Beam Portals Research Articles

Effective beam directions using radiobiologically optimized IMRT of node positive breast cancer

The purpose of this study was to investigate the optimal coplanar beam directions when treating an early breast cancer with locoregional lymphatic spread with a few radiobiologically optimized intensity modulated beams. Also to determine the increase in the probability of complication-free cure with the number of beam portals and the smallest number required to perform a close to optimal treatment for this tumour site. Four test patients with stage II left-sided breast cancer were studied with heart, lung and contralateral breast as principal organs at risk. The clinical target volume consisted of the breast tissue remaining after surgery, the axillary, the internal mammary as well as the supraclavicular lymph nodes. Through an exhaustive search of all possible beam directions the most effective coplanar beams with one to four intensity modulated photon beam portals were investigated. Comparisons with uniform beam treatment techniques and up to 12 intensity modulated beams were also made. The different plans were optimized using the probability of complication-free tumour cure, P +, as biological objective function. When using two intensity modulated beam directions three major sets of suitable directions were identified denoted by A, P and T. A corresponds to an anterior oblique pair of beams around 25° and 325°, P is a perpendicular lateral pair at around 50° and 130° whereas T is a more conventional tangential pair at around 155° and 300°. Interestingly, these configurations identify simply three major effective beam directions namely at 30°±20°, 145°±20° and 310°±15°. For the three intensity modulated beam technique a combination of these three effective beam directions generally covered the global maximum of the probability of complication-free tumour control. The improvement in complication-free cure probability with two optimally selected intensity modulated beams is around 10% when compared to a uniform beam technique with three to four beam portals. This increase is mainly due to a reduction by almost 1% in the probability of injury to the heart and an increase of 6% in the probability of local tumour control. When three or four biologically optimized beam portals are used a further increase in the probability of complication-free cure of about 6% can often be obtained. This improvement is caused by a small decrease in the probability of injury to the heart, left lung and other surrounding normal tissue, as well as a slight further increase in the probability of tumour control. The increase in the treatment outcome is minimal when more than four intensity modulated beams are employed. A small increase in dose homogeneity in the target volume and a slight decrease in the normal tissue volume receiving high dose may be seen, but without appreciably improving the complication-free cure probability. For a stage II breast cancer, three and in more complex cases four optimally oriented beams are sufficient to reach close to the maximum probability of complication-free tumour control when biologically optimized intensity modulated dose delivery is used. Angle of incidence optimization may then be advantageous starting from the given most effective three beam directions.

392 Influence of number of multileaf collimator segments and beam portals on the treatment outcome in radiobiologically optimized radiation therapy

392 Influence of number of multileaf collimator segments and beam portals on the treatment outcome in radiobiologically optimized radiation therapy

SU‐FF‐T‐105: The Impact of Multileaf Collimator Rotation in IMRT Planning

Purpose: Collimator angle is a degree of freedom that has seldom been explored in IMRT inverse planning. This work investigates the impact of multi‐leaf collimator rotation (IMLCR) on a dosimetric scoring function and lays the groundwork for optimal collimator angle selection in IMRT planning. Method and Materials: The scoring function models the maximum deliverable dose at an arbitrary beam orientation and is calculated using a sampling method. To calculate the scoring function, the beam portal is divided into a grid of beamlets based on collimator movement constraints. The intensity of each beamlet is set at the maximum intensity that does not violate the tolerance of any structure intersected by that beamlet. The score is calculated from the fraction of the dose prescription deliverable by each beamlet. The IMLCR is measured as the increase in the scoring function over all collimator angles. Results: For large tumors (not fully coverable by the beam portal), the mean IMLCR is 22.2%. Collimator rotation enables more exposure of the tumor to the beam, and thus increases the irradiation of the tumor. For small tumors (fully coverable by the beam portal) with nearby critical structures, the mean IMLCR is 9.3%. Collimator rotation allows critical structures to be better isolated from tumor irradiation. When the tolerance of the nearby structures is high relative to the tumor's prescription, collimator rotation has little impact for small tumors. Conclusion: The variation observed in the dosimetric scoring function indicates that collimator rotation can significantly raise the maximum deliverable dose at a given gantry angle. When the tumor is large and/or has nearby critical structures (tolerance <=50% of the tumor's prescription), collimator rotation has the greatest impact and is worth considering. Collimator angle selection could play an important role in improving the quality of IMRT treatment plans.

