Optimizing beam weights and wedge filters with the concept of the super-omni wedge

This study introduces a new concept, the super-omni wedge, and proposes an algorithm for optimizing beam weights, wedge angles, and wedge orientations on the basis of this new concept. The super-omni wedge is a generalization of the omni wedge. Instead of combining one open beam and two orthogonal wedged beams, it uses two orthogonal pairs of nominal wedged beams to generate a wedged dose distribution with an arbitrary wedge angle and an arbitrary wedge orientation. The orientations of a pair of nominal wedges are opposite each other. In this way, the effective wedge orientation can vary from 0 degrees to 360 degrees rather than being restricted to one quadrant. When the concept of the super-omni wedge is used, the optimization of beam weights, wedge angles, and wedge orientations for J beams is transformed into the optimization of beam weights for 4J beams. A quadratic dose-based objective function is defined, and the method of sequential quadratic programming is used to find the 4J beam weights that minimize it. After the weights of the nominal wedged beams have been determined, the beams can be delivered in one of four methods: Directly, by using the omni wedge technique, by using the universal wedge technique, and by using the virtual wedge technique. When tested with two clinical cases, the algorithm achieved homogeneous dose distributions in target volumes while meeting the constraints to the organs at risk. A prominent feature of the algorithm is that there is no need to manually preselect the orientations of nominal wedges.     

Optimization of combined electron and photon beams for breast cancer

Recently, intensity-modulated radiation therapy and modulated electron radiotherapy have gathered a growing interest for the treatment of breast and head and neck tumours. In this work, we carried out a study to combine electron and photon beams to achieve differential dose distributions for multiple target volumes simultaneously. A Monte Carlo based treatment planning system was investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We compared breast treatment plans generated using this home-grown optimization and dose calculation software for different treatment techniques. Five different planning techniques have been developed for this study based on a standard photon beam whole breast treatment and an electron beam tumour bed cone down. Technique 1 includes two 6 MV tangential wedged photon beams followed by an anterior boost electron field. Technique 2 includes two 6 MV tangential intensity-modulated photon beams and the same boost electron field. Technique 3 optimizes two intensity-modulated photon beams based on a boost electron field. Technique 4 optimizes two intensity-modulated photon beams and the weight of the boost electron field. Technique 5 combines two intensity-modulated photon beams with an intensity-modulated electron field. Our results show that technique 2 can reduce hot spots both in the breast and the tumour bed compared to technique 1 (dose inhomogeneity is reduced from 34% to 28% for the target). Techniques 3, 4 and 5 can deliver a more homogeneous dose distribution to the target (with dose inhomogeneities for the target of 22%, 20% and 9%, respectively). In many cases techniques 3, 4 and 5 can reduce the dose to the lung and heart. It is concluded that combined photon and electron beam therapy may be advantageous for treating breast cancer compared to conventional treatment techniques using tangential wedged photon beams followed by a boost electron field.     

A comparative dosimetric study on tangential photon beams,intensity-modulated radiation therapy (IMRT) and modulated electron radiotherapy (MERT) for breast cancer treatment

Recently, energy- and intensity-modulated electron radiotherapy (MERT) has garnered a growing interest for the treatment of superficial targets. In this work. we carried out a comparative dosimetry study to evaluate MERT, photon beam intensity-modulated radiation therapy (IMRT) and conventional tangential photon beams for the treatment of breast cancer. A Monte Carlo based treatment planning system has been investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We have compared breast treatment plans generated using this home-grown treatment optimization and dose calculation software forthese treatment techniques. The MERT plans were planned with up to two gantry angles and four nominal energies (6, 9, 12 and 16 MeV). The tangential photon treatment plans were planned with 6 MV wedged photon beams. The IMRT plans were planned using both multiple-gantry 6 MV photon beams or two 6 MV tangential beams. Our results show that tangential IMRT can reduce the dose to the lung, heart and contralateral breast compared to conventional tangential wedged beams (up to 50% reduction in high dose volume or 5 Gy in the maximum dose). MERT can reduce the maximum dose to the lung by up to 20 Gy and to the heart by up to 35 Gy compared to conventional tangential wedged beams. Multiple beam angle IMRT can significantly reduce the maximum dose to the lung and heart (up to 20 Gy) but it induces low and medium doses to a large volume of normal tissues including lung, heart and contralateral breast. It is concluded that MERT has superior capabilities to achieve dose conformity both laterally and in the depth direction, which will be well suited for treating superficial targets such as breast cancer.     

