C Chui

Intensity-modulated tangential beam irradiation of the intact breast

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

Implementation of a Monte Carlo dosimetry method for patient‐specific internal emitter therapy

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose-response relationship, as well as to avoid critical organ toxicity. This work describes the implementation of a dosimetry method that accounts for the radionuclide decay spectrum, and patient-specific activity and density distributions. The dosimetry algorithm is based on a Monte Carlo procedure that simulates photon and electron transport and scores energy depositions within the patient. The necessary input information may be obtained from a registered set of CT and SPECT or PET images. The algorithm provides the absorbed dose rate for the radioactivity distribution provided by the SPECT or PET image. The algorithm was benchmarked by reproducing dosimetric quantities using the Medical Internal Radionuclide Dose (MIRD) Committees Standard Man phantom and was used to calculate absorbed dose distributions for representative case studies.

A stereotactic method for the three-dimensional registration of multi-modality biologic images in animals: NMR, PET, histology, and autoradiography.

The objective of this work was to develop and then validate a stereotactic fiduciary marker system for tumor xenografts in rodents which could be used to co-register magnetic resonance imaging (MRI), PET, tissue histology, autoradiography, and measurements from physiologic probes. A Teflon fiduciary template has been designed which allows the precise insertion of small hollow Teflon rods (0.71 mm diameter) into a tumor. These rods can be visualized by MRI and PET as well as by histology and autoradiography on tissue sections. The methodology has been applied and tested on a rigid phantom, on tissue phantom material, and finally on tumor bearing mice. Image registration has been performed between the MRI and PET images for the rigid Teflon phantom and among MRI, digitized microscopy images of tissue histology, and autoradiograms for both tissue phantom and tumor-bearing mice. A registration accuracy, expressed as the average Euclidean distance between the centers of three fiduciary markers among the registered image sets, of 0.2 +/- 0.06 mm was achieved between MRI and microPET image sets of a rigid Teflon phantom. The fiduciary template allows digitized tissue sections to be co-registered with three-dimensional MRI images with an average accuracy of 0.21 and 0.25 mm for the tissue phantoms and tumor xenografts, respectively. Between histology and autoradiograms, it was 0.19 and 0.21 mm for tissue phantoms and tumor xenografts, respectively. The fiduciary marker system provides a coordinate system with which to correlate information from multiple image types, on a voxel-by-voxel basis, with sub-millimeter accuracy--even among imaging modalities with widely disparate spatial resolution and in the absence of identifiable anatomic landmarks.

Beam characteristics of a new generation 50 MeV racetrack microtron

The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x-ray and electron beams. Both x-ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.

WE‐E‐224A‐02: The Role of External Beam in Brachytherapy

Combination of permanent low dose‐rate interstitial implantation (LDR‐BRT) and external beam radiotherapy(EBRT) has been used in the treatment of clinically localized prostate cancer. Patients treated with this regimen initially receive an I‐125 implant prescribed to 110 Gy followed, two months later, by 50.4 Gy in 28 fractions using intensity modulated external beam radiotherapy. While a high radiationdose is delivered to the prostate in this setting, the actual biologic dose equivalence compared to monotherapy is not commonly invoked. I shall describe methodology for obtaining the fused dosimetry of this combined treatment and assigning a dose equivalence which in turn can be used to develop desired normal tissue and target constraints for biologic‐based treatment planning. Furthermore, I shall argue that LDR‐EBRT treatments, when properly designed, may confer significant advantages in terms of: a) escalating the dose without normal tissue penalties, b) avoid the question of organ motion, and c) decrease significantly the size of the PTV.

TU‐EE‐A1‐04: Evaluation of Tumor Motion Effects On Dose Distribution for Hypofractionated Radiotherapy of Non‐Small‐Cell Lung Cancer