The Clinical Value of Non-Coplanar Photon Beams in Biologically Optimized Intensity Modulated Dose Delivery on Deep-Seated Tumours

The aim of the present study is to compare the merits of different radiobiologically optimized treatment techniques using few-field planar and non-coplanar dose delivery on an advanced cancer of the cervix, with rectum and bladder as principal organs at risk. Classically, the rational for using non-coplanar beams is to minimize the overlap of beam entrance and exit regions and to find new beam directions avoiding organs at risk, in order to reduce damage to sensitive normal tissues. Two four-beam configurations have been extensively studied. The first consists of three evenly spaced coplanar beams and a fourth non-coplanar beam. A second tetrahedral-like configuration, with two symmetric non-coplanar beams at the same gantry angle and two coplanar beams, with optimized beam directions, was also tested. The present study shows that when radiobiologically optimized intensity modulated beams are applied to such a geometry, only a marginal increase in the treatment outcome can be achieved by non-coplanar beams compared to the optimal coplanar treatment. The main reason for this result is that the high dose in the beam-overlap regions is already optimally reduced by biologically optimized intensity modulation in the plane. The large number of degrees of freedom already incorporated in the treatment by the use of intensity modulation and radiobiological optimization, leads to the saturation of the benefit acquired by a further increase in the degrees of freedom with non-coplanar beams. In conclusion, the use coplanar of radiobiologically optimized intensity modulation simplifies the dose delivery, reducing the need for non-coplanar beam portals.

222 poster Conformal radiotherapy of prostate lesions using a cobalt unit with customised beam portals and adjacent MSCT imaging: a feasibility study

222 poster Conformal radiotherapy of prostate lesions using a cobalt unit with customised beam portals and adjacent MSCT imaging: a feasibility study

On-line correction of beam portals in the treatment of prostate cancer using an endorectal balloon device

Background: Reproducible target volume assessment is required in order to optimize portal field margins in the treatment of prostate cancer. The benefits of an endorectal balloon on target volume assessment remain unclear. Material and methods: Nine patients were treated with a daily placed air filled rectal balloon. Portal films and computer-associated tomography during the treatment were used to determine the position of the structures of interest. Comparative planning with or without a balloon was performed in order to determine rectal wall exposure to radiation. Results: The range of movements during treatment predicting the position of the prostate in relation to the symphysis was 0.05–0.59 cm in the lateral direction, 0.27–2.2 cm in the antero-posterior direction, and 0.33–1.8 cm in the crano-caudal direction, as compared to the position of the prostate predicted by the balloon ranging from 0.18 to 0.76 cm in the lateral direction, 0.22–1.68 cm in the antero-posterior direction, and 0.58–2.99 cm in the crano-caudal direction. Planning target volumes (PTV) margins as defined by the position of the balloon were 10 mm in the antero-posterior direction, 6 mm in the lateral direction, and 16 mm in the crano-caudal direction. The volume of rectal wall exposed to radiation was reduced from 40 (±12%) to 25% (±19%) with an endorectal balloon ( P<0.05). Conclusions: Daily online correction with portal vision for external beam set-up is improved by an endorectal balloon device, leading to improved PTV margins and reduced radiation exposure of the rectal wall.