Portal dose image prediction for dosimetric treatment verification in radiotherapy. II. An algorithm for wedged beams

A method is presented for calculation of a two-dimensional function, T(wedge)(x,y), describing the transmission of a wedged photon beam through a patient. This in an extension of the method that we have published for open (nonwedged) fields [Med. Phys. 25, 830-840 (1998)]. Transmission functions for open fields are being used in our clinic for prediction of portal dose images (PDI, i.e., a dose distribution behind the patient in a plane normal to the beam axis), which are compared with PDIs measured with an electronic portal imaging device (EPID). The calculations are based on the planning CT scan of the patient and on the irradiation geometry as determined in the treatment planning process. Input data for the developed algorithm for wedged beams are derived from (the already available) measured input data set for transmission prediction in open beams, which is extended with only a limited set of measurements in the wedged beam. The method has been tested for a PDI plane at 160 cm from the focus, in agreement with the applied focus-to-detector distance of our fluoroscopic EPIDs. For low and high energy photon beams (6 and 23 MV) good agreement (approximately 1%) has been found between calculated and measured transmissions for a slab and a thorax phantom.     

Optimization of intensity-modulated very high energy (50-250 MeV) electron therapy

This work evaluates the potential of very high energy (50-250 MeV) electron beams for dose conformation and identifies those variables that influence optimized dose distributions for this modality. Intensity-modulated plans for a prostate cancer model were optimized as a function of the importance factors, beam energy and number of energy bins, number of beams, and the beam orientations. A trial-and-error-derived constellation of importance factors for target and sensitive structures to achieve good conformal dose distributions was 500, 50, 10 and I for the target, rectum, bladder and normal tissues respectively. Electron energies greater than 100 MeV were found to be desirable for intensity-modulated very high energy electron therapy (VHEET) of prostate cancer. Plans generated for lower energy beams had relatively poor conformal dose distributions about the target region and delivered high doses to sensitive structures. Fixed angle beam treatments utilizing a large number of fields in the range 9-21 provided acceptable plans. Using more than 21 beams at fixed gantry angles had an insignificant effect on target coverage, but resulted in an increased dose to sensitive structures and an increased normal tissue integral dose. Minor improvements in VHEET plans utilizing a 'small' number (< or =9) of beams may be achieved if, in addition to intensity modulation, energy modulation is implemented using a small number (< or =3) of beam energies separated by 50 to 100 MeV. Rotation therapy provided better target dose homogeneity but unfortunately resulted in increased rectal dose, bladder dose and normal tissue integral dose relative to the 21-field fixed angle treatment plan. Modulation of the beam energy for rotation therapy had no beneficial consequences on the optimized dose distributions. Lastly, selection of beam orientations influenced the optimized treatment plan even when a large number of beams (approximately 15) were employed.     

Target motion tracking with a scanned particle beam

Treatment of moving targets with scanned particle beams results in local over- and under-dosage due to interplay of beam and target motion. To mitigate the impact of respiratory motion, a motion tracking system has been developed and integrated in the therapy control system at Gesellschaft für Schwerionenforschung. The system adapts pencil beam positions as well as the beam energy according to target motion to irradiate the planned position. Motion compensation performance of the tracking system was assessed by measurements with radiographic films and a 3D array of 24 ionization chambers. Measurements were performed for stationary detectors and moving detectors using the tracking system. Film measurements showed comparable homogeneity inside the target area. Relative differences of 3D dose distributions within the target volume were 1 +/- 2% with a maximum of 4%. Dose gradients and dose to surrounding areas were in good agreement. The motion tracking system successfully preserved dose distributions delivered to moving targets and maintained target conformity.     

Polymer gel measurement of dose homogeneity in the breast: comparing MLC intensity modulation with standard wedged delivery

Polymer gel dosimetry has been used to measure the radiotherapy dose homogeneity in a breast phantom for two different treatment methods. The first 'standard' method uses two tangential wedged fields while the second method has three static fields shaped by multileaf collimators (MLCs) in addition to the standard wedged fields to create intensity modulated fields. Gel dose distributions from the multileaf modulation treatment show an improved dose uniformity in comparison to the standard treatment with a decreased volume receiving doses over 105%.     

