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PubMed1999

Evaluation of polymer gels and MRI as a 3-D dosimeter for intensity-modulated radiation therapy.

BANG gel (MGS Research, Inc., Guilford, CT) has been evaluated for measuring intensity-modulated radiation therapy (IMRT) dose distributions. Treatment plans with target doses of 1500 cGy were generated by the Peacock IMRT system (NOMOS Corp., Sewickley, PA) using test target volumes. The gels were enclosed in 13 cm outer diameter cylindrical glass vessels. Dose calibration was conducted using seven smaller (4 cm diameter) cylindrical glass vessels irradiated to 0-1800 cGy in 300 cGy increments. Three-dimensional maps of the proton relaxation rate R2 were obtained using a 1.5 T magnetic resonance imaging (MRI) system (Siemens Medical Systems, Erlangen, Germany) and correlated with dose. A Hahn spin echo sequence was used with TR = 3 s, TE = 20 and 100 ms, NEX = 1, using 1 x 1 x 3 mm3 voxels. The MRI measurements were repeated weekly to identify the gel-aging characteristics. Ionization chamber, thermoluminescent dosimetry (TLD), and film dosimetry measurements of the IMRT dose distributions were obtained to compare against the gel results. The other dosimeters were used in a phantom with the same external cross-section as the gel phantom. The irradiated R2 values of the large vessels did not precisely track the smaller vessels, so the ionization chamber measurements were used to normalize the gel dose distributions. The point-to-point standard deviation of the gel dose measurements was 7.0 cGy. When compared with the ionization chamber measurements averaged over the chamber volume, 1% agreement was obtained. Comparisons against radiographic film dose distribution measurements and the treatment planning dose distribution calculation were used to determine the spatial localization accuracy of the gel and MRI. Spatial localization was better than 2 mm, and the dose was accurately determined by the gel both within and outside the target. The TLD chips were placed throughout the phantom to determine gel measurement precision in high- and low-dose regions. A multidimensional dose comparison tool that simultaneously examines the dose-difference and distance-to-agreement was used to evaluate the gel in both low-and high-dose gradient regions. When 3% and 3 mm criteria were used for the comparisons, more than 90% of the TLD measurements agreed with the gel, with the worst of 309 TLD chip measurements disagreeing by 40% of the criteria. All four MRI measurement session gel-measured dose distributions were compared to evaluate the time behavior of the gel. The low-dose regions were evaluated by comparison with TLD measurements at selected points, while high-dose regions were evaluated by directly comparing measured dose distributions. Tests using the multidimensional comparison tool showed detectable degradation beyond one week postirradiation, but all low-dose measurements passed relative to the test criteria and the dose distributions showed few regions that failed.

Biophysical Phenomena↗
PubMed2000

Intensity-modulated radiation therapy in head and neck cancers: The Mallinckrodt experience.

The purpose of the study was to investigate the feasibility and the optimization of tomotherapy-based intensity-modulated radiation therapy (IMRT) in patients with head and neck cancer. From February 1997 to November 1997, 17 patients with squamous cell carcinoma of the head and neck were treated with IMRT. Patients were immobilized with a noninvasive mask and treated using a serial tomotherapy device on a 6 MV linear accelerator. Treatment planning was performed on a Peacock inverse planning system and prescription optimization was used to achieve the best plan for target coverage and parotid sparing. The treatment planning system process has a dosimetric characteristic of delivering different doses to different target structures simultaneously in each daily treatment; therefore, the biological equivalent dose was implemented using the linear-quadratic model to adjust the total dose to the target volume receiving a daily dose of less than 1.9 Gy. All eight patients with gross disease (six in the nasopharynx, two in the tonsil) and one patient with recurrent nasopharyngeal carcinoma received concurrent cisplatin chemotherapy. Six postoperative patients were treated with irradiation alone. Median follow-up was 2.2 years (range 2.6-1.8 years). All patients completed the prescribed treatment without unexpected interruption. Acute side effects were comparable to those of patients treated with conventional beam arrangements during the same period. No patient required gastrostomy during irradiation. The preliminary experience showed that the noninvasive immobilization mask yielded high positioning reproducibility for our patients. To spare the parotid gland, which is in the proximity of the target, a fraction of the target volume may not receive the prescribed dose. In the best-achievable plan of our studied cohort, only 27% +/- 8% of parotid gland volumes were treated to more than 30 Gy, while an average of 3.3% +/- 0.6% of the target volume received less than 95% of the prescribed dose. This is mainly related to the steep dose gradient in the region where the target abuts the parotid gland. The inverse planning system allowed us the freedom of weighting normal tissue-sparing and target coverage to select the best-achievable plan. Local control was achieved in eight patients with gross tumor; six were treated postoperatively. Of three reirradiated patients, two had symptomatic improvement but persistent disease, and one is without evidence of disease. In summary, a system for patient immobilization, setup verification, and dose optimization for head and neck cancer with parotid sparing without significantly compromising target coverage is being implemented for a tomotherapy-based IMRT plan at the Mallinckrodt Institute of Radiology. The initial clinical experience in tumor control is promising, and no severe adverse acute side effects have been observed. Further refining of delivery technology and the inverse planning system, gaining clinical experience to address target definition and dose inhomogeneity within the targets, and understanding the partial volume effect on normal tissue tolerance are needed for IMRT to excel in the treatment of head and neck cancer. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 92-103, (2000).