Pseudo beam’s-eye–view as applied to beam orientation selection in intensity-modulated radiation therapy

Purpose: To introduce the concept of pseudo beam’s-eye–view (pBEV), to establish a framework for computer-assisted beam orientation selection in intensity-modulated radiation therapy (IMRT), and to evaluate the utility of the proposed technique. Methods and Materials: To facilitate the selection of beam orientations for IMRT treatment planning, a scoring of beam direction was introduced. The score function was based on the maximum target dose deliverable by the beam without exceeding the tolerance doses of the critical structures. For the score function calculation, the beam portal at given gantry and couch angles was divided into a grid of beamlets. Each beamlet crossing the target was assigned the maximum intensity that could be used without exceeding the dose tolerances of the organs at risk (OARs) and normal tissue. Thereafter, a score was assigned to the beam according to the target dose delivered. The beams for the treatment were selected among those with the highest scores. In a sense, this technique is similar to the beam’s-eye–view approach used in conventional radiation therapy, except that the evaluation by a human is replaced by a score function, and beam modulation is taken into account. Results: The pBEV technique was tested on two clinical cases: a paraspinal treatment and a nasopharyngeal cancer with both coplanar and noncoplanar beam configurations. The plans generated under the guidance of pBEV for the paraspinal treatment offered superior target dose uniformity and reduced OAR doses. For the nasopharyngeal cancer case, it was also found that the pBEV-selected coplanar and noncoplanar beams significantly improved the target coverage without compromising the sparing of the OARs. Conclusions: The pBEV technique developed in this work provides a comprehensive tool for beam orientation selection in IMRT. It is especially valuable for complicated cases, where the target is surrounded by several sensitive structures and where it is difficult to select a set of good beam orientations. The pBEV technique has considerable potential for simplifying the IMRT treatment planning process and for maximizing the technical capacity of IMRT.

Interstitial Brachytherapy With or Without External Beam Irradiation in Head and Neck Cancer: Institute Rotary Cancer Hospital Experience

Iridium-192 interstitial brachytherapy is practiced infrequently in developing countries, even where head and neck cancer is a major neoplasm and the technique could provide good results. This report from India is presented as an audit to validate the benefit of brachytherapy.One hundred and six head and neck cancer patients were treated by interstitial brachytherapy alone (n = 29) or combined with external irradiation (n = 77). The oral cavity and the oropharynx together constituted 82% of the sites of implanted tumours; 75% were T1–2N0 status. Brachytherapy was carried out using afterloaded plastic catheters and the Paris dose prescription system was followed. External cobalt-60 beam portals covered the primary and the neck.The median duration of follow-up was 22 months. The median dose of brachytherapy used alone was 60 Gy. With combined treatment, the median external radiotherapy and brachytherapy doses were 50 Gy and 25 Gy respectively. The median brachytherapy dose rate was 0.5 Gy/h. Primary and nodal recurrences were recorded in 41/106 (38.7%) and 18/106 (17.0%) patients at median intervals of 15 and 13 months respectively. Implant site failure was more common after combined treatment than with brachytherapy alone (42.8% versus 27.5%), but it did not reach statistical significance in this analysis (P = 0.15). Kaplan–Meier actuarial 5-year estimates showed 52% and 87% disease-free and overall survivals.Iridium-192 interstitial implants in suitably selected head and neck cancer patients can improve the radiotherapeutic results, with the promise of organ conservation in 50%. In India, the practice should be established in more radiotherapy centres and could be utilized in 10 000–25 000 head and neck cancer patients annually.

