The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers.

The purpose of this study was the investigation of perturbation factors for microionization chambers in small field dosimetry and the influence of penumbra for different spot sizes. To this purpose, correlated sampling was implemented in the EGSnrc Monte Carlo (MC) user code cavity: CScavity. CScavity was first benchmarked against results in the literature for an NE2571 chamber. An efficiency increase of 17 was attained for the calculation of a realistic chamber perturbation factor in a water phantom. Calculations have been performed for microionization chambers of type PinPoint 31006 and PinPoint 31016 in full BEAMnrc linac simulations. Investigating the physical backgrounds of the differences for these small field settings, perturbation factors have been split up into (1) central electrode perturbation, (2) wall perturbation, (3) air-to-water perturbation (chamber volume air-to-water) and (4) water volume perturbation (water chamber volume to 1 mm(3) voxel). The influence of different spot sizes, position in penumbra, measuring depth and detector geometry on these perturbation factors has been investigated, in a 0.8 x 0.8 cm(2) field setting. p(cel) for the PP31006 steel electrode shows a variation of up to 1% in the lateral position, but only 0.4% for the PP31016 with an Al electrode. The air-to-water perturbation in the optimal scanning direction for both profiles and depth is most influenced by the radiation field, and only to a small extent the chamber geometry. The PP31016 geometry (shorter, larger radius) requires less total perturbation within the central axis of the field, but results in slightly larger variations off axis in the optimal scanning direction. Smaller spot sizes (0.6 mm FWHM) and sharper penumbras, compared to larger spot sizes (2 mm FWHM), result in larger perturbation starting in the penumbra. The longer geometries of the PP31006/14/15 exhibit in the non-optimal scanning direction large variations in total perturbation (p(tot) 1.201(4) (0.6 mm spot, 3 mm off axis, type A MC uncertainty) to 0.803(4) (5 mm off axis)) mainly due to volume perturbation. Therefore in IMRT settings, when the detector is not always in the optimal scanning direction, the PP31016 geometry requires less extreme perturbation (max p(tot) 1.130(3)) and shows less variation. However, these results suggest that small variations in positioning, spot size or MLC result in large differences in perturbation factors. Therefore even these 0.016 cm(3) ionization chambers are limited in their use for a field setting of 0.8 x 0.8 cm(2), as used in this investigation.

Monte Carlo modeling of the ModuLeaf miniature MLC for small field dosimetry and quality assurance of the clinical treatment planning system.

The purpose of this investigation was the verification of both the measured data and quality of the implementation of the add-on ModuLeaf miniature multileaf collimator (ML mMLC) into the clinical treatment planning system for conformal stereotactic radiosurgery treatment. To this end the treatment head with ML mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The 6 MV photon beams used in the setup were first benchmarked with a set of measurements. A total ML mMLC transmission of 1.13% of the 10 x 10 cm2 open field dose was measured and reproduced with the BEAMnrc/DOSXYZnrc code. Correspondence between calculated and measured output factors (OFs) was within 2%. Correspondence between MC and measured profiles was within 2% dose and 2 mm distance, only for the smallest 0.5 x 0.5 cm2 field the results were within 3% dose. In the next step, the MC model was compared with Gafchromic film measurements and Pinnacle(3) 7.4 f (convolution superposition algorithm) calculated dose distributions, using a gamma evaluation comparison, for a multi-beam patient setup delivered to a Lucytrade mark phantom. The gamma evaluation of the MC versus Gafchromic film resulted in 3.4% of points not fulfilling gamma

The influence of cooling rate on the accuracy of normoxic polymer gel dosimeters.

