Bernard Lachance

Reasoning about geological space

Topological relationships between geological objects are of great interest for mining and petroleum exploration. Indeed, adjacency, inclusion and intersection are common relationships between geological objects such as faults, geological units, fractures, mineralized zones and reservoirs. However, in the context of 3D modeling, actual geometric data models used to store those objects are not designed to manage explicit topological relationships. For example, with Gocad? software, topological analyses are possible but they require a series of successive manipulations and are time consuming. This paper presents the development of a 3D topological query prototype, TQuery, compatible with Gocad? modeling platform. It allows the user to export Gocad? objects to a data storage model that regularizes the topological relationships between objects. The development of TQuery was oriented towards the use of volumetric objects that are composed of tetrahedrons. Exported data are then retrieved and used for 3D topological and spatial queries. One of the advantages of TQuery is that different types of objects can be queried at the same time without restricting the operations to voxel regions. TQuery allows the user to analyze data more quickly and efficiently and does not require a 3D modeling specialist to use it, which is particularly attractive in the context of a decision-making aid. The prototype was tested on a 3D GeoModel of a continental red-bed copper deposit in the Silurian Robitaille Formation (Transfiguration property, Quebec, Canada).

Automatic generation of anatomy-based MLC fields in aperture-based IMRT

We have developed an algorithm to automatically generate anatomy-based MLC fields. For each beam, a first field is adjusted to the projection of the target in a beams eye view, allowing subsequent fields to be derived from this conformal field by removing the overlapping surface of each organ at risk, respectively. The projections are based on a surface sampling of the anatomical structures. On top of the MLC mechanical constraints, verification constraints are imposed on the MLC segments, in order to get reliable dosimetry using a commercial dose calculation engine. Thus, in each direction, the apertures cross-section must be greater than a specified threshold, in our case 2 cm. Furthermore, junctions are not tolerated in order to avoid underdosage, for instance from the tongue-and-groove effect. The use of such MLC fields simplifies the verification process. The performance of the algorithm is illustrated for head and neck, thorax and prostate cases. Only a fraction of a second of CPU time is required to perform the segmentation for each beam.

A new penumbra generator for electron fields matching.

Abutment of two or more electron fields to irradiate extended areas may lead to significant dose inhomogeneities in the junction region. This paper describes the geometric and dosimetric characteristics of a device developed to modify the penumbra of an electron beam and thereby improve the dose uniformity in the overlap region when fields are abutted. The device is a Lipowitz metal block placed on top of the electron applicators insertion plate and positioned to stop part of the electron beam on the side of field abutment. The air-scattered electrons beyond the block increase the penumbra width from about 1.4 to 2.7-3.4 cm with an SSD of 100 cm. The modified penumbra is broad and almost linear at all depths for the 9 and 12 MeV electron beams used in this study. Film dosimetry was used to obtain beam profiles and isodose distributions of single modified beams and matched fields of 9 and 12 MeV as well as matched fields of both energies. Computer simulation was used to optimize the skin gap to be used and to quantify the dose uniformity as a function of the field separation for both modified and nonmodified beams. Results are presented for various field configurations. Without the penumbra generator, lateral setup errors of 2-3 mm may introduce dose variations of 20% or more in the junction region. Similar setup errors cause less than 5% dose variations when the penumbra generator is used to match the fields. The potential of the technique for the irradiation of curved surfaces is presented. A possible method for implementing the modified penumbra into a conventional treatment planning system is evaluated.

Dosimetric evaluation of three adaptive strategies for prostate cancer treatment including pelvic lymph nodes irradiation