Purpose: Hypofractionated Radiotherapy (HFRT), 1–4(12–30 Gy) fractions of intensity modulated radiation therapy(IMRT)treatments, may be advantageous in treating early‐stage non‐small‐cell lungcancer(NSCLC). But motion of dynamic multileaf collimator (DMLC) leaves relative to respiration‐induced tumor motion could cause substantial differences between doses predicted from the planning scan and delivered doses. Such effects average out for conventionally fractionated IMRT (20–40 fractions, 1.8–2 Gy/fx) but must be evaluated for HFRT. Method and Materials We performed numerical simulations to investigate effects of respiratory motion on HFRT IMRT of 20 Gy × 3 fractions. Using the clinical treatment plans and DMLC leaf motion files for 9 NSCLC patients (11 tumors), the planned Clinical Target Volume (CTV) and Gross Tumor Volume (GTV) dose distributions were retrospectively compared with the distributions calculated for simulated periodic motion for amplitudes, periods and directions typical of normal respiration (0.2–1.26 cm, 3–8 sec, superior‐inferior and anterior‐posterior). Results: For the largest amplitude (1.26 cm, excursion ∼2.5 cm), the average ± standard deviation of the ratio of simulated to planned values of mean dose, minimum dose, D95 and V95 was 0.98±0.01, 0.88±0.09, 0.94±0.05 and 0.94±0.07 respectively for the CTV and 0.99±0.004, 0.99±0.03, 0.98±0.02 and 1.0±0.008 for the GTV. For 0.9 cm amplitude, corresponding ratios were 1.0±0.004, 0.97±0.07, 0.98±0.02, and 0.99±0.02 for CTV, and 1.0±0.004, 0.99±0.05, 0.99±0.01, and 1.0±0.01 for GTV. There was minimal dependence on motion period and initial phases. Graphical dose distributions showed respiration induced broadening of the beam penumbras in the motion direction. More severe effects were seen with intensity patterns that are more highly modulated than typical for clinical NSCLC HFRT fields. Conclusion: For our IMRTlungcancer HFRT technique, motion effects on target coverage are minimal for amplitudes below 1 cm but could be significant for larger amplitudes, more highly modulated fields and smaller field margins.

SU-FF-T-79: Accurate and Efficient Monte Carlo Dose Calculation for Electron Beams

Purpose: To develop a Monte Carlo dose calculation engine for electron beams that is feasible for routine clinical treatment planning.Method and Materials: The dose calculation engine consists of a description of the clinical beams and a dose calculation module. A 12‐component multi‐source model was used to characterize the phase‐space of clinical beams. There are 6 components each for electrons and photons, corresponding to the 3 scrapers, x‐jaws, y‐jaws, and the direct component respectively. In addition, we have developed a method to account for the presence of an arbitrary shaped cutout by modifying the last component of the standard beam model. For the dose calculation module, implementation of the Super‐Monte Carlo method accelerates the calculation by using electron and photon tracks pre‐calculated in water to avoid the computationally intensive sampling processes. These tracks are replayed in the patient computermodel as defined by CT. To account for inhomogeneities, the step size and scattering angle were adjusted according to the CT voxel values and material indexes. The dose calculation engine was verified by comparing with film measurements in several different geometries. Results: The results agreed with film measurements to within 2–5% percent both in homogeneous and heterogeneous phantoms. Our method is faster than the analog Monte Carlo calculation by a factor of 6 to 10 and is comparable in performance to a commercial system. The modified beam models for arbitrary cutouts can be derived in a few seconds. The disk storage for pre‐calculated tracks is about 5.5 GB and 125 MB for the standard beam models.Conclusion: The developed Monte Carlo calculation engine is accurate and efficient. The disk space and computational time required are well within clinical acceptability. It is a highly promising dose calculation tool for routine clinical applications. This work supported in part by NCI grant P01‐CA59017.

SU‐FF‐T‐421: Acceleration of Photon Monte Carlo Dose Calculation Using Multi‐Resolution Voxel Transport Geometry

Purpose: A multi‐resolution voxel geometry transport scheme was introduced to improve the efficiency of Monte Carlo dose calculation for photon beams. Method and Materials: Dual resolution voxel geometries were established with the coarse voxel size defined as a multiple of the finest voxel dimension. For each sub‐step of secondary photons, if the step size is greater than the nearest coarse voxel boundary, the particle is transported in coarse resolution voxel geometry. When the residual step size is less than the nearest coarse voxel boundary, the transport of the particle is switched back to the fine resolution voxel geometry. Electrons are always transported in fine resolution geometry. Timing was compared with the full Monte Carlo carried out on the fine voxel geometry, using fine voxel size of 4, 2, 1 mm and different coarse voxel size up to 2 cm. This multi‐resolution scheme was implemented in our in‐house photon Monte Carlo code (pMC2) and was assessed for accuracy using three heterogeneous phantom test cases. Results: For the 4‐mm, 2‐mm and 1‐mm voxel resolutions, the calculation time for the same particle histories can be decreased by 14%, 26%, and 42% respectively by using the dual resolution voxel transport geometry (for coarse voxel dimension of 2 cm). For the inhomogeneous test problems, the multi‐resolution transport scheme agrees with the analog Monte Carlo dose calculation within 1%, even when there is high density material such as iron involved. The inter‐resolution transport geometry swapping over head is negligible. Conclusion: This multi‐resolution transport scheme proposed in this study is more efficient than regular photon Monte Carlo dose calculation. The efficiency gain is more significant when the finer calculation resolution is required. This acceleration scheme doesnt affect the accuracy and can be applied in any Monte Carlo dose calculation system.