Adult↗
PubMed1995

Optimization of intensity-modulated 3D conformal treatment plans based on biological indices.

To overcome the limitations of the intensity modulation optimization techniques based on dose criteria, we introduce a method for optimizing intensity distributions in which we employ an objective function based on biological indices. The objective function also includes constraints on dose and dose-volume combinations to ensure that the results are consistent with the physician's judgement. We apply a variant of the steepest-descent method to optimize the objective function. The method is three-dimensional and incorporates scattered radiation in the optimization process using an iterative scheme employing the pencil beam convolution method. Previously we had shown that the inverse technique of obtaining optimum intensity distributions, for which the objectives are defined in terms of a desired uniform dose to the target volume and desired upper limits of dose to normal organs, produces satisfactory approximations of the desired dose distributions for prostate plans. However, for lung, the performance of this technique was considerably inferior. Our conclusion was that, in general, it is not sufficient to specify the objectives of optimization purely in terms of a desired pattern of dose and that the objectives should also incorporate biology, perhaps in the form of biological indices. We demonstrate that the biology-based approach produces lung plans that are superior to those produced when only dose-based objectives are used. For the treatment of prostate, the two methods produce comparable dose distributions.

Adenocarcinoma↗
PubMed2000

Conformal irradiation of concave-shaped PTVs in the treatment of prostate cancer by simple 1D intensity-modulated beams.

BACKGROUND: In the case of concave-shaped PTVs including prostate (P) and seminal vesicles (SV), intensity-modulated radiation therapy (IMRT) should improve the therapeutic ratio of the treatment of prostate cancer. PURPOSE: Comparing IMRT by simple 1D modulations with conventional 3D conformal therapy (i.e. non-IMRT) in the treatment of concave-shaped PTVs including P+SV. MATERIALS AND METHODS: For five patients having a concave-shaped PTV (P+SV) previously treated at our Institute with conformal radiotherapy, conventional 3- and 4-fields conformal plans were compared with IMRT plans in terms of biological indices. IMRT plans were generated by using five equi-spaced beams with a partial shielding of the rectum obtainable with our single-absorber modulation technique (Fiorino C, Lev A, Fusca M, Cattaneo GM, Rudello F, Calandrino R. Dynamic beam modulation by using a single dynamic absorber. Phys. Med. Biol. 1995;40:221-240). The modulation was one-dimensional and the shape of the beams was at single minimum in correspondence with the 'core' of the rectum; the beam intensity in the minimum was set equal to 20 or 40% of the open beam intensity. All plans were simulated on the CADPLAN TPS using a pencil-beam based algorithm (with 18 MV X-rays). Tumour control probability (TCP) and normal tissue complication probabilities (NTCPs) (for rectum, bladder and femoral head) were calculated for all situations when varying the isocentre dose from 60 to 90 Gy. Dose distributions were corrected taking dose fractionation into account through the linear-quadratic model; for the TCP/NTCP estimations the Webb-Nahum and the Lyman-Kutcher models were respectively applied. Three different scores were considered: (a) increase of TCP while keeping rectum NTCP equal to 5% (TCP(5%)); (b) increase of the uncomplicated tumour control probability (P+); (c) increase of the biological-based scoring function (S+), developed by Mohan et al. (Mohan R, Mageras GS, Baldwin B, Clinically relevant optimization of 3D conformal treatments. Med. Phys. 1992;19:933-944). The impact of the uncertainty in the knowledge of the parameters of the biological models was investigated for TCP(5%). RESULTS: (a) The average gain in TCP(5%) when considering IMRT against non-IMRT conformal plans was 7.3% (range 5.0-13.5%); (b) the average increase of P+ was 3.4% (range: 1. 0-8.5%); and (c) the average increase of S+ was 5.4% (range 2.9-12. 4%). The largest gain was found for one patient (patient 5) showing a significantly larger overlapping between PTV and rectum. CONCLUSIONS: Simple 1D-IMRT may clearly improve the therapeutic ratio in the treatment of concave-shaped PTVs including P and SV. In the range of clinically suitable values, the impact of the uncertainty of the parameters n and sigma(alpha) does not significantly alter the main results concerning the gain in TCP(5%). The reported gain in terms of P+ and S+ should be considered with great caution because of the intrinsic uncertainties of the model's parameters and, for bladder, because the 'true' DVH (considering variations of the shape and dimension due to variable filling) may be very different from the DVH calculated on a single CT scan. Further investigations should consider inversely-optimised 1D and 2D-IMRT plan in order to compare them in terms of cost-benefit.