Polymer gel dosimeters offer a wide range of applications in the three-dimensional verification of complex radiation dose distributions such as in intensity-modulated radiotherapy (IMRT). With the release of polymer gel dosimeters that can be fabricated in normal atmospheric ('normoxic') conditions, the gel manufacturing process has been significantly simplified. Gel dosimeters are calibrated by use of a series of calibration vials irradiated with known doses or by use of a calibration phantom with a known dose distribution. The overall accuracy of the polymer gel dosimeters is determined by different dosimetric properties. In this study, we show the influence of the temperature history during storage of the gel dosimeter on the dose response curve for two gel dosimeters using the monomers acrylamide/N,N'-methylene-bis-acrylamide (nPAG) and methacrylic acid (nMAG) respectively and bis[tetrakis(hydroxymethyl)phosphonium]sulphate (THP) as antioxidant in both gel dosimeters. This study reveals that differences in temperature history after fabrication of normoxic polymer gel dosimeters may compromise the dosimetric accuracy. It was found that the acrylamide based gel dosimeter (nPAG) is less dependent on the post-manufacture temperature history than the methacrylic acid based gel dosimeter (nMAG). The importance of an equal temperature history for the gel dosimeter and calibration vials is emphasized by this study. A reproducibility study has also been performed on the nPAG gel dosimeter when additional efforts are made to control the temperature changes upon cooling.

Decoupling initial electron beam parameters for Monte Carlo photon beam modelling by removing beam-modifying filters from the beam path.

A new method is presented to decouple the parameters of the incident e(-) beam hitting the target of the linear accelerator, which consists essentially in optimizing the agreement between measurements and calculations when the difference filter, which is an additional filter inserted in the linac head to obtain uniform lateral dose-profile curves for the high energy photon beam, and flattening filter are removed from the beam path. This leads to lateral dose-profile curves, which depend only on the mean energy of the incident electron beam, since the effect of the radial intensity distribution of the incident e- beam is negligible when both filters are absent. The location of the primary collimator and the thickness and density of the target are not considered as adjustable parameters, since a satisfactory working Monte Carlo model is obtained for the low energy photon beam (6 MV) of the linac using the same target and primary collimator. This method was applied to conclude that the mean energy of the incident e- beam for the high energy photon beam (18 MV) of our Elekta SLi Plus linac is equal to 14.9 MeV. After optimizing the mean energy, the modelling of the filters, in accordance with the information provided by the manufacturer, can be verified by positioning only one filter in the linac head while the other is removed. It is also demonstrated that the parameter setting for Bremsstrahlung angular sampling in BEAMnrc ('Simple' using the leading term of the Koch and Motz equation or 'KM' using the full equation) leads to different dose-profile curves for the same incident electron energy for the studied 18 MV beam. It is therefore important to perform the calculations in 'KM' mode. Note that both filters are not physically removed from the linac head. All filters remain present in the linac head and are only rotated out of the beam. This makes the described method applicable for practical usage since no recommissioning process is required.

Ultrasonic velocities of concentric laminated uric acid stones.

In concentric laminated uric acid samples two different sound velocities are observed with very different values, the one in the organic matrix varies from 670 to 1170 m s(-1), the other in the crystalline layers between 3200 and 5300 m s(-1). This large difference illustrates the importance of the internal stone structure on the fracture behaviour of urinary stones and reveals the weakness of the actual disintegration models, attributing the destructive effect of shock waves on the differences in acoustical impedance at the water/stone interface and not on the differences in acoustical impedance at the matrix/crystalline interfaces inside the urinary calculus.

Neoadjuvant hormonal therapy before radical prostatectomy decreases the number of positive surgical margins in stage T2 prostate cancer: interim results of a prospective randomized trial. The Belgian Uro-Oncological Study Group.

PURPOSE: We investigated the effect of neoadjuvant treatment before radical prostatectomy for clinically localized prostate cancer. MATERIALS AND METHODS: A total of 130 patients with stages T2b and T3 prostate cancer was randomized in a multicenter study: 62 underwent immediate radical prostatectomy and 65 received 560 mg. estramustine phosphate daily for 6 weeks preoperatively. RESULTS: For clinical stage T2b tumors the neoadjuvant treatment resulted in a significant decrease in positive surgical margins compared to the nonpretreated group. This difference was not found for clinical stage T3 tumors. The impact on progression and survival still must be analyzed. CONCLUSIONS: Neoadjuvant treatment can be beneficial for clinical stage T2 prostate cancer. Optimal treatment for stage T3 tumors remains controversial.

Importance of the implosion of ESWL-induced cavitation bubbles.

