source distributions in brachytherapy to obtain uniform dose rates in spherical tumor models.

Interstitial brachytherapy involves implanting many small radioactive sources into a tumor, with the goal of delivering a uniform radiation dose to the target volume. As a guide for the optimal placement of these sources, we assumed a spherical tumor irradiated by a continuously distributed radiation source. The solution of the ensuing integral equation shows that the source density is very low near the center of the sphere, increases rapidly toward the surface and becomes infinite at the surface. Integration of the source density over a given spherical sub-volume shows that only about 6% of the total activity is contained in the central core up to 50% of the tumor radius, while about one-half of the activity has to be placed in the outer spherical shell having a thickness of one-tenth of the tumor radius. Since attenuation is not taken into account, the results are applicable to highly penetrating radiation of isotopes such as 192Ir and 137Cs and tumor radii of a few cm. This situation is approximated in the high dose rate (HDR) treatment of the prostate using 192Ir. The results are in good agreement with the recommendations given in the traditional Paterson-Parker tables for radium and cesium treatments and a numerical solution to the problem.

A device for precision positioning and alignment of room lasers to diminish their contribution to patient setup errors.

This report presents an analysis of patient setup errors resulting from inaccurately positioned wall lasers. It suggests that laser beams should agree within 0.2 degree or better with the machine axes that they are delineating. For typical simulator and treatment rooms having wall-to-isocenter distances of 3 m, this requirement is satisfied when the beam-emitting aperture is mounted within about 1.0 cm from the intersection of the respective machine axis with the wall. To achieve the required precision, we developed and clinically tested a simple, inexpensive tool, the Laser Placer (LP). The essential component of the LP is a cube with mirror surfaces that is aligned with the machine axes using built-in spirit levels and the light field and cross hairs of the collimator. Wall, ceiling, and sagittal lasers are installed and aligned according to reflections of their beams by the cube, and reference lines provided by the LP. Measurements showed that, even in new accelerator installations performed by highly experienced technicians, wall lasers are often mounted off target by more than 1.5 cm. Such inaccuracies can contribute systematic errors of 2 mm or more to the random setup errors attributable to interfraction movement in patient anatomy. To keep setup errors to a minimum, medical physicists should check beam orthogonality in addition to beam congruence at isocenter as recommended by the TG-40 quality assurance protocol from the American Association of Physicists in Medicine.

Comprehensive evaluation of a commercial macro Monte Carlo electron dose calculation implementation using a standard verification data set.

A commercial electron dose calculation software implementation based on the macro Monte Carlo algorithm has recently been introduced. We have evaluated the performance of the system using a standard verification data set comprised of two-dimensional (2D) dose distributions in the transverse plane of a 15 X 15 cm2 field. The standard data set was comprised of measurements performed for combinations of 9-MeV and 20-MeV beam energies and five phantom geometries. The phantom geometries included bone and air heterogeneities, and irregular surface contours. The standard verification data included a subset of the data needed to commission the dose calculation. Additional required data were obtained from a dosimetrically equivalent machine. In addition, we performed 2D dose measurements in a water phantom for the standard field sizes, a 4 cm X 4 cm field, a 3 cm diameter circle, and a 5 cm X 13 cm triangle for the 6-, 9-, 12-, 15-, and 18-MeV energies of a Clinac 21EX. Output factors were also measured. Synthetic CT images and structure contours duplicating the measurement configurations were generated and transferred to the treatment planning system. Calculations for the standard verification data set were performed over the range of each of the algorithm parameters: statistical precision, grid-spacing, and smoothing. Dose difference and distance-to-agreement were computed for the calculation points. We found that the best results were obtained for the highest statistical precision, for the smallest grid spacing, and for smoothed dose distributions. Calculations for the 21EX data were performed using parameters that the evaluation of the standard verification data suggested would produce clinically acceptable results. The dose difference and distance-to-agreement were similar to that observed for the standard verification data set except for the portion of the triangle field narrower than 3 cm for the 6- and 9-MeV electron beams. The output agreed with measurements to within 2%, with the exception of the 3-cm diameter circle and the triangle for 6 MeV, which were within 5%. We conclude that clinically acceptable results may be obtained using a grid spacing that is no larger than approximately one-tenth of the distal falloff distance of the electron depth dose curve (depth from 80% to 20% of the maximum dose) and small relative to the size of heterogeneities. For judicious choices of parameters, dose calculations agree with measurements to better than 3% dose difference and 3-mm distance-to-agreement for fields with dimensions no less than about 3 cm.