Modulated Photon Beams Research Articles

Comparison of Air-Gaps Effect in a Small Cavity on Dose Calculation for 6 MV Linac.

Background: Many authors stated that cavities or air-gaps were the main challenge of dose calculation for head and neck with flattening filter medical linear accelerator (Linac) irradiation.Objective: The study aimed to evaluate the effect of air-gap dose calculation on flattening-filter-free (FFF) small field irradiation.Material and Methods: In this comparative study, we did the experimental and Monte Carlo (MC) simulation to evaluate the presence of heterogeneities in radiotherapy. We simulated the dose distribution on virtual phantom and the patient’s CT image to determine the air-gap effect of open small field and modulated photon beam, respectively. The dose ratio of air-gaps to tissue-equivalent was calculated both in Analytical Anisotropic Algorithm (AAA) and MC.Results: We found that the dose ratio of air to tissue-equivalent tends to decrease with a larger field size. This correlation was linear with a slope of -0.198±0.001 and -0.161±0.014 for both AAA and MC, respectively. On the other hand, the dose ratio below the air-gap was field size-dependent. The AAA to MC dose calculation as the impact of air-gap thickness and field size varied from 1.57% to 5.35% after the gap. Besides, patient’s skin and oral cavity on head and neck case received a large dose discrepancy according to this study. Conclusion: The dose air to tissue-equivalent ratio decreased with smaller air gaps and larger field sizes. Dose correction for AAA calculation of open small field size should be considered after small air-gaps. However, delivered beam from others gantry angle reduced this effect on clinical case.

TriB-RT: Simultaneous optimization of photon, electron and proton beams

Purpose. To develop a novel treatment planning process (TPP) with simultaneous optimization of modulated photon, electron and proton beams for improved treatment plan quality in radiotherapy. Methods. A framework for fluence map optimization of Monte Carlo (MC) calculated beamlet dose distributions is developed to generate treatment plans consisting of photon, electron and spot scanning proton fields. Initially, in-house intensity modulated proton therapy (IMPT) plans are compared to proton plans created by a commercial treatment planning system (TPS). A triple beam radiotherapy (TriB-RT) plan is generated for an exemplary academic case and the dose contributions of the three particle types are investigated. To investigate the dosimetric potential, a TriB-RT plan is compared to an in-house IMPT plan for two clinically motivated cases. Benefits of TriB-RT for a fixed proton beam line with a single proton field are investigated. Results. In-house optimized IMPT are of at least equal or better quality than TPS-generated proton plans, and MC-based optimization shows dosimetric advantages for inhomogeneous situations. Concerning TriB-RT, for the academic case, the resulting plan shows substantial contribution of all particle types. For the clinically motivated case, improved sparing of organs at risk close to the target volume is achieved compared to IMPT (e.g. myelon and brainstem −37%) at cost of an increased low dose bath (healthy tissue V 10% +22%). In the scenario of a fixed proton beam line, TriB-RT plans are able to compensate the loss in degrees of freedom to substantially improve plan quality compared to a single field proton plan. Conclusion. A novel TPP which simultaneously optimizes photon, electron and proton beams was successfully developed. TriB-RT shows the potential for improved treatment plan quality and is especially promising for cost-effective single-room proton solutions with a fixed beamline in combination with a conventional linac delivering photon and electron fields.

Effects of gas-filled temporary breast tissue expanders on radiation dose from modulated rotational photon beams