PURPOSE The movements of the prostate relative to the pelvic lymph nodes during intensity-modulated radiation therapy treatment can limit margin reduction and affect the protection of the organs at risk (OAR). In this study, the authors performed an analysis of three adaptive treatment strategies that combine information from both bony and gold marker registrations. The robustness of those treatments against the interfraction prostate movements was evaluated. METHODS A retrospective study was conducted on five prostate cancer patients with 7-13 daily cone-beam CTs (CBCTs). The clinical target volumes (CTVs) consisting of pelvic lymph nodes, prostate, and seminal vesicles as well as the OARs were delineated on each CBCT and the initial CT. Three adaptive strategies were analyzed. Two of these methods relied on a two-step patient positioning at each fraction. First step: a bony registration was used to deliver the nodal CTV prescription. Second step: a gold marker registration was then used either to (1) complete the dose delivered to the prostate (complement); (2) or give almost the entire prescription to the prostate with a weak dose gradient between the targets to compensate for possible motions (gradient). The third method (COR) used a pool of precalculated plans based on images acquired at previous treatment fractions. At each new fraction, a plan is selected from that pool based on the daily position of prostate center-of-mass. The dosimetric comparison was conducted and results are presented with and without the systematic shift in the prostate position on the CT planning. The adaptive strategies were compared to the current clinical standard where all fractions are treated with the initial nonadaptive plan. RESULTS The minimum daily prostate D95% is improved by 2%, 9%, and 6% for the complement, the gradient, and the COR approaches, respectively, compared to the nonadaptive method. The average nodal CTV D95% remains constant across the strategies, except for the gradient approach where a reduction of 7% is observed. However, a correction of the systematic shift reduced the problem, and the adaptive strategies remain robust against the prostate movement across the fraction. The bladder V55Gy is reduced by 35% on average for the adaptive strategies. CONCLUSIONS Because they offer increased CTV coverage and OAR sparing, adaptive methods may be suitable candidates for simple and efficient adaptive treatment strategies for prostate cancer. Margin reduction and systematic error correction in the prostate position improve the protection of the OAR and the dose coverage. A cumulative dose to simulate a complete treatment would show real effects and allow a better comparison between each method.

Evaluation of an electron Monte Carlo dose calculation algorithm for treatment planning

The purpose of this study is to evaluate the accuracy of the electron Monte Carlo (eMC) dose calculation algorithm included in a commercial treatment planning system and compare its performance against an electron pencil beam algorithm. Several tests were performed to explore the systems behavior in simple geometries and in configurations encountered in clinical practice. The first series of tests were executed in a homogeneous water phantom, where experimental measurements and eMC‐calculated dose distributions were compared for various combinations of energy and applicator. More specifically, we compared beam profiles and depth‐dose curves at different source‐to‐surface distances (SSDs) and gantry angles, by using dose difference and distance to agreement. Also, we compared output factors, we studied the effects of algorithm input parameters, which are the random number generator seed, as well as the calculation grid size, and we performed a calculation time evaluation. Three different inhomogeneous solid phantoms were built, using high‐ and low‐density materials inserts, to clinically simulate relevant heterogeneity conditions: a small air cylinder within a homogeneous phantom, a lung phantom, and a chest wall phantom. We also used an anthropomorphic phantom to perform comparison of eMC calculations to measurements. Finally, we proceeded with an evaluation of the eMC algorithm on a clinical case of nose cancer. In all mentioned cases, measurements, carried out by means of XV‐2 films, radiographic films or EBT2 Gafchromic films. were used to compare eMC calculations with dose distributions obtained from an electron pencil beam algorithm. eMC calculations in the water phantom were accurate. Discrepancies for depth‐dose curves and beam profiles were under 2.5% and 2 mm. Dose calculations with eMC for the small air cylinder and the lung phantom agreed within 2% and 4%, respectively. eMC calculations for the chest wall phantom and the anthropomorphic phantom also showed a positive agreement with the measurements. The retrospective dosimetric comparison of a clinical case, which presented scatter perturbations by air cavities, showed a difference in dose of up to 20% between pencil beam and eMC algorithms. When comparing to the pencil beam algorithm, eMC calculations are definitely more accurate at predicting large dose perturbations due to inhomogeneities. PACS numbers: 87.55.de, 87.55.kd

SU-E-T-593: Comparison of Three IMRT Adaptive Methods: The Case of Prostate Cancer