Optically guided intensity modulated radiotherapy

Background and Purpose: Previously, we reported on development of an optically guided system for 3D conformal intracranial radiotherapy using multiple noncoplanar fixed fields. In this paper we report on the extension of our system for stereotactic fractionated radiotherapy to include intensity modulated static ports.Methods and Materials: A 3D treatment plan with maximum beam separation is developed in the stereotactic space established by an optically guided system. Gantry angles are chosen such that each beam has a unique entrance and exit pathway, avoids the critical structures, and has a minimal beam's eye view projection. Once, a satisfactory treatment plan is found using this geometric approach an inverse treatment plan is developed using the beam portals established previously. The purpose of adding inverse planing is two fold, on the one hand it allows further reduction of margins around the PTV, while on the other hand it affords the possibility of conformal avoidance of critical structures that are close to or abut the PTV.Results: The use of the optically guided system in conjunction with intensity modulated noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit smaller 90, 70, and 50% of prescription dose isodose volumes, improved PITV ratios, comparable or improved EUD, smaller NTDmean for the critical structures, and an inhomogeneity index that is within generally accepted limits.Conclusion: Because optically guided technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, the planning target volume needs to be corrected only for the limitations of image resolution. Intensity modulated static beam radiotherapy planning then provides the user the ability to further reduce margins on the PTV and to conform very closely to this smaller target volume, and enhances the normal tissue sparing, and high degree of conformality possible with 3D conformal radiotherapy. In addition, since optically guided technology affords improved patient localization and online monitoring of patient position during treatment delivery it allows for safe and efficient delivery of intensity modulated radiotherapy.

Computer-assisted selection of coplanar beam orientations inintensity-modulated radiation therapy*

In intensity-modulated radiation therapy (IMRT), the incident beam orientations are often determined by a trial anderror search. The conventional beam's-eye view (BEV) tool becomes less helpfulin IMRT because it is frequently required that beams go through organs at risk(OARs) in order to achieve a compromise between the dosimetric objectives ofthe planning target volume (PTV) and the OARs. In this paper, we reporta beam's-eye view dosimetrics (BEVD) technique to assist in the selection ofbeam orientations in IMRT. In our method, each beam portal is divided into agrid of beamlets. A score function is introduced to measure the `goodness' ofeach beamlet at a given gantry angle. The score is determined by the maximumPTV dose deliverable by the beamlet without exceeding the tolerance doses ofthe OARs and normal tissue located in the path of the beamlet. The overallscore of the gantry angle is given by a sum of the scores of all beamlets. Fora given patient, the score function is evaluated for each possible beamorientation. The directions with the highest scores are then selected as thecandidates for beam placement. This procedure is similar to the BEV approachused in conventional radiation therapy, except that the evaluation by a humanis replaced by a score function to take into account the intensity modulation.This technique allows one to select beam orientations without the excessivecomputing overhead of computer optimization of beam orientation. It alsoprovides useful insight into the problem of selection of beam orientation andis especially valuable for complicated cases where the PTV is surrounded byseveral sensitive structures and where it is difficult to select a set of`good' beam orientations. Several two-dimensional (2D) model cases were usedto test the proposed technique. The plans obtained using the BEVD-selectedbeam orientations were compared with the plans obtained using equiangularspaced beams. For all the model cases investigated, the use of BEVD-selected beamorientations improved the dose distributionssignificantly. These examples indicate that the technique has considerablepotential for simplifying the IMRT treatment planning process and allows forbetter utilization of the technical capacity of IMRT.

Radiation-induced lung disease and the impact of radiation methods on imaging features.

Although radiologic findings in radiation-induced lung disease are well described in the literature, the influence exerted on these findings by different radiation methods is not well understood. Radiation treatment of non-small cell lung cancer varies depending on the location and extent of disease. Irradiation with oblique beam angles results in unusual distribution of radiation-induced lung disease. Small cell lung cancer is treated with irradiation concurrent with or following chemotherapy, and portal arrangements are controversial. In breast cancer, use of tangential beam portals may induce radiation pneumonitis or fibrosis at the peripheral lung anterolaterally. Use of supraclavicular portals may produce lesions in the lung apex that appear similar to pulmonary tuberculosis. In esophageal cancer, radiation portals with a 5-6-cm margin above and below the tumor are generally recommended, and computed tomography (CT) frequently demonstrates radiation-related lung damage adjacent to the mediastinum. In mediastinal tumors, the mantle field includes all the major lymph node regions above the diaphragm. Radiation pneumonitis varies from minimal to extremely marked change in the paramediastinal areas and in both apices. CT is more sensitive to radiation-induced lung disease than chest radiography and demonstrates related changes earlier. Furthermore, it more clearly depicts the precise distribution and pattern of disease. Familiarity with the imaging findings in radiation-induced lung disease produced by different radiation methods will help radiologists interpret abnormalities seen at chest radiography and CT in affected patients.