Gas-filled temporary tissue expanders (TTEs), implanted to assist in post mastectomy breast reconstructions, are expected to produce increased dosimetric uncertainty in breast radiotherapy treatments, due to their containing both a substantial metallic component and a comparatively large volume of gas. This study therefore builds on previous investigations of the dosimetric effects of gas-filled TTEs in static photon and electron beams, by examining the effects of these implants on dose distributions from common modulated rotational treatment techniques; volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT). Radiochromic film measurements were used to evaluate the accuracy of VMAT and HT dose calculations, for a humanoid phantom augmented with a sample Aeroform CO2-filled TTE (AirXpanders Inc, San Jose, USA) as well as purpose-designed and 3D printed “breast tissue.” Results showed that the TomoTherapy Hi-Art VoLO convolution-superposition algorithm (Accuray Inc, Sunnyvale, USA) produced comparatively accurate calculations of treatment dose within this complex phantom, including immediately anterior and posterior to the TTE. The Varian Eclipse Acuros (AXB) algorithm generally showed better agreement with the film measurement than the Varian Eclipse AAA algorithm (Varian Medical Systems, Palo Alto, USA), although the film measurements showed regions of 5% to 10% disagreement with both AAA and AXB in the dosimetrically-challenging region on the anterior side of the implant. Although the Aeroform CO2-filled TTE has substantial and obvious effects on the downstream dose from a static photon beam, the results of this study showed how inverse-planning of modulated rotational radiotherapy treatments can produce modulated fluence distributions that compensate for the dramatic density heterogeneities in the implant. Despite some disagreements with the planned dose, all film measurements showed that the use of inverse-planned modulated rotational photon beams resulted in comparatively homogeneous coverage of the radiotherapy target, in the complex patient-like phantom with a gas-filled TTE. Due to the importance of matching each planned fluence distribution to the density distribution within each TTE, careful use of available 3D imaging techniques is advisable, when modulated rotational radiotherapy treatments are delivered to patients with gas-filled TTEs.

Clinical implementation of combined modulated electron and photon beams with conventional MLC for accelerated partial breast irradiation

PurposeTo report the clinical implementation of a novel external beam radiotherapy technique for accelerated partial breast irradiation treatments based on combined electron and photon modulated beams radiotherapy (MERT+IMRT) with conventional MLC. Materials and methodsA group of patients was selected to test the viability of the technique. The prescribed dose was 38.5Gy, following a hypofractionated schema, and the structures were defined following the NSABP-B39/RTOG-0413 protocol. The plans were calculated with an in-house Monte Carlo based planning system to consider explicitly the particle interactions with the MLC. An ad-hoc breast phantom was designed for a specific QA protocol. A reduced SSD was used for electron beams. Toxicity and cosmetic effects were assessed at every follow-up visit. ResultsAll the plans achieved the dosimetric objectives and fulfilled the specific quality assurance protocol. Treatment delivery did not entail additional drawbacks for the clinical routine. Moderate or severe grade of toxicity was not reported, and the cosmetic results were comparable to those obtained with other APBI techniques. ConclusionsResults showed that MERT+IMRT with the MLC is a feasible and secure technique, and easy to be extended to other centers with the implementation of the adequate software for planning.

SU‐E‐T‐593: Outcomes and Toxicities From a Clinical Trial of APBI Using MERT+IMRT with the Same XMLC

Purpose:We present the results from a clinical trial of accelerated partial breast irradiation (APBI), using mixed modulated photon and electron beams (MERT+IMRT) with the same photon multileaf collimator (xMLC).Methods:Seven patients were enrolled in the first year of the APBI clinical trial. Patients were selected following the conditions included in the NSABP B‐39/RTOG 0413 protocol. The targets and clinically relevant normal structures were contoured on the CT images following this protocol for APBI‐EBRT. All treatments were delivered using combined modulated electron and photon beams by means of the same xMLC installed in a SIEMENS Primus linac, with a reduced SSD equal to 60 cm for electron beams. The plans were performed with a treatment planning system based on full Monte Carlo simulations, called CARMEN, developed by our group. Simultaneously, an alternative IMRT plan was calculated with the commercial TPS PINNACLE v8.0m (Philips), and both plans were compared. An ad‐hoc breast phantom with semi‐spherical geometry called NAOMI was designed for a specific QA protocol. Patients received a total dose of 38.5 Gy, delivered in 10 fractions over 5 consecutive days, with a twice‐a‐day hypofractionated schema.Follow‐up visits during 2.5 years on average were repeated at 1 month post‐treatment, every 3 months for the first year, and every 6 months for the second year. Toxicity was scored according to National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE version 3.0).Results:This APBI technique achieved high loco‐regional control rates and showed low acute toxicity (grade 1 of CTCAE) and no toxicities from first month onwards. Photographic assessment of cosmesis showed skin excellent results.Conclusion:The clinical results achieved with MERT+IMRT by using the same xMLC are comparable or even better than those obtained with other APBI techniques, thanks to a software solution without any additional equipment or specific device.

