G. Pittomvils

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 <or= 1 for a 2%/2 mm criterion, the Pinnacle(3) 7.4 f versus Gafchromic results 3.8% and Pinnacle versus MC less than 1%. For specific patients with lesions of 8 cc and 0.2 cc, Monte Carlo and Pinnacle simulations of the plans were performed and compared using DVH evaluation. DVHs corresponded within 2% dose and 2% volume.

Measurement Techniques, Modeling Strategies and Pitfalls to Avoid when Implementing a Mini MLC in a Non Dedicated Planning System*

Background and Purpose:Ghent University Hospital investigated the feasibility of the Pinnacle® system for planning intracranial stereotactic treatments. The aim was to perform precise dose computation using the collapsed cone engine for treatment delivery with the Moduleaf mini-MLC mounted on an Elekta accelerator.Material and Methods:The Moduleaf® was commissioned using dose rate corrected data recorded by a diamond detector and using data measured by cylindrical chambers each limited to restricted field sizes.Results:Automatic modeling resulted in clinical relevant dose errors up to 10%. Using manual modeling in Pinnacle®, for clinical applicable fields a 2%/2 mm agreement between modeled data and measurements was obtained.Conclusion:The overall accuracy of the collapsed cone algorithm is within tolerances for single fraction stereotactic treatments.Hintergrund und Ziel:Am Universitätsklinikum Gent wurde die Anwendung eines Pinnacle®-Planungssystems für die intrakranielle stereotaktische Bestrahlung untersucht. Das Ziel bestand darin, für die Bestrahlung mit einem Moduleaf-Mini-MLC, der an einem Elekta-Beschleuniger® befestigt ist, mit dem „Collapsed cone“-Rechenalgorithmus eine präzise Dosisberechnung zu erstellen.Material und Methoden:Hierzu wurden Messungen mit einem Diamantdetektor durchgeführt, die bezüglich der Dosisleistung korrigiert wurden, sowie Messungen mit einer zylindrischen Ionisationskammer für kleine Felder.Ergebnisse:Die automatische Modellierung führte zu Fehlern von bis zu 10%, was bereits klinisch relevant ist. Mit der manuellen Modellierung in Pinnacle war es möglich, zwischen modellierten Daten und Messungen eine Übereinstimmung von 2%/2 mm zu erreichen.Schlussfolgerung:Die Gesamtgenauigkeit des „Collapsed-cone“-Algorithmus liegt innerhalb der Toleranzgrenzen für stereotaktische Einzeldosisbehandlungen.

EP-1535: Electron Skin Irradiation: refinement of an abutting field technique

Purpose or Objective: The Calypso 4D Localization System consists in an electromagnetic detection of implanted Beacon transponders in order to continuously track their moves. The use of this system requires a specific couch top and the introduction in the treatment beam of an electromagnetic array. The purpose of this study is to quantify the dosimetric impact of the new material introduction in photon beams.