The damage induced by an extracorporeal shock wave lithotripter is observed with a fiber optic stress sensing technique. When a stone is placed in the focus, besides the expected stress induced by the incoming shock wave emitted by the ESWL apparatus, a second delayed stress is observed some hundreds of microseconds later. The second stress is induced by a shock wave generated at the collapse of a cavitation bubble. Partial reflection of the incoming shock wave at the stone boundary is at the origin of the large cavitation bubble formation. Sensing fiber fracture results always from the second shock wave due to the collapse of the cavitation bubble. Thus the largest stress is generated at the collapse. When no target is placed in the focus of the lithotripter, no large cavitation bubble is formed and no delayed shock wave is observed. Our results demonstrate unambiguously the decisive role of cavitation in ESWL procedures.

Macroscopic ESWL-induced cavitation: in vitro studies.

The rarefaction shock wave results in a liquid failure at the target/fluid interface. In the wake of the reflected ESWL-induced shock wave, a macroscopic cavity is generated in filtered water. The cavity implosion induces a large shock wave, divulging the bubble existence and lifetime. The existence of this shock wave is revealed by the diffraction of a He-Ne laser beam. The induced cavitation bubbles are registered by a camera, illuminated by an externally triggered stroboscope (exposure time 10 microseconds). The radius of the large cavity bubble, generated at the stone surface, is time dependent as theoretically predicted.

The influence of internal stone structure upon the fracture behaviour of urinary calculi.

In vitro extracoporeal shock wave lithotripsy (ESWL) on different types of urinary calculi, in combination with microfocus x-ray and microphotography, illustrates the importance of the internal stone structure. Calculi with a rough surface layered structure (calcium oxalate monohydrate) and untextured calculi (cystine) are characterized by a low stone fragility, whereas coarse-grain calculi (calcium oxalate dihydrate, struvite), and calculi with a smooth surface layered structure (uric acid), are very fragile. Shell-like fragmentation in layered calculi, with smooth surface of the crystalline laminations, suggests that the stone matrix influenced the propagation of the shock wave energy inside the stone.

In vitro analysis of urinary calculi: type differentiation using computed tomography and bone densitometry.

Thirty-six urinary stones were studied by computed tomography and bone densitometry in order to find a correlation between the chemical composition of the stones and the densitometric data. In vitro, the more common cases of lithiasis can be divided into 3 main groups: uric acid, cystine and the calcium salts, e.g. calcium oxalate monohydrate, calcium oxalate dihydrate and calcium phosphate. Struvite stones fall between the second and third groups. Computed tomography and bone densitometry can differentiate between the groups, provided that the value of the "stone mineral or calcium content" is correlated with the true calculated volume of the stone (resulting in the specific stone density) instead of the projection area (resulting in the area density). This is only possible in vitro. The in vivo application of these technologies in the treatment of stones of unknown composition could provide important information, but major restrictive factors pose difficulties. Further clinical studies are necessary, especially to define the role of bone densitometry.

Stone geometry and structure dependence on extracorporeal shock wave lithotripsy.

The in vitro fragmentation effects of electromagnetic-induced shock wave lithotripsy (Siemens Lithostar) were studied on parallel laminated schists in relation to the incoming shock wave energy and the incoming angle of the incident shock wave. Well above the threshold energy, the total amount of acoustic energy required for fragmentation remained unchanged regardless of the power setting of the high-tension supply. The amount of energy transmitted to the stone was limited by a total reflection angle, restricting the available transmission surface. These observations determine the effect of shock waves on calculi with an irregular shape such as staghorn stones. Lithotripsy-induced cavitation has been recognized, but no immediate influence on disintegration has become apparent until now.

High pressure versus low pressure electromagnetic extracorporeal lithotripsy.