Planning target volumes for radiotherapy: how much margin is needed?

Purpose: The radiotherapy planning target volume (PTV) encloses the clinical target volume (CTV) with anisotropic margins to account for possible uncertainties in beam alignment, patient positioning, organ motion, and organ deformation. Ideally, the CTV-PTV margin should be determined solely by the magnitudes of the uncertainties involved. In practice, the clinician usually also considers doses to abutting healthy tissues when deciding on the size of the CTV-PTV margin. This study calculates the ideal size of the CTV-PTV margin when only physical position uncertainties are considered. Methods and Materials: The position of the CTV for any treatment is assumed to be described by independent Gaussian distributions in each of the three Cartesian directions. Three strategies for choosing a CTV-PTV margin are analyzed. The CTV-PTV margin can be based on: 1. the probability that the CTV is completely enclosed by the PTV; 2. the probability that the projection of the CTV in the beam’s eye view (BEV) is completely enclosed by the projection of the PTV in the BEV; and 3. the probability that a point on the edge of the CTV is within the PTV. Cumulative probability distributions are derived for each of the above strategies. Results: Expansion of the CTV by 1 standard deviation (SD) in each direction results in the CTV being entirely enclosed within the PTV 24% of the time; the BEV projection of the CTV is enclosed within the BEV projection of the PTV 39% of the time; and a point on the edge of the CTV is within the PTV 84% of the time. To have the CTV enclosed entirely within the PTV 95% of the time requires a margin of 2.8 SD. For the BEV projection of the CTV to be within the BEV projection of the PTV 95% of the time requires a margin of 2.45 SD. To have any point on the surface of the CTV be within the PTV 95% of the time requires a margin of 1.65 SD. Conclusion: In the first two strategies for selecting a margin, the probability of finding the CTV within the PTV is unrelated to dose variations in the CTV. In the third strategy, the specified confidence limit is correlated with the minimum target dose. We recommend that the PTV be calculated from the CTV using a margin of 1.65 SD in each direction. This gives a minimum CTV dose that is greater than 95% of the minimum PTV dose. Additional sparing of adjoining healthy structures should be accomplished by modifying beam portals, rather than adjusting the PTV. Then, the dose distributions more accurately reflect the clinical compromise between treating the tumor and sparing the patient.

Optimal electron and combined electron and photon therapy in the phase space of complication-free cure

The possibility of using intensity-modulated high-energy electrons beams alone or in combination with photon beams to treat tumours located at depths from 5 cm to 25 cm has been investigated. A radiobiologically based optimization algorithm using the probability of complication-free tumour control has been used to calculate the optimal dose distributions. Two different target volumes have been used; one advanced cervical cancer with locally involved lymph nodes and one astrocytoma in the upper brain hemisphere. Treatments with only electron beams and also combinations between electron and photon beams have been investigated. The dependence of the expected treatment outcome on the beam energy and directions was investigated, and to some extent on the number of beam portals. It is shown that the beam direction intervals resulting in a high expected treatment outcome increase with increasing electron energy and also with some electron-photon combinations. For an eccentrically placed, not too deeply situated tumour surrounded by sensitive normal tissue it is shown that the expected treatment outcome can be improved by using electron beams in combination with photon beams compared with using two photon beams, and using two electron beams results in almost as high an expected treatment outcome. The possibility of improving the dose conformity from electron beams by adding photon fields parallel or orthogonal to the electron beams is demonstrated.