SU‐FF‐T‐352: Predicting the Parameters of a Prostate IMRT Objective Function Based On Dose Statistics Under Fixed Parameter Settings

Purpose: To simplify the trial‐and‐error process of adjusting objective function parameters (e.g. weights, dose limits) in prostate IMRT planning, we present a feasibility study showing that machine learning followed by a sensitivity‐driven greedy search can quickly and automatically determine parameters that lead to a plan meeting the clinical requirements. Method and Materials: The training database is composed of 39 plans treated effectively under a five‐field 8640cGy prostate IMRT protocol. For each plan, the output features include the clinical parameter values, and the input features include simple dose statistics resulting from optimizing the objective function starting from several fixed parameter values. Given the same parameters, these dose statistics vary between patients, representing different degrees of competition between the PTV and OARs. We predict a new plan based on the 3 plans in the training database that have the most similar input features. Starting from such a “pre‐plan”, a sensitivity‐based automatic parameter search is applied to improve the plans deficiencies. Results: Experiments on the 39‐patient dataset showed that a clinically acceptable prostate IMRT plan could be automatically determined within 15 iterations of optimization (using simplified dose calculations). Compared with the clinical ground truth, the mean absolute differences for PTV V95, PTV Dmin, rectwall V54 and rectwall Dmax are 1.1%, 2.4%, 2.6% and 0.9%, respectively. The adjusted plans met DVH constraints defining clinical acceptability and were comparable to manually‐determined plans. Conclusion: Prior knowledge gained through historical plans can facilitate rapid parameter selection for dose‐volume‐based IMRT objectives. The combination of machine learning and automatic sensitivity‐based parameter searching may also be applicable to other types of objective functions, and has the potential to ease the manual burden of IMRT planning in more complex sites. Supported by National Cancer Institute grant 5P01CA59017‐13 and the NSF Center for Subsurface Sensing and Imaging Systems, grant EEC‐9986821.

SU‐FF‐T‐15: A Kerma‐Dose Hybrid Scheme for Variance Reduction in Monte Carlo Dose Calculations of Photon Beams

Purpose: To improve the performance of Monte Carlo dose calculation for photon beams, a kerma‐dose hybrid dose calculation scheme is introduced in this work. Material and methods: Due to the high scoring efficiency of kerma, the statistical uncertainty of kerma can be reduced to a lower level than for dose with the same number of histories. However, kerma only equals dose in regions where charged particle equilibrium (CPE) exists. Our approach is to first determine the regions where CPE exists and then replace the dose by kerma to improve statistical accuracy in these voxels. To perform this hybrid kerma‐dose calculation, the kerma and the energy imbalance function are simultaneously recorded for each voxel at the time of dose calculation. The energy imbalance function (Q) is defined as Q =Σ i E i −Σ j ( edep j ) , where Ei is the initial energy of the i‐th charged particle starting its track in the voxel and edepj is the energy deposited by the j‐th charged particle passing through this voxel. In a voxel where CPE exists, Q should be close to zero. In the current work, dose is replaced by kerma in voxels where Q is less than 10% of kerma. Results: Calculations were done in a multi‐layer slab geometry and a CT voxel phantom. In both geometries, the results show that the energy imbalance function is sufficiently sensitive to distinguish regions where CPE exists. The replacement of dose by kerma significantly reduces the statistical uncertainty (by a factor of 3) and the isodose curves become much smoother in those regions. Conclusions: The proposed hybrid scheme can significantly reduce the statistical uncertainty in the dose calculation while accurately accounting for the inhomogeneity effect caused by loss of CPE.