PO-0697: Clinical implementation of APBI using combined modulated electron and photon beams by means of the same MLC device

PO-0697: Clinical implementation of APBI using combined modulated electron and photon beams by means of the same MLC device

Combined modulated electron and photon beams planned by a Monte-Carlo-based optimization procedure for accelerated partial breast irradiation

The purpose of this study was to present a Monte-Carlo (MC)-based optimization procedure to improve conventional treatment plans for accelerated partial breast irradiation (APBI) using modulated electron beams alone or combined with modulated photon beams, to be delivered by a single collimation device, i.e. a photon multi-leaf collimator (xMLC) already installed in a standard hospital. Five left-sided breast cases were retrospectively planned using modulated photon and/or electron beams with an in-house treatment planning system (TPS), called CARMEN, and based on MC simulations. For comparison, the same cases were also planned by a PINNACLE TPS using conventional inverse intensity modulated radiation therapy (IMRT). Normal tissue complication probability for pericarditis, pneumonitis and breast fibrosis was calculated. CARMEN plans showed similar acceptable planning target volume (PTV) coverage as conventional IMRT plans with 90% of PTV volume covered by the prescribed dose (Dp). Heart and ipsilateral lung receiving 5% Dp and 15% Dp, respectively, was 3.2–3.6 times lower for CARMEN plans. Ipsilateral breast receiving 50% Dp and 100% Dp was an average of 1.4–1.7 times lower for CARMEN plans. Skin and whole body low-dose volume was also reduced. Modulated photon and/or electron beams planned by the CARMEN TPS improve APBI treatments by increasing normal tissue sparing maintaining the same PTV coverage achieved by other techniques. The use of the xMLC, already installed in the linac, to collimate photon and electron beams favors the clinical implementation of APBI with the highest efficiency.

Dosimetric verification of 6 and 18 MV intensity modulated photon beams using a dedicated fluoroscopic electronic portal imaging device (EPID)

Background and purpose Intensity modulated radiation therapy offers a dose distribution improvement by modulating the two-dimensional X-ray fluence. This increases the treatment complexity from the planning phase to the verification one. Pre-treatment dosimetric verifications of each treatment field are strictly necessary, films and EPIDs are generally used for this purpose. In this paper we investigated the dosimetric characteristics of a commercially available fluoroscopic EPID “I’mRT QA” (Scanditronix-Wellhöfer) (I-QA) based on charge-coupled device camera (CCD) and we present a new method for optimising the EPID’s response. Materials and methods I-QA is an optically sealed EPID and it is designed for on-line two-dimensional measurements of relative dose distribution. Dose profiles measured in water with diodes and dose distributions measured in water-equivalent phantoms with films were compared with those obtained with the I-QA for homogeneous and intensity modulated 6 and 18 MV photon beams. Results and conclusions I-QA measurements depend on the field size and on the two-dimensional energy spectrum of the beam. The incoming beam was modified positioning a series of lead and plastic (RW3) slabs above the fluorescent screen to obtain a homogeneous response of the I-QA over the whole sensitive area. The thickness of lead and RW3 sheets was optimised to get the best matching between diodes, films and the I-QA measurements for each energy. Gamma index evaluations showed a correspondence between I-QA measurements and diode ones within 3% or 2 mm for homogeneous and simple modulated fields, and within 5% or 3 mm for complex modulated fields.

TU‐E‐ValB‐02: Using Magnetically Collimated Electrons and Narrow Intensity Modulated Tangential Photons for Accelerated Partial Breast Irradiation

Purpose: Previous studies have shown that enface magnetic collimation lowers entrance dose and reduces lateral scattering for electron beams. However, dose uniformity is limited at depth for single beam delivery. This study investigates whether adding narrow intensity modulated tangential photon beams would solve the problem and create a new technique for accelerated partial breast irradiation. Method and Materials: Magnetically collimated electron beams were measured and modeled empirically for treatment planning and combined with photon beams on a commercial treatment planning system (Pinnacle 7.6, Philips Medical System). Treatment plans were generated for conformal irradiation of the partial breast using a pair of intensity modulated tangential photon beams plus enface magnetically collimated electron beams. The photon beams were planned via inverse planning with the beam weights of the electron beam adjusted simultaneously. The first MLC segment of the photon beams was fixed to cover the entire planning target volume in the beam's eye view to minimize the effect of target motion. Final treatment plans were analyzed for dose uniformity, for the conformity of the target volume and also for the dose to normal tissues including the skin. Results: The combined beam approach significantly improved dose conformity, e.g., narrower separation between peripheral isodose lines, as compared with using either electron beams or a pair of tangential fields alone. The use of enface electron beams significantly improved the target dose uniformity to better than 10%. Due to magnetic collimation, the skin dose was on average lowered by 18% over the use of conventional electron beam for the technique. Conclusion: Intensity modulated tangential photon beams combined with magnetically collimated electron beams offers a new technique for accelerated partial breast irradiation.