A practical method to calculate head scatter factors in wedged rectangular and irregular MLC shaped beams for external and internal wedges

Factor based methods for absorbed dose or monitor unit calculations are often based on separate data sets for open and wedged beams. The determination of basic beam parameters can be rather time consuming, unless equivalent square methods are applied. When considering irregular wedged beams shaped with a multileaf collimator, parametrization methods for dosimetric quantities, e.g. output ratios or wedge factors as a function of field size and shape, become even more important. A practical method is presented to derive wedged output ratios in air (S(c,w)) for any rectangular field and for any irregular MLC shaped beam. This method was based on open field output ratios in air (S(c)) for a field with the same collimator setting, and a relation f(w) between S(c,w) and S(c). The relation f(w) can be determined from measured output ratios in air for a few open and wedged fields including the maximum wedged field size. The function f(w) and its parametrization were dependent on wedge angle and treatment head design, i.e. they were different for internal and external wedges. The proposed method was tested for rectangular wedged fields on three accelerators with internal wedges (GE, Elekta, BBC) and two accelerators with external wedges (Varian). For symmetric regular beams the average deviation between calculated and measured S(c,w) / S(c) ratios was 0.3% for external wedges and about 0.6% for internal wedges. Maximum deviations of 1.8% were obtained for elongated rectangular fields on the GE and ELEKTA linacs with an internal wedge. The same accuracy was achieved for irregular MLC shaped wedged beams on the accelerators with MLC and internal wedges (GE and Elekta), with an average deviation < 1% for the fields tested. The proposed method to determine output ratios in air for wedged beams from output ratios of open beams, combined with equivalent square approaches, can be easily integrated in empirical or semi-empirical methods for monitor unit calculations.     

Monitor unit calculations for wedged asymmetric photon beams

Algorithms for calculating monitor units (MUs) in wedged asymmetric high-energy photon beams as implemented in treatment planning systems have their limitations. Therefore an independent method for MU calculation is necessary. The aim of this study was to develop an empirical method to determine MUs for points at the centre of wedged fields, asymmetric in two directions. The method is based on the determination of an off-axis factor (OAF) that corrects for the difference in dose between wedged asymmetric and wedged symmetric beams with the same field size. Measurements were performed in a water phantom irradiated with 6 and 18 MV photon beams produced by Elekta accelerators, which are fitted with an internal motorized wedge that has a complex shape. The OAF perpendicular to the wedge direction changed significantly with depth for the 18 MV beam. Dose values measured for a set of 18 test cases were compared with those calculated with our method. The maximum difference found was 6.5% and in 15 cases this figure was smaller than 2.0%. The analytical method of Khan and the empirical method of Georg were also tested and showed errors up to 12.8%. It can be concluded that our simple formalism is able to calculate MUs in wedged asymmetric fields with an acceptable accuracy in most clinical situations.     

A simplified intensity modulated radiation therapy technique for the breast

A simplified intensity modulated radiation therapy (sIMRT) technique for the breast is presented. The technique aims to produce a uniform dose distribution in the entire breast volume. Using the standard tangential beam arrangement, we first determine for each pencil beam the midpoint of the segment that intersects the treatment volume. The dose to the midpoint from the open field is then calculated. The intensity of the pencil beam is determined as proportional to the inverse of the open field dose. With this intensity modulated beam, the dose delivered to the midpoint of each pencil beam segment that intersects the treatment volume is now equalized. The dose distribution in the entire treatment volume is nearly as uniform as can be achieved under the given beam arrangement. Fifteen left breast patients were planned with the sIMRT technique. For comparison, the same group of patients was also planned with the standard wedged pair technique and the full-fledged volume-based IMRT (vIMRT) technique. Both the sIMRT and the vIMRT techniques achieved more homogeneous dose in the treatment volume than the standard plan. Doses to the heart, the ipsilateral lung, and the contralateral breast were also reduced. The planning time and the treatment time for the sIMRT technique were comparable to that of the standard technique, and significantly less than that required by the vIMRT technique. The sIMRT technique is practical for large-scale implementation in a busy clinic without requiring significant increase of resources.     