Algorithms↗
PubMed1997

Modulated beam conformal therapy for head and neck tumors.

PURPOSE: The goal of modulated-beam conformal therapy is to reduce the dose to healthy tissue and sensitive structures around a uniformly irradiated target volume. Multiple intensity-modulated fields offer improved tissue-sparing dose distributions. New computer-based systems for planning and delivering such treatments may soon be available from different commercial sources that will make the formulation of an intensity-modulated treatment plan and its execution widely available at any treatment facility that has the resources to acquire the necessary equipment. This work reports on a study of the integration of two such systems. METHODS AND MATERIALS: Treatment planning was done using a commercially available inverse planning algorithm based on simulated annealing. The plans arbitrarily assumed nine coplanar x-ray beams at nonopposed gantry angles. Intensity modulation was computed for each beam. The modulated field at each gantry angle was broken down into a series of uniform (nonmodulated) subfields, which could be delivered as a sequence to produce the desired dose distribution. Because a large number of subfields was delivered, a multileaf collimator (MLC) was used for field shaping. This allowed rapid and accurate field shaping for treatments made up of several hundred subfields. Computer control of the MLC and linear accelerator allowed delivery of doses less than .01 Gy per subfield. Treatment was delivered on a prototype, computer-controlled accelerator and MLC system. Resulting dose distributions were analyzed using film and an anatomically specific, homogeneous phantom. RESULTS: The treatment plans were evaluated using dose-volume histogram analysis. The plans provided acceptably uniform irradiation of the target volume without exceeding dose tolerances for nearby critical structures. The plans were successfully delivered by a prototype dynamic MLC. The time needed to deliver a sequence of subfields at one gantry angle ranged from 0.7 to 2.0 min. Isodoses from film agreed reasonably well with planned isodose distributions. CONCLUSIONS: It is feasible to plan and deliver fixed gantry, modulated-beam conformal therapy for head and neck tumors with systems being developed commercially. The planned dose distributions exhibit significant potential for sparing closely spaced normal tissue structures in the head and neck.

Algorithms↗
PubMed1997

Field margin reduction using intensity-modulated x-ray beams formed with a multileaf collimator.

PURPOSE: In axial, coplanar treatments with multiple fields, the superior and inferior ends of a planning target volume (PTV) are at risk to get underdosed due to the overlapping penumbras of all treatment fields. We have investigated a technique using intensity modulated x-ray beams that allows the use of small margins for definition of the superior and inferior field borders while still reaching a minimum PTV-dose of 95% of the isocenter dose. METHODS AND MATERIALS: The applied intensity modulated beams, generated with a multileaf collimator, include narrow (1.1-1.6 cm) boost fields to increase the dose in the superior and inferior ends of the PTV. The benefits of this technique have been assessed using 3D treatment plans for 10 prostate cancer patients. Treatment planning was performed with the Cadplan 3D planning system (Varian-Dosetek). Dose calculations for the narrow boost fields have been compared with measurements. The application of the boost fields has been tested on the MM50 Racetrack Microtron (Scanditronix Medical AB), which allows fully computer-controlled setup of all involved treatment fields. RESULTS: Compared to our standard technique, the superior-inferior field length can be reduced by 1.6 cm, generally yielding smaller volumes of rectum and bladder in the high dose region. For the narrow boost fields, calculated relative dose distributions agree within 2% or 0.2 cm with measured dose distributions. For accurate monitor unit calculations, the phantom scatter table used in the Cadplan system had to be modified using measured data for square fields smaller than 4 x 4 cm2. The extra time needed at the MM50 for the setup and delivery of the boost fields is usually about 1 min. CONCLUSION: The proposed use of intensity modulated beams yields improved conformal dose distributions for treatment of prostate cancer patients with a superior-inferior field size reduction of 1.6 cm. Treatments of other tumor sites can also benefit from the application of the boost fields.

