J. Pouliot

Seed misplacement and stabilizing needles in transperineal permanent prostate implants

BACKGROUND AND PURPOSE Seed misplacement occurring in transperineal permanent implants contributes to the degradation in dose coverage. It has been suggested that needles could be used to immobilize the prostate and help reduce misplacement. This study investigates the effects of parallel stabilizing needles on seed misplacement. MATERIALS AND METHODS A group of ten patients implanted with stabilizing needles was compared with a group of 20 patients implanted without stabilization. Measurements were performed on the displacement of individual seeds and needles. The needle measurements are: insertion angle, the ratio of post-implant over pre-implant lengths and the clustering tendency, a measure of relative misplacement among the seeds of the same needle. RESULTS No difference was observed in seed misplacement. No difference was observed in needle insertion angle, a measure which was expected to improve with the use of stabilizing needles. CONCLUSION None of the expected effects from the use of parallel stabilizing needles have been observed. This method of prostate contention appears to be without benefits. Seed misplacement is most pronounced along the insertion axis and is caused by friction between prostatic tissues and implantation needles. Reducing friction could be a promising alternative to prostate contention in trying to reduce misplacement.

The mass dependence of CsI(Tl) scintillation response to heavy ions

Abstract The response of CsI(Tl) scintillators to heavy ions is investigated as a function of E , A , and Z . In addition to the expected dependence of light output on Z and E , we observe a significant dependence on mass number. A simple parameterization of the quenching in terms of a few physical variables permits characterization of the light output for a variety of nuclear species with a single quenching constant.

Monte Carlo simulations of prostate implants to improve dosimetry and compare planning methods

The objective of this study is to use Monte Carlo simulations to assess the sensitivity of implant planning methods to seed misplacement. A model of seed misplacement is first developed. It is based upon data gathered after a study on source migration performed on 30 patients treated with I-125 transperineal implants. It consists of applying elementary transformations to every needle in a loading plan to produce a distorted implant mimicking the effect of migration. After being validated, the model has been used to tune the inverse planning system in use at our institution. The new planning system is now used clinically and actual results are compared with those predicted by simulations. Simulations were also used to compare our planning method with others. The new planning system increased the average postimplant dose-volume histogram DVH(160) from 82% to 93%, which is the value predicted by the simulations. This improvement is due to an increased dose margin providing coverage even in the presence of migration. At the same time, the dose to the urethra remained at 267 Gy because of a special protection feature included in the planning system. Some other implant planning methods are not as robust [average DVH(160) ranging from 76% to 85%] and deliver a higher dose to the urethra (close to 400 Gy). To conclude, a simple model of source migration can provide realistic feedback about sensitivity to migration of planning methods. It allowed a significant clinical improvement at our institution. The improved inverse planning system provided better coverage with fewer seeds (but equal total activity) than a manual method. Hence, a properly tuned inverse planning system has the potential to deliver the less sensitive plans. The model also helped demonstrate that planning methods are not equally robust to migration and that they should not be evaluated solely by the plans they produce, but also by their clinical (or simulated) results.

A variable speed translating couch technique for total body irradiation

We have developed a variable speed translating patient couch system for the delivery of total body irradiation (TBI). For a whole body Rando-type phantom, dose variation at mid-plane relative to the prescription point (navel) can be as high as 15% (neck or legs) with a constant velocity. By taking into account variations in body thickness, the intensity modulation radiation therapy, resulting from variable velocities, effectively delivers a uniform dose distribution at mid plane. The couch control user interface, technical aspects and dose planning optimization procedure for determining velocity distribution are described.

Energy-light relation for CsI(T1) scintillators in heavy ion experiments at intermediate energies

Abstract In this paper an original energy relation for light scintillation in a CsI(T1) detector is established. It is dependent on the charge and mass of the particle (fragment) and is a direct light-to-energy function suitable for the calibration of CsI(T1) detectors used in heavy-ion studies at intermediate energies. Resulting calibration with this function and energy spectra for light ions, obtained from a heavy ion experiment, are presented.