PURPOSE Independant movements of prostate and pelvic lymph nodes during IMRT can limit margin reduction and affect the protection of organs-at-risk (OAR). In this study we perform an analysis of three adaptive treatments that combine information from both bony and gold marker registrations. The efficiency of those treatments against interfraction prostate movements was evaluated. METHODS A retrospective study was conducted on four prostate cancer patients with 5 to 10 daily CBCTs. Clinical target volumes (CTVs) consisting of pelvic lymph nodes, prostate and seminal vesicles (SV) and OAR were delineated on each CBCT and on the initial CT. Three adaptive methods were analyzed. Two methods relied on a double patient positioning at each fraction. For these, nodal CTVs prescription was delivered on bony registration. Gold markers match was then used either to: (1) complete the dose delivered to prostate and SV (Complement); (2) give almost the entire prescription to prostate and SV with slow gradient between targets to compensate for motions (Controlled Gradient). The third method (COR) used a pool of pre-calculated plans from anterior fractions where the plan with the prostate center of mass closest to the daily anatomy was chosen. Adaptive techniques were compared to the standard non-corrected CT plan. RESULTS With adaptive techniques, prostate and SV V(100%) is improved by 4% compared to non-corrective method. This improvement does not Result in dose increase to OAR. EUD to bladder is reduced with an average reduction of 15% for COR. CONCLUSION Because of their increase in CTV coverage and OAR sparing, COR and Complement methods may be good candidates for simple and effective adaptive treatment strategies for prostate cancer. Further improvements may be obtained by tuning CTV margins. A cumulative dose to simulate a complete treatment will show real effects and allow better comparison between each method. Ministere de la Sante et des Services Sociaux.

SU-GG-T-417: Evaluation of a Commercial Monte Carlo Dose Calculation Algorithm for Electron Beam Treatment Planning

Purpose: To evaluate the accuracy of the electron Monte Carlo (eMC) dose calculation algorithm included in the Eclipse treatment planning system. Method and Materials: Multiple tests were planned to explore the system behaviour in configurations encountered in clinical practices. The first series of test were performed in a homogeneous water phantom. Measured and eMC calculated dose distributions were compared for different combinations of energy/applicator/depth. Three different inhomogeneous solid phantoms simulating high and low density materials were constructed to explore various heterogeneity conditions; a small air cylinder, a lung phantom and a chest wall phantom. The test under study is the lung inhomogeneity which consists of cork slabs. In all cases, comparisons are made between measurements, carried out by means of XV‐2 films, and eMC and Pinnacle computations. Finally, the evaluation of the eMC algorithm for a clinical case, a nose cancer, was conducted. Results: The eMC validation in the water phantom and in the lung phantom is shown to be accurate. The maximum observed discrepancy between measurements and eMC is 2.5%. For the lung case, Pinnacle does not correctly model the electron scattering. The retrospective study of the clinical case, which presents scatter perturbations by air cavities, shows planar dose difference up to 20% between Pinnacle and eMC. Conclusion: eMC algorithm showed good agreements with measurements in simple homogeneous and heterogeneous phantoms. Comparatively to the electron pencil beam algorithms of Pinnacle, the eMC calculations more precisely predict large dose perturbations due to inhomonegenities.

Poster — Thur Eve — 12: Evaluation of an Electron Monte Carlo Dose Calculation Algorithm for Treatment Planning

Purpose: To evaluate the accuracy of the electron Monte Carlo (eMC) dose calculation algorithm included in the Eclipse treatment planning system. Method and Materials: Multiple tests were planned to explore the system behaviour in configurations encountered in clinical practices. The first series of test were performed in a homogeneous water phantom. Measured and eMC calculated dose distributions were compared for different combinations of energy/applicator/depth. Three different inhomogeneous solid phantoms simulating high and low density materials were constructed to explore various heterogeneity conditions: a small air cylinder, a lung phantom and a chest wall phantom. In this work, the lung phantom, which consists of cork slabs, is described in details. In all cases, comparisons are made between measurements, carried out by means of XV‐2 films, and eMC and Pinnacle computations. Finally, the evaluation of the eMC algorithm for clinical case, a nose cancer, was conducted. Results: The eMC validation in the water phantom and in the lung phantom is shown to be accurate. The maximum observed discrepancy between measurements and eMC is only 2.5%. For the lung case, Pinnacle does not correctly model the electron scattering. The retrospective study of the clinical case, which presents scatter perturbations by air cavities, shows planar dose difference up to 20% between Pinnacle and eMC. Conclusion: eMC algorithm showed good agreements with measurements in simple homogeneous and heterogeneous phantoms. Comparatively to the electron pencil beam algorithms of Pinnacle, the eMC calculations more precisely predict large dose perturbations due to inhomogeneities.