Humans↗
PubMed1999

Beam intensity modulation to reduce the field sizes for conformal irradiation of lung tumors: a dosimetric study.

PURPOSE: In conformal radiotherapy of lung tumors, penumbra broadening in lung tissue necessitates the use of larger field sizes to achieve the same target coverage as in a homogeneous environment. In an idealized model configuration, some fundamental aspects of field size reduction were investigated, both for the static situation and for a moving tumor, while maintaining the dose homogeneity in the target volume by employing a simple beam-intensity modulation technique. METHODS AND MATERIALS: An inhomogeneous phantom, consisting of polystyrene, cork, and polystyrene layers, with a 6 x 6 x 6 cm3 polystyrene cube inside the cork representing the tumor, was used to simulate a lung cancer treatment. Film dosimetry experiments were performed for an AP-PA irradiation technique with 8-MV or 18-MV beams. Dose distributions were compared for large square fields, small square fields, and intensity-modulated fields in which additional segments increase the dose at the edge of the field. The effect of target motion was studied by measuring the dose distribution for the solid cube, displaced with respect to the beams. RESULTS: For the 18-MV beam, the field sizes required to establish a sufficient target coverage are larger than for the 8-MV beam. For each beam energy, the mean dose in cork can significantly be reduced (at least a factor of 1.6) by decreasing the field size with 2 cm, while keeping the mean target dose constant. Target dose inhomogeneity for these smaller fields is limited if the additional edge segments are applied for 8% of the number of monitor units given with the open fields. The target dose distribution averaged over a motion cycle is hardly affected if the target edge does not approach the field edge to within 3 mm. CONCLUSIONS: For lung cancer treatment, a beam energy of 8 MV is more suitable than 18 MV. The mean lung dose can be significantly reduced by decreasing the field sizes of conformal fields. The smaller fields result in the same biological effect to the tumor if the mean target dose is kept constant. Intensity modulation can be employed to maintain the same target dose homogeneity for these smaller fields. As long as the target (with a 3 mm margin) stays within the field portal, application of a margin for target motion is not necessary.

Film Dosimetry↗
PubMed1999

Intensity-modulated tangential beam irradiation of the intact breast.

PURPOSE: To evaluate the potential benefits of intensity modulated tangential beams in the irradiation of the intact breast. METHODS AND MATERIALS: Three-dimensional treatment planning was performed on five left and five right breasts using standard wedged and intensity modulated (IM) tangential beams. Optimal beam parameters were chosen using beams-eye-view display. For the standard plans, the optimal wedge angles were chosen based on dose distributions in the central plane calculated without inhomogeneity corrections, according to our standard protocol. Intensity-modulated plans were generated using an inverse planning algorithm and a standard set of target and critical structure optimization criteria. Plans were compared using multiple dose distributions and dose volume histograms for the planning target volume (PTV), ipsilateral lung, coronary arteries, and contralateral breast. RESULTS: Significant improvements in the doses to critical structures were achieved using intensity modulation. Compared with a standard-wedged plan prescribed to 46 Gy, the dose from the IM plan encompassing 20% of the coronary artery region decreased by 25% (from 36 to 27 Gy) for patients treated to the left breast; the mean dose to the contralateral breast decreased by 42% (from 1.2 to 0.7 Gy); the ipsilateral lung volume receiving more than 46 Gy decreased by 30% (from 10% to 7%); the volume of surrounding soft tissue receiving more than 46 Gy decreased by 31% (from 48% to 33%). Dose homogeneity within the target volume improved greatest in the superior and inferior regions of the breast (approximately 8%), although some decrease in the medial and lateral high-dose regions (approximately 4%) was also observed. CONCLUSION: Intensity modulation with a standard tangential beam arrangement significantly reduces the dose to the coronary arteries, ipsilateral lung, contralateral breast, and surrounding soft tissues. Improvements in dose homogeneity throughout the target volume can also be achieved, particularly in the superior and inferior regions of the breast. It remains to be seen whether the dosimetric improvements achievable with IMRT will lead to significant clinical outcome improvements.