Vergleich verschiedener Optimierungskonzepte für die intensitätsmodulierte Strahlentherapie

Intensity modulated irradiation beams provide a substantial progress for radiotherapy. Both, the conformity of the dose distribution with the target volume and sparing of organs at risk are improved. The determination of the fluence profiles is an optimization problem. It depends on the formulation of the objective function, whether a meaningful solution can be achieved. The parameters to maximize uncomplicated tumor control directly, are not yet known with sufficient accuracy. In this work, the usual method of minimizing the quadratic deviation between the calculated and desired dose distributions is compared to methods where clinically proven dose constraints are used directly. For the first approach it is necessary to adapt weighting factors interactively in order to achieve the desired result. Algorithms based on the use of dose limits are faster and allow to simulate usual clinical strategies like palliation or curative treatment very easily. The differences of the dose distributions for this strategies are described as a function of the number of beam portals.

Beam characteristics and clinical possibilities of a new compact treatment unit design combining narrow pencil beam scanning and segmental multileaf collimation

Not until the last decade has flexible intensity modulated three-dimensional dose delivery techniques with photon beams become a clinical reality, first in the form of heavy metal transmission blocks and other beam compensators, then in dynamic and segmented multileaf collimation, and most recently by scanning high-energy narrow electron and photon beams. The merits of various treatment unit and bremsstrahlung target designs for high-energy photon therapy are investigated theoretically for two clinically relevant target sites, a cervix and a larynx cancer both in late stages. With an optimized bremsstrahlung target it is possible to generate photon beams with a half-width of about 3 cm at a source to axis distance (SAD) of 100 cm and an initial electron energy of 50 MeV. By making a more compact treatment head and shortening the SAD, it is possible to reduce the half-width even further to about 2 cm at a SAD of 70 cm and still have sufficient clearance between the collimator head and the patient. One advantage of a reduced SAD is that the divergence of the beam for a given field size on the patient is increased, and thus the exit dose is lowered by as much as 1%/cm of the patient cross section. A second advantage of a reduced SAD is that the electron beam on the patient surface will be only about 8 mm wide and very suitable for precision spot beam scanning. It may also be possible to reduce the beamwidth further by increasing the electron energy up to about 60 MeV to get a photon beam of around 15 mm half-width and an electron beam as narrow as 5 mm. The compact machine will be more efficient and easy to work with, due to the small gantry and the reduced isocentric height. For a given target volume and optimally selected static multileaf collimator, it is no surprise that the narrowest possible scanned elementary bremsstrahlung beam generates the best possible treatment outcome. In fact, by delivering a few static field segments with individually optimized scan patterns, it is possible to combine the advantage of being able to fine tune the fluence distribution by the scanning system with the steeper dose gradients that can be delivered by a few static multileaf collimator segments. It is demonstrated that in most cases a few collimator segments are sufficient and often a single segment per beam portal may suffice when narrow scanned photon beams are employed, and they can be delivered sequentially with a negligible time delay. A further advantage is the increase of therapeutically useful photons and improved patient protection, since the pencil beam is only scanned where the leaf collimator is open. Consequently, some of the problems associated with dynamic multileaf collimation such as the tongue and groove and edge leakage effects are significantly reduced. Fast scanning beam techniques combined with good treatment verification systems allow interesting future possibilities to counteract patient and internal organ motions in real time.

Scanned intensity modulations for 50 MV photons

Optimization of the dose distributions by individual beam compensation is a useful tool in conformal radiation therapy. Intensity modulation by electromagnetic scanning of a narrow elementary beam allows fast dose delivery and causes little change in beam quality compared with other methods, especially for high energies such as 50 MV. Intensity modulated beams from the MM50 accelerator were measured and compared with calculations based on Monte Carlo simulations. Good agreement between measurements and calculations were found, typically within 1% for central dose profiles. The steepest wedge angle that was produced with the scanning beam technique was of or for a cm field, slightly varying with depth. The elementary 50 MV photon `pencil beam' for a full range, high- z bremsstrahlung target, is a wide dose distribution at 10 cm depth in water which limits the modulation gradient and hence the complexity of the modulation by the scanning of a photon pencil beam only. Scanned wedge beam distributions were modelled in the treatment planning system and a pelvic treatment with three fields was used to illustrate a clinical application. The resulting dose volume data were compared for different radiation qualities but with similar beam portals. `Energy modulation' by field matching with lower photon energies was performed to sharpen the penumbra towards organs at risk.