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.

200 Quantification of differences between planned and delivered IMRT dose distributions in terms of complication-free tumor cure

Biological treatment optimization aim at improving radiation therapy by accounting for the radiobiological tumour and normal tissues response properties when optimizing the dose delivery. Generally traditional methods, using only dosimetrical measures, disregard the nonlinear radiation response of different tumours and normal tissues. The accumulated knowledge on tissue response to radiation, in the form of more accurate dose response relations, cell survival models and their associated biological parameters, alongside with the tools for biological treatment plan optimization, has allowed the present investigation on the potential merits of biologically based treatment optimization in radiation therapy.With a more widespread implementation of intensity modulated radiation therapy in the clinic, there is an increasing demand for faster and safer treatment delivery techniques. In this thesis biological treatment plan optimization, using the probability to achieve complication free tumour control as the quantifier for treatment outcome, was applied to radiation therapy of early breast cancer and advanced cervix cancer. It is shown that very conformal dose distributions can generally be produced with 3 or 4 optimally orientated coplanar intensity modulated beams, without having clinically significant losses in treatment outcome from the optimal dose distribution.By using exhaustive search methods, the optimal coplanar beam directions for intensity modulated photon beams for early breast cancer and the optimal non-coplanar directions for an advanced cervix cancer were investigated. Although time consuming, exhaustive search methods have the advantage of revealing most features involving interactions between a small number of beams and how this may influence the treatment outcome. Thus phase spaces may serve as a general database for selecting an almost optimal treatment configuration for similar patients. Previous knowledge acquired with physically optimized uniform beam radiation therapy may not apply when intensity modulated biological optimization is used. Thus unconventional treatment directions were sometimes found.

Clinical experience in breast irradiation with intensity modulated photon beams

Since 2002, twenty-five patients affected by breast cancer, for whom extensive lung/heart involvement was expected from conventional techniques, were irradiated with intensity modulated photon beams (IMRT). For 10 patients the internal mammary lymphatic chain was included in the target, 6 patients received treatment of the thoracic wall after mastectomy and 9 patients were treated after breast conserving surgery. Dose prescription ranged between 48.6 and 50 Gy at 1.8 or 2 Gy per fraction. The target volume (PTV) ranged from 194 to 2121 cm3. For all cases a Dose Volume Histogram analysis has been conducted on omolateral lung, heart, contralateral lung and breast, healthy tissue and PTV. For PTV the volume receiving more than 90% of the prescribed dose (V90%) was 95.8±1.8% while V107%=5.3±2.8%. The mean dose computed for the heart was 10.4±2.9 Gy, for the omolateral lung: 13.3±2.8 Gy, for the contralateral breast: 3.4±1.8 Gy, for the contralateral lung: 4.6±2.7 Gy. For the omolateral lung V20Gy=23.1±7.0% and V45Gy=0.8±0.9 Gy. With a short mean follow up of 10.4 months, no pulmonary or cardiac complications were observed. IMRT proved to be technically feasible on a clinical basis for the treatment of the whole breast, including internal mammary chain, or of the thoracic wall after mastectomy.