Breast↗
PubMed1999

Dose, volume, and function relationships in parotid salivary glands following conformal and intensity-modulated irradiation of head and neck cancer.

PURPOSE: To determine the relationships between the three-dimensional dose distributions in parotid glands and their saliva production, and to find the doses and irradiated volumes that permit preservation of the salivary flow following irradiation (RT). METHODS AND MATERIALS: Eighty-eight patients with head and neck cancer irradiated with parotid-sparing conformal and multisegmental intensity modulation techniques between March 1994 and August 1997 participated in the study. The mean dose and the partial volumes receiving specified doses were determined for each gland from dose-volume histograms (DVHs). Nonstimulated and stimulated saliva flow rates were selectively measured from each parotid gland before RT and at 1, 3, 6, and 12 months after the completion of RT. The data were fit using a generalized linear model and the normal tissue complication probability (NTCP) model of Lyman-Kutcher. In the latter model, a "severe complication" was defined as salivary flow rate reduced to < or =25% pre-RT flow at 12 months. RESULTS: Saliva flow rates data were available for 152 parotid glands. Glands receiving a mean dose below or equal to a threshold (24 Gy for the unstimulated and 26 Gy for the stimulated saliva) showed substantial preservation of the flow rates following RT and continued to improve over time (to median 76% and 114% of pre-RT for the unstimulated and stimulated flow rates, respectively, at 12 months). In contrast, most glands receiving a mean dose higher than the threshold produced little saliva with no recovery over time. The output was not found to decrease as mean dose increased, as long as the threshold dose was not reached. Similarly, partial volume thresholds were found: 67%, 45%, and 24% gland volumes receiving more than 15 Gy, 30 Gy, and 45 Gy, respectively. The partial volume thresholds correlated highly with the mean dose and did not add significantly to a model predicting the saliva flow rate from the mean dose and the time since RT. The NTCP model parameters were found to be TD50 (the tolerance dose for 50% complications rate for whole organ irradiated uniformly) = 28.4 Gy, n (volume dependence parameter) = 1, and m (the slope of the dose/response relationship) = 0.18. Clinical factors including age, gender, pre-RT surgery, chemotherapy, and certain medical conditions were not found to be significantly associated with the salivary flow rates. Medications (diuretics, antidepressants, and narcotics) were found to adversely affect the unstimulated but not the stimulated flow rates. CONCLUSIONS: Dose/volume/function relationships in the parotid glands are characterized by dose and volume thresholds, steep dose/response relationships when the thresholds are reached, and a maximal volume dependence parameter in the NTCP model. A parotid gland mean dose of < or =26 Gy should be a planning goal if substantial sparing of the gland function is desired.

Adult↗
PubMed2000

Relative profile and dose verification of intensity-modulated radiation therapy.

PURPOSE: To develop a quality assurance (QA) procedure to assess the intensity profile and dosimetry for intensity-modulated (IM) treatment fields using electronic portal imaging devices (EPIDs). METHODS AND MATERIALS: A series of rapidly acquired (approximately 1/sec) portal images are summed and converted to dose. For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile forming a series of error values. The standard deviation, sigma, of the errors, a measure of the goodness of the match, is minimized by applying a normalization and uniform scatter subtraction from the measured profile. For dose verification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID readings. Seventy prostate treatment fields were used in a phantom study to verify these approaches. Sensitivity was studied by creating artificial mismatches. RESULTS: The average sigma for relative profile verification is 3.3% (percentage of average intended intensity) whereas artificial mismatches resulted in sigma values from 5% to 27%. The average isocentric dose calculated from EPID readings is 1.001 relative to the planned dose with a standard deviation of 0.018. CONCLUSIONS: An EPID can be used for profile verification and absolute isocentric dose measurement for IM fields.