Observation study of electronic portal images for off-line verification

PURPOSE The goals of this study were to evaluate the use of electronic portal imaging device (EPID) paper images as off-line verification tools and to assess the feasibility of replacing portal films by EPID printed images. MATERIALS AND METHODS Electronic portal images were acquired using a video-based imaging system. After contrast enhancement, these images were printed and compared to portal films when prescribed, and judged about their usefulness for off-line verification. A total of 2025 images were acquired from 322 fields on 137 patients. The images were shown to eight radiation oncologists and two senior residents in radiation oncology, each one of them judging fields relevant to his (her) daily practice. The questions asked were related to the choice of important anatomical structures and the visibility of such structures, the usefulness of the printed images, the comparison with portal films and the possible replacement of such films by paper images. RESULTS Answers to the different questions were treated as quantitative scores. For the visibility question, means and standard deviations were calculated for each individual structure, then a global score was obtained for a given treatment site. Means and standard deviations were also computed for the comparison question. Proportions and confidence intervals were used for the other questions. The results show that EPID paper images are useful for some treatment sites such as breast, thorax, prostate, abdomen, pelvis (other than rectum) and axilla. The image quality remains insufficient for some other sites such as head and neck and spine. CONCLUSION Although global anatomical landmarks scores are good, the usefulness score is not always as high because some essential anatomical structures scores must be taken into account. There is also a strong habit factor related to acceptance of EPID printed images as verification tools. As long as they see more and more images, radiation oncologists can more easily visualize anatomical structures and are less stringent when evaluating the efficiency of EPID paper images as off-line verification tools.

Dissipative binary mechanisms in 24Mg + 12C collisions at 25A and 35A MeV☆

Abstract A study of the most violent collisions in the 24Mg+12C reactions at 25A and 35A MeV has been carried out. Experimental data, for those events in which the total charge of the system has been detected, are compared to simulations based on statistical fragmentation codes. For violent events, a binary mechanism appears to be competing successfully with compound nucleus formation.

Breakup of highly excited 35Cl projectiles on a gold target at 30 A MeV: An exclusive analysis

Abstract Projectile excitation and breakup have been studied in the reaction 35 Cl+ 197 Au at 30 A MeV. Differential cross sections and energy distributions were measured. The absolute cross sections of 98 different exit channels made of as many as 7 coincident charged fragments have been determined. Such excludive measurements allow event-by-event reconstruction of the excited primary nuclei. By selecting specific exit channels, we studied the decay of hot nuclei over a wide range of excitation energy. The behaviour of the excited nuclei is consistent with a statistical description. The fragmentation mechanism was also investigated. Although the lowest excited channels are well reproduced by a standard sequential-binary-decay model, evidence is observed for an evolution with excitation energy of the time scale for particle emission. Model calculations have been performed which show that the lifetime of the excited chlorine projectile should decrease by a factor of 4 when the excitation energy increases from 2.3 A to 4.3 A MeV.

Statistical and sequential breakup of 24Mg in peripheral reactions at intermediate energies

Abstract The fragmentation of the projectile 24 Mg, excited in peripheral collisions on 197 Au and 12 C targets, has been investigated at 25A MeV and 35A MeV with a large scintillator detector array. Projectile breakup events were selected in the off-line analysis, by requiring that the total detected charge in one event be equal to that of the projectile. The projectile-like nucleus (PLN) excitation energies have been reconstructed and compared to results at 50A MeV and 70A MeV. The branching ratios of 2-, 4-, 5-, and 6-fold decays show a linear dependence, characteristic of statistical decay. Global variable analysis in terms of relative angles was applied to the six-alpha exit channel and the corresponding data have been compared to theoretical calculations based on simultaneous and sequential binary decay. The latter have been divided into sequential fission and sequential evaporation decays. The data has been found to be consistent with a sequential evaporation decay mechanism for a wide range of the intermediate-energy domain.

Statistical signatures of the quasi-projectile breakup at 70A MeV

Abstract Exit channel cross sections in quasi-elastic and transfer reactions have been measured in the 24 Mg+ 197 Au reaction at 70 A MeV. The Q -value dependence of the cross section, and the linear relation between the logarithm of P n / P 2 an d E − 1 2 , show that even at 70 A MeV, deexcitation is statistical. The absence of entrance channel effects indicates the thermalization of the quasi-projectile formed via transfer reactions.