The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams

In this paper the performance of the LA48 linear ion chamber array (PTW, Freiburg, Germany) for characterization of intensity-modulated (IM) beams was investigated. First, some elementary properties were explored. A series of beam penumbras and output factors for small rectangular fields were measured at 6 and 18 MV, and the results were compared with data obtained using a diamond detector. The energy and dose rate dependence of the array response were examined, and the leakage current was assessed. In a second step, profiles were measured for two clinically delivered IM beams and for a dynamic wedge. The interplay between the sharpening of the penumbra by the upper metal electrode plate of the array and the volume averaging of the 4 x 4 mm ion chamber elements results in precise measurements, even in regions of high dose gradient. It is true, however, that the metal electrodes imply a small energy spectrum dependence in the array response. The dose rate dependence is found to be negligible. All of this makes, the LA48 linear array a suitable device for analysing dose distributions of clinical IM beams.     

Scatter dose for wedged fields

The behaviour of scatter dose in 4 and 8 MV wedged x-ray beams has been studied by calculating scatter-to-primary dose ratios (SPR) and comparing these with SPR for non-wedged beams. On the central axis the SPR for wedged and non-wedged beams differ only by a few per cent, a difference which increases slightly with wedge angle and field size. In other points within the field the differences are larger but generally less than 3% of the total dose on the central axis at the same depth. The product rule for points that do not lie in a principal plane is valid within the same limits as for non-wedged beams.     

Multilayer Gafchromic film detectors for breast skin dose determination in vivo

Assessment of skin dose delivered to patients from radiotherapy x-ray beams should be performed both inside and outside the prescribed treatment fields. A multilayer Gafchromic film detector which has high sensitivity for detection of radiation can be used to measure skin dose in a two-dimensional map over the skin surface if required. This is an advantage over other detectors, which only provide point dose estimates. A study of 25 patients undergoing breast irradiation was performed to analyse the ability of the multilayer detector to analyse skin dose and to assess both in-field and out-of-field radiation doses delivered during tangent field breast irradiation. Results show that the main contributor to total skin dose within the treatment field was delivered by exit dose. However, outside the field, most dose was delivered by entry beams. Patients with smaller breast separations where found, in general, to receive a higher total skin dose from entry and exiting beams at the central axis. Results also showed that a significant skin dose was delivered outside the treatment field and the main cause of this dose was from electron contamination from entry beams. The multilayer Gafchromic film detector provided adequate skin dose assessment within one fraction of treatment for in vivo results.     

Radiotherapy of stage IEA primary breast lymphoma: case report

A 47-year-old woman was referred for the treatment to our Hospital because of a palpable nodule in the upper medial quadrant of her right breast. After tumor excision, pathohistological examination showed a follicular center cell lymphoma grade 2, B-cell type (CD20+, bc16+, CD10+, bcl2+). The final diagnosis was stage IEA primary extranodal non-Hodgkin s breast lymphoma. The involved breast was irradiated isocentrically with two opposite 6-megavolt (MeV) photon beams delivered from the linear accelerator (tangential fields) using asymmetric collimator opening. Radiation volume, inclinations of the medial and lateral field, and the part of the underlying chest wall and lung parenchyma were determined during the radiotherapy simulation process. The total irradiation dose was 44 Gy delivered in single daily doses of 2 Grays (Gy). After breast photon irradiation, a boost to the tumor bed was performed by a direct 12 MeV electron beam, with a total dose of 6 Gy delivered over three days. Since primary non-Hodgkin lymphoma of the breast is rather rare, there has been no uniform approach to its treatment. The advantage of applying the asymmetric collimator jaw opening in breast radiotherapy is the instant reduction of the dose at margin fields, resulting in both the protection of neighboring lung parenchyma and the good coverage of planned target volume.     