Pre-treatment verification of intensity modulated photon beams with films and electronic portal imaging – Two years of clinical experience

Aim of this report was to summarise clinical experience in the field of pre-treatment dosimetric verification of intensity-modulated photon beams (IRMT). From May 2001 to July 2003, 50 patients were irradiated according to IMRT techniques with 6 MV photon beams produced by a Varian Clinac equipped with a 80 leaves multileaf collimator. Dose plans were computed using commercial treatment planning systems, Nucletron Helax-TMS for static cases and Varian Eclipse-Helios for dynamic cases. Pre-treatment dosimetric verification was carried out on a field-per-field basis measuring 2D absolute dose distributions in solid water at 10 cm depth using films or an electronic portal imaging device (EPID). Verification measurements were compared with expected dose maps, and differences were evaluated by means of both a point-to-point analysis and the Gamma Index.Irradiated target volumes (30 head and neck, 8 breast, 12 other patients) ranged from 111 to 2121 cm3 with a mean of 652 ± 378 cm3. Twentynine dose plans were delivered with dynamic technigue and 44 with static technique. On averge, 5.9 ± 1.3 fields were applied per plan, with 12.1 ± 1.6 segments per field in the static mode. Averaging over the whole number of fields we obtained a mean difference (on a pixel-by-pixel basis and per 100 MU delivered) of −0.22 ± 0.64 cGy between calculation and measurement, with a standard deviation of 1.93 ± 0.65 cGy. The mean value for the Gamma Index evaluation was 0.47 ± 0.10, with a mean standard deviation of 0.35 ± 0.17. The fraction of pixels lying inside the field and showing a gamma index larger than 1 was 5.7 % for the triplet Eclipse-film-dynamic delivery and 9.9 % for the triplet Helax-TMS-EPID-static delivery. The employed IMRT treatments proved that this technique is feasible and dosimetrically accurate. Treatment verification stability and dosimetric analysis of treated plans are highly satisfactory and allow the safe introduction of this modality in the spectrum of techniques offered to a large class of patients.

Number and Orientation of Beams in Inversely Planned Intensity-Modulated Radiotherapy of the Female Breast and the Parasternal Lymph Nodes

Intensity-modulated radiotherapy (IMRT) provides better sparing of normal tissue. We evaluated the optimum beam configuration for IMRT based on inverse treatment planning in adjuvant radiotherapy for breast cancer in a case of left-sided tumor. In addition to radiotherapy planning with the conventional technique of tangential wedged 6-MV photon beams and an oblique 15-MeV electron beam, we performed inversely planned IMRT with the step-and-shoot-technique. Dose calculation was carried out using the treatment planning system Virtuos with the inverse optimization module KonRad adapted to it. IMRT plans were generated for 2 to 16 beams. The results were compared with conventional techniques. For a maximum treatment time of 20 minutes, it is shown that IMRT with 12 modulated photon beams and 7 intensity steps is best suited for treatment in the presented case. Compared with a conventional technique with photons combined with electrons, dose conformality and homogeneity of the planning target volume was increased. The mean heart dose was reduced from 9.1 Gy to 6.1 Gy. The volume of heart irradiated with a dose higher than 30 Gy was reduced from 7.6% to 1.9%, and the volume of the left lung from 13.6% to 11.5% as well. Inverse optimization for IMRT with multiple beams is feasible in the adjuvant treatment of breast cancer. Because of the reduction of the high-dose area of a substantial cardiac volume, it is superior to conventional techniques in cases where the parasternal lymph nodes should be integrated into the target volume. Here, a clinical advantage might be detectable.

High energy electron beams in intensity modulated radiation therapy

The aims of the thesis were the development of the phase space evolution (PSE) model for electron beam treatment planning, investigation of techniques to improve electron beam dose distributions, and investigation of the added value of high energy (⩽50 MeV) electron beams in intensity modulated radiotherapy. The PSE model is capable of calculating dose distributions by taking into account density variations in full three dimensions and is accurate and fast compared to Fermi–Eyges based pencil beam models or Monte Carlo models. A relatively simple model that requires only measurements as input is adequate to describe the initial electron beam. The PSE model can also be used for the calculation of pencil beam kernels for optimization of intensity modulated electron beam treatments. Techniques, such as intensity modulation, energy modulation, and mixing with photons are suggested to improve dose distributions resulting from electron beam irradiations from a single direction. It is shown that, by the combination of intensity modulated electron and photon beams from one direction, dose distributions can have steep dose falloffs similar to unmodulated electron beams, and narrow penumbras similar to photon beams. The added value of high energy electron beams was investigated in a comparative treatment planning study using optimized intensity modulated electron and photon beams. For the studied clinical cases use of intensity modulated photon beams gives significant improvements over conventional treatments. Inclusion of electron beams could only marginally improve the expected treatment outcome, although dose volume histograms of organs at risk show improvements, especially at lower dose levels (e.g., <40 Gy) in the treatments with electrons.