Algorithms↗
PubMed1999

Comparison of three-dimensional conformal radiation therapy and intensity-modulated radiation therapy systems.

The use of three-dimensional conformal radiation therapy (3DCRT) has now become common practice in radiation oncology departments around the world. Using beam's eye viewing of volumes defined on a treatment planning computed tomography scan, beam directions and beam shapes can be selected to conform to the shape of the projected target and minimize dose to critical normal structures. Intensity-modulated radiation therapy (IMRT) can yield dose distributions that conform closely to the three-dimensional shape of the target volume while still minimizing dose to normal structures by allowing the beam intensity to vary across those shaped fields. Predicted dose distributions for patients with tumors of the prostate, nasopharynx, and paraspinal region are compared between plans made with 3DCRT programs and those with inverse-planned IMRT programs. The IMRT plans are calculated for either static or dynamic beam delivery methods using multileaf collimators. Results of these comparisons indicate that IMRT can yield significantly better dose distributions in some situations at the expense of additional time and resources. New technologies are being developed that should significantly reduce the time needed to plan, implement, and verify these treatments. Current research should help define the future role of IMRT in clinical practice.

Costs and Cost Analysis↗
PubMed1999

Tomotherapy.

Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.

Dose Fractionation, Radiation↗
PubMed1995

Dynamic beam modulation by using a single computer-controlled absorber.

The authors have developed an apparatus able to generate ID intensity-modulated beams, using only one moving absorber within the irradiation field. A procedure for deriving optimized absorber-speed profiles in order to produce the desired fluence/dose profiles has been suggested. Experimental tests show that the system should be sufficiently reliable in reproducing modulated beam profiles of different shape: expected relative doses against measured relative doses have been found to be in agreement in a number of situations within 3% using a nonfocused device. A better agreement should be expected using a focused apparatus (which is currently being developed). Beam modulation by single absorber cannot modulate the beam fluence in any was one wishes, due to physical constraints, which depend on the absorber and field widths and on the shape of the desired fluence profile. However, the authors show that this simple and low-cost tool could offer, with a sufficient degree of accuracy, the possibility of modulating the beam fluence with a high degree of versatility. In particular, a procedure or performing tissue-missing compensation by single-absorber dynamic beam modulation is suggested. Moreover, 'strongly' modulated beam profiles can be created, showing that this simple technique could also have some interesting applications in the field of conformal radiotherapy by non-uniform dose delivery.

Algorithms↗
PubMed1996

A method to study the characteristics of 3D dose distributions created by superposition of many intensity-modulated beams delivered via a slit aperture with multiple absorbing vanes.

Highly conformal dose distributions can be created by the superposition of many radiation fields from different directions, each with its intensity spatially modulated by the method known as tomotherapy. At the planning stage, the intensity of radiation of each beam element (or bixel) is determined by working out the effect of superposing the radiation through all bixels with the elemental dose distribution specified as that from a single bixel with all its neighbours closed (the 'independent-vane' (IV) model). However, at treatment-delivery stage, neighbouring bixels may not be closed. Instead the slit beam is delivered with parts of the beam closed for different periods of time to create the intensity modulation. As a result, the 3D dose distribution actually delivered will differ from that determined at the planning stage if the elemental beams do not obey the superposition principle. The purpose of this paper is to present a method to investigate and quantify the relation between planned and delivered 3D dose distributions. Two modes of inverse planning have been performed: (i) with a fit to the measured elemental dose distribution and (ii) with a 'stretched fit' obeying the superposition principle as in the PEACOCK 3D planning system. The actual delivery has been modelled as a series of component deliveries (CDs). The algorithm for determining the component intensities and the appropriate collimation conditions is specified. The elemental beam from the NOMOS MIMiC collimator is too narrow to obey the superposition principle although it can be 'stretched' and fitted to a superposition function. Hence there are differences between the IV plans made using modes (i) and (ii) and the raw and the stretched elemental beam, and also differences with CD delivery. This study shows that the differences between IV and CD dose distributions are smaller for mode (ii) inverse planning than for mode (i), somewhat justifying the way planning is done within PEACOCK. Using a stretched elemental beam is a useful adjustment to improve the accuracy of inverse planning but the 3D dose distribution actually delivered will display characteristics of the collimation.

Humans↗
PubMed1998

The effects of intra-fraction organ motion on the delivery of dynamic intensity modulation.