Optimization of 3D conformal electron beam therapy in inhomogeneous media by concomitant fluence and energy modulation

The possibilities of using simultaneous fluence and energy modulation techniques in electron beam therapy to shape the dose distribution and almost eliminate the influences of tissue inhomogeneities have been investigated. By using a radiobiologically based optimization algorithm the radiobiological properties of the tissues can be taken into account when trying to find the best possible dose delivery. First water phantoms with differently shaped surfaces were used to study the effect of surface irregularities. We also studied water phantoms with internal inhomogeneities consisting of air or cortical bone. It was possible to improve substantially the dose distribution by fluence modulation in these cases. In addition to the fluence modulation the most suitable single electron energy in each case was also determined. Finally, the simultaneous use of several preselected electron beam energies was also tested, each with an individually optimized fluence profile. One to six electron energies were used, resulting in a slow improvement in complication-free cure with increasing number of beam energies. To apply these techniques to a more clinically relevant situation a post-operative breast cancer patient was studied. For simplicity this patient was treated with only one anterior beam portal to clearly illustrate the effect of inhomogeneities like bone and lung on the dose distribution. It is shown that by using fluence modulation the influence of dose inhomogeneities can be significantly reduced. When two or more electron beam energies with individually optimized fluence profiles are used the dose conformality to the internal target volume is further increased, particularly for targets with complex shapes.

Exploration of new treatment modalities offered by high energy (up to 50 MeV) electrons and photons

Background and purpose: A number of deep seated tumours are difficult to treat conformally with photon beams mainly due to the almost exponential dose decrease with depth. Materials and methods: In order to improve the conformity of these treatments a number of useful characteristics of high energy (above 20 MeV) electron beams of the MM50 Racetrack Microtron have been systematically investigated and clinically applied. Results: A typical characteristic of electron beams with energies up to 20 MeV is the sharp dose fall-off with depth. At higher energies this effect is less pronounced but may be improved by adding a small fraction of photons with a matching dose gradient (wedge). With this technique, high energy electrons can be used close to sensitive organs down to 17 cm depth. Another physical characteristic of high energy electrons is the sharp penumbra at depths down to 4–5 cm and the possibility to use opposed electron beams in order to enhance the dose centrally or near the centre of a body. Skin sparing by delivering a part of the absorbed dose with photons through the same beam portal as the electrons has also been systematically studied. These characteristics of the high-energy electron beams have been utilised in the optimisation of some clinical treatments. Conclusions: Electron beams in this high energy region give increased possibilities to achieve dose conformity. Enhanced conformity can be obtained especially if electrons and photons are combined to augment some specific characteristics of the electron beams.

Automatic detection of single MLC leaf positions with corrections for penumbral effects and portal imager dose rate characteristics

A method has been developed to determine the individual settings of the MLC leaves from the image of the beam portal. The portal image is analysed by a two-step approach: (i) The general orientation of the collimator leaves is determined. (ii) Next, a one-dimensional edge operator (generalized Laplacian) is applied in a scanline fashion to extract an array holding the image coordinates of the field edge. The prescribed position of each leaf is then compared with the position of the steepest gradient behind that leaf as determined from the portal image. To obtain adequate precision in the determination of the leaf positions, the dose rate dependency of the electronic image detector, the increased transmission through the leaf tips and flanges, and the properties of the edge detector used for image processing were investigated. From these studies, a set of correction terms were obtained that are used to correctly calculate the position of the field edge from the prescribed MLC leaf settings. If not corrected, a leaf position-dependent discrepancy in the range -0.5 to 1.0 mm will arise. The results from tests of this method, applying the correction terms, show that the leaf positions can be determined with a precision better than 0.1 mm (one standard deviation). The reproducibility of the leaf positions was found to be better than 0.1 mm.

4 - The optimal number of beam portals in radiation therapy

4 - The optimal number of beam portals in radiation therapy