Critical appraisal of treatment techniques based on conventional photon beams, intensity modulated photon beams and proton beams for therapy of intact breast

Purpose: To analyse different treatment techniques with conventional photon beams, intensity modulated photon beams, and proton beams for intact breast irradiation for patients in whom conventional irradiation would cause potentially dangerous lung irradiation. Material and methods: Five breast cancer patients with highly concave breast tissue volume around the lung were considered at planning level in order to assess the suitability of different irradiation techniques. Three-dimensional dose distributions for conventional two-field tangential photon treatment, two-field intensity modulated radiotherapy (IMRT), three-field non-IMRT, three-field IMRT, and single-field proton treatment were investigated, aiming at assessing the possibility to reduce lung irradiation below risk levels. Analysis of dose–volume histograms and related physical and biological parameters (significant minimum, maximum and mean doses, conformity indexes and equivalent uniform dose (EUD)) for planned target volume (PTV) and lung was carried out. Dose plans were compared with the conventional two-field tangential photon technique. Results: PTV coverage was comparable for non-IMRT and IMRT techniques (EUD from 47.1 to 49.4 Gy), and improved with single-field proton treatment (EUD=49.8 Gy). Lung irradiation was reduced, in terms of mean dose, with three-field (9.5 Gy) and proton technique (3.5 Gy), with respect to the conventional two-field treatment (12.9 Gy); also a reduction of the lung volume irradiated at high doses was observed. Better results could be achieved with protons. In addition, cardiac irradiation was also reduced with those techniques. Conclusions: Geometrically difficult breast cancer patients could be irradiated with a three-field non-IMRT technique thus reducing the dose to the lung which is proposed as standard for this category of patients. Intensity modulated techniques were only marginally more successful than the corresponding non-IMRT treatments, while protons offer excellent results.

56 oral A method for MU calculation in dynamic MLC intensity modulated photon beams

56 oral A method for MU calculation in dynamic MLC intensity modulated photon beams

Breast-conserving radiation therapy using combined electron and intensity-modulated radiotherapy technique

Background and purpose: To explore the feasibility of a multi-modality breast-conserving radiation therapy treatment technique to reduce high dose to the ipsilateral lung and the heart when compared with the conventional treatment technique using two tangential fields. Materials and methods: An electron beam with appropriate energy was combined with four intensity modulated photon beams. The direction of the electron beam was chosen to be tilted 10–20° laterally from the anteroposterior direction. Two of the intensity-modulated photon beams had the same gantry angles as the conventional tangential fields, whereas the other two beams were rotated 15–25° toward the anteroposterior directions from the first two photon beams. An iterative algorithm was developed which optimizes the weight of the electron beam as well as the fluence profiles of the photon beams for a given patient. Two breast cancer patients with early-stage breast tumors were planned with the new technique and the results were compared with those from 3D planning using tangential fields as well as 9-field intensity-modulated radiotherapy (IMRT) techniques. Results: The combined electron and IMRT plans showed better dose conformity to the target with significantly reduced dose to the ipsilateral lung and, in the case of the left-breast patient, reduced dose to the heart, than the tangential field plans. In both the right-sided and left-sided breast plans, the dose to other normal structures was similar to that from conventional plans and was much smaller than that from the 9-field IMRT plans. The optimized electron beam provided between 70 to 80% of the prescribed dose at the depth of maximum dose of the electron beam. Conclusions: The combined electron and IMRT technique showed improvement over the conventional treatment technique using tangential fields with reduced dose to the ipsilateral lung and the heart. The customized beam directions of the four IMRT fields also kept the dose to other critical structures to a minimum.

Intensity modulated photon beams subject to a minimal surface smoothingconstraint

A method to smooth intensity-modulated photon beams is presented which can beapplied in conjunction with any optimization algorithm. The method employs anadditional soft constraint to minimize the area of the surface defined by thephoton fluence.

Tumor dose enhancement and normal tissue protection using magnetic field modulated photon beams in radiation therapy

Tumor dose enhancement and normal tissue protection using magnetic field modulated photon beams in radiation therapy