A low pressure and an upgraded high pressure electromagnetic lithotriptor were compared for the treatment of calculi within the ureter. The upgrading allows for increased energy density onto the stone surface. We targeted 139 ureteral calculi (average diameter 8.1 mm.) with the low pressure system. The mean number of shock waves was 7,308, 9,226 and 5,304 for calculi in the upper, mid and lower ureter, respectively, and the energy density corresponded to 0.222, mJ./mm.2, 0.224 mJ./mm.2 and 0.223 mJ./mm.2, respectively. Immediate fragmentation was achieved in 39 of 51 upper (77%), 21 of 42 mid (50%) and 22 of 46 lower (49%) ureteral calculi. The upgraded shock wave tube was used in 52 calculi (average diameter 6.1 mm.). The mean number of shock waves was 6,250, 14,150 and 3,400 for calculi in the upper, mid and lower ureter, respectively, and the energy density corresponded to 0.32 mJ./mm.2, 0.33 mJ./mm.2 and 0.30 mJ./mm.2, respectively. Immediate fragmentation was achieved in 11 of 16 upper (69%), 4 of 6 mid (67%) and 7 of 30 lower (23%) ureteral calculi. The comminution rate did not significantly increase, which implies that the focal energy density is not the only decisive parameter in fragmentation by extracorporeal shock wave lithotripsy.

Effect of alkalinization on calcium oxalate monohydrate calculi during extracorporeal shock wave lithotripsy: in vivo experiments.

Previous in vitro experiments demonstrated the reduced microhardness of calcium oxalate monohydrate (COM) calculi, relative to dry values, when saturated with an alkaline solution (pH = 9.5). Nineteen patients with a COM calculus in the distal ureter which had been resistant to prior extracorporeal shock wave lithotripsy in situ, were treated when the stone was surrounded by alkaline urine. The urine of 14 patients was alkalinized orally by administration of acetazolamine and citrate solution; in 5 other patients direct percutaneous irrigation of sodium bicarbonate via a nephrostomy tube was carried out. The urinary pH just before lithotripsy was greater than or equal to 9 in 17/19 patients. 4,000 shock waves, averaging 18.1 kV generated by the Siemens Lithostar, were delivered onto the calculus. No significant increase of comminution rate was apparent at radiographic control immediately after the treatment and only in half of the cases was evacuation obtained within 3 months.

High energy pulsed dye laser lithotripsy: management of ureteral calcium oxalate monohydrate calculi.

High energy pulsed dye laser lithotripsy (Candela MDL-2000), with energy output upgraded to a maximum of 140 mJ. at the laser fiber tip using the 320 mu core fiber, was compared to the initially commercialized device, with the energy output fixed at 60 mJ. using the 200 mu core fiber (Candela MDL-1). A total of 31 treatments in 28 patients was performed with the Candela MDL-1 device. Complete disintegration or at least fragmentation to spontaneously passable fragments occurred in 18 of 31 cases (58%). Only in 11 of the 24 calcium oxalate monohydrate calculi (46%) was fragmentation achieved. Another 73 laser lithotripsies in 72 patients were performed with the Candela MDL-2000 device. Complete disintegration or at least fragmentation to spontaneously passable fragments was achieved in 67 of 73 treatments (92%). Calcium oxalate monohydrate calculi were successfully treated in 41 of 45 procedures (91%). There was no response to the laser treatment in the only cystine calculus.

Combined extracorporeal shock wave lithotripsy and percutaneous alkalinization in uric acid calculi.

The combination of percutaneous alkaline irrigation and lithotripsy was performed in 5 cases of uric acid calculi obstructing the ureter. Shock wave lithotripsy accelerates the dissolution of uric acid stones.

Anesthetic requirements during electromagnetic extracorporeal shock wave lithotripsy.

An important aspect of modern extracorporeal shock wave lithotripsy is the ability to perform the procedure without anesthesia. Between June 1987 and April 1990, a total of 7,500 treatments were performed in our Lithotripsy Unit, using the Lithostar (Siemens AG, Erlangen, FRG): moreover 80% of the treatments were carried out as an anesthesia-free outpatient service. All treatments were reviewed for anesthetic requirements. Epidural anesthesia was performed in 74/7,500 (0.98%) treatments: during the initial period, the first 70 procedures were systematically performed under epidural anesthesia, and in 4 later cases of simultaneous endoscopic stone manipulation. Local skin infiltration was applied at the coupling site in 658/7,500 (8.7%) treatments. With increasing experience and technological improvement, 6,229/7,500 (83%) procedures were performed later on with only mild sedation: 1 mg of lorazepam orally 30 min before the treatment. Intravenous sedation was required in 510/7,500 (6.8%) cases because of painful local irritation, especially when treating calculi close to sites where shock waves can progress along the ribs. General anesthesia was required in all 28 children (0.4%).