Computer-optimized treatment plans, aimed to enhance tumour control and reduce normal tissue complication, generally require non-uniform beam intensities. One of the techniques for delivering intensity-modulated beams is the use of dynamic multileaf collimation, where the beam aperture moves and the field shape changes during irradiation. When intensity-modulated beams are delivered with dynamic collimation, the problem of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static field treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra, the interplay of the movement of the beam aperture and the movement of the patient anatomy can create 'hot' and 'cold' spots throughout the field. The mechanism for creating these effects is not well understood. This paper provides a simple analytical model which illustrates the fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiation. Numerical analysis was carried out which calculates the cumulative primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variations in the target can be greater than 100% of the desired beam intensity. The magnitude of the photon intensity variations is strongly dependent on the speed of the beam aperture relative to the speed of the target motion, and the width of the scanning beam relative to the amplitude of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction organ motion are discussed.

Biophysical Phenomena↗
PubMed1998

Configuration options for intensity-modulated radiation therapy using multiple static fields shaped by a multileaf collimator.

One-dimensional (1D) intensity-modulated beams (IMBs) can be generated by multiple static fields (MSFs) created by a multileaf collimator (MLC) with the radiation switched off between field re-settings (Bortfeld-Boyer method). Each component irradiation is of equal fluence. This paper presents and analyses the formulae for the number of physically allowed combinations of leaf settings which generate any given IMB. The formulae are general to an IMB with any number of local minima and extend from the well-known N! formula for a single-peaked IMB with N left-leaf (L-leaf) and N right-leaf (R-leaf) positions. A 'combination' is a set of N L-leaf and R-leaf pairings. A 'physically allowed combination' is one in which no L-leaf is paired with an R-leaf to its left. The physically allowed combinations are grouped by specific properties into classes in which the well-known techniques of 'leaf-sweep' and 'close-in' are just two members. Consideration of these properties leads to a new suggestion of the 'forced-baseline' configuration in which the first intensity increment is delivered for the full field width and there remain choices concerning the delivery of the rest of the IMB within which two different possibilities are 'one-out-of-sync leaf-sweep' and 'minimum leaf travel'. The extension to 2D is briefly introduced.

Models, Theoretical↗
PubMed1998

Configuration options for intensity-modulated radiation therapy using multiple static fields shaped by a multileaf collimator. II: constraints and limitations on 2D modulation.

This paper addresses the technique of using multiple static multileaf-collimator-shaped field components to create a two-dimensional intensity-modulated beam (2D IMB). It addresses the physical constraints on the problem of determining the optimum field-component leaf configurations under the circumstances that (i) the static field components are shaped by leaves alone and (ii) the 2D intensity distribution is delivered by exactly N field components when there are N rising-intensity equal-fluence increments in the 1D channel containing the maximum fluence in the 2D IMB. This corresponds to the least inefficient delivery. In general it is noted that an optimum solution (set of field-component leaf configurations) with zero tongue-and-groove underdose may not exist (depending on the distribution) and an exhaustive search for the set of leaf configurations with the minimum tongue-and-groove underdose is impossible for realistically sized problems. Against this background iterative methods to examine a limited search space are shown to yield an optimum solution with zero tongue-and-groove underdose for certain intensity distributions. These searches are not robust and can be defeated. The problem of finding an optimum solution may be generally insoluble for some 2D IMBs under the conditions (i) and (ii). If, however, a larger number of field components is permitted and/or the accelerator jaws may also be used, in addition to the multileaves, then an optimum solution with zero tongue-and-groove underdose can always be found with lower efficiency.

Biophysical Phenomena↗
PubMed1997

The optimum intensities for multiple static multileaf collimator field compensation.

A method of determining the optimum beam intensities for compensation using multiple static multileaf collimator fields is presented. In this method a histogram of the number of beam pixels against beam intensity is generated for the intensity-modulated beam (IMB). The intensity of each beam to be used is chosen to minimize the mean square deviation between each bin in the histogram and the closest beam intensity. This method has been applied to sample IMBs possessing one maximum and two maxima. For both cases, the use of uniform beam intensity increments is shown to be close to optimal. In the case with two maxima, the efficacy of irradiating both peaks simultaneously, rather than separately, has been studied and shown to be of potential benefit. The optimum intensities for an IMB for breast radiotherapy are also presented.

Humans↗