全乳腺放射治疗3种照射技术对比

目的评价不同全乳腺放射治疗技术的剂量学优缺点。方法选取乳腺癌保乳术后患者10例,均为女性,年龄25～58岁,中位年龄42岁。用常规切线野、野中野调强、多野调强3种技术设计全乳腺放射治疗计划并进行剂量学对比。结果与常规切线野计划比较,野中野调强计划99%靶区体积含盖剂量从（4640±72）cGy增加到（4753±25）cGy;剂量不均匀指数从1.104±0.017下降到1.060±0.008。常规切线野计划与野中野调强计划在危及器官受量上的差异无统计学意义。野中野调强计划与多野调强计划在靶区剂量分布上的差异无统计学意义;与野中野调强计划比较,多野调强计划中心脏接受高于10Gy剂量的体积（V10）从（13.0±8.5）%增加到（53.3±22.7）%;同侧肺的V10从（25.2±3.4）%增加到（42.7±3.7）%;对侧乳腺、对侧肺、非特异正常组织的V5分别从（1.9±3.1）%增加到（32.6±2.3）%、从0到（18.5±8.3）%、从（9.9±1.0）%到（32.1±3.6）%。结论野中野调强技术较常规切线野技术明显改善靶区剂量分布;在此基础上多野调强技术未能明显进一步改善靶区剂量,但增加正常组织照射。     

Online compensation for target motion with scanned particle beams: simulation environment

Target motion is one of the major limitations of each high precision radiation therapy. Using advanced active beam delivery techniques, such as the magnetic raster scanning system for particle irradiation, the interplay between time-dependent beam and target position heavily distorts the applied dose distribution. This paper presents a simulation environment in which the time-dependent effect of target motion on heavy-ion irradiation can be calculated with dynamically scanned ion beams. In an extension of the existing treatment planning software for ion irradiation of static targets (TRiP) at GSI, the expected dose distribution is calculated as the sum of several sub-distributions for single target motion states. To investigate active compensation for target motion by adapting the position of the therapeutic beam during irradiation, the planned beam positions can be altered during the calculation. Applying realistic parameters to the planned motion-compensation methods at GSI, the effect of target motion on the expected dose uniformity can be simulated for different target configurations and motion conditions. For the dynamic dose calculation, experimentally measured profiles of the beam extraction in time were used. Initial simulations show the feasibility and consistency of an active motion compensation with the magnetic scanning system and reveal some strategies to improve the dose homogeneity inside the moving target. The simulation environment presented here provides an effective means for evaluating the dose distribution for a moving target volume with and without motion compensation. It contributes a substantial basis for the experimental research on the irradiation of moving target volumes with scanned ion beams at GSI which will be presented in upcoming papers.     

调整等中心技术在乳腺癌三维适形放疗中的应用

目的：比较乳腺癌放疗中，采用传统切线野、适形和调整等中心三种照射技术对改善靶区剂量均匀性和保护正常组织的作用．以改进放射治疗方法。方法：随机选择6位行全乳放疗的乳腺癌患者，为每位患者各设计上述的三种照射技术的治疗计划．给予90％PTV靶体积45Gy的处方剂量，分别比较它们的靶区平均剂量、剂量均匀性指数、重要器官平均剂量．以及剂量分布图和DVH图。结果：三种技术靶区剂量均匀性方差分析差异具有统计学意义（P〈0．01），调整等中心技术的靶区均匀性最好，而传统的切线照射技术最差。三种技术在保护重要器官上其差异也具有统计学意义（P〈0．01），调整等中心技术对肺组织保护得最好。结论：通过调整靶区等中心能够提高靶区剂量均匀性，有效降低重要器官的受量，提高乳腺癌的控制率，而且此法简单易行，值得推广。     

Selecting beam weight and wedge filter on the basis of dose gradient analysis

This study proposes an algorithm for selecting beam weight, wedge angle, and wedge orientation for three-dimensional radiation therapy treatment planning. According to dose gradient analysis, the necessary and sufficient condition for achieving a homogeneous dose over the target volume is that the total vector sum of the dose gradients of all beams be zero everywhere in the target volume. This study presents equations for calculating the beam weight, wedge angle, and collimator angle (because the collimator angle determines wedge orientation when beam direction is known) for treatment plans using two angled beams or three coplanar or noncoplanar beams. It also provides suggestions for calculations of treatment plans using more than three beams, for which many feasible solutions will be available. When tested using two clinical cases, this algorithm achieved homogeneous dose distributions over target volumes. With this algorithm, repeated manual adjustments are reduced, and the quality and efficiency of treatment planning are improved.