Roberto Mini

A multiple source model for 6 MV photon beam dose calculations using Monte Carlo

A multiple source model (MSM) for the 6 MV beam of a Varian Clinac 2300 C/D was developed by simulating radiation transport through the accelerator head for a set of square fields using the GEANT Monte Carlo (MC) code. The corresponding phase space (PS) data enabled the characterization of 12 sources representing the main components of the beam defining system. By parametrizing the source characteristics and by evaluating the dependence of the parameters on field size, it was possible to extend the validity of the model to arbitrary rectangular fields which include the central 3 x 3 cm2 field without additional precalculated PS data. Finally, a sampling procedure was developed in order to reproduce the PS data. To validate the MSM, the fluence, energy fluence and mean energy distributions determined from the original and the reproduced PS data were compared and showed very good agreement. In addition, the MC calculated primary energy spectrum was verified by an energy spectrum derived from transmission measurements. Comparisons of MC calculated depth dose curves and profiles, using original and PS data reproduced by the MSM, agree within 1% and 1 mm. Deviations from measured dose distributions are within 1.5% and 1 mm. However, the real beam leads to some larger deviations outside the geometrical beam area for large fields. Calculated output factors in 10 cm water depth agree within 1.5% with experimentally determined data. In conclusion, the MSM produces accurate PS data for MC photon dose calculations for the rectangular fields specified.

An efficient framework for photon Monte Carlo treatment planning

Currently photon Monte Carlo treatment planning (MCTP) for a patient stored in the patient database of a treatment planning system (TPS) can usually only be performed using a cumbersome multi-step procedure where many user interactions are needed. This means automation is needed for usage in clinical routine. In addition, because of the long computing time in MCTP, optimization of the MC calculations is essential. For these purposes a new graphical user interface (GUI)-based photon MC environment has been developed resulting in a very flexible framework. By this means appropriate MC transport methods are assigned to different geometric regions by still benefiting from the features included in the TPS. In order to provide a flexible MC environment, the MC particle transport has been divided into different parts: the source, beam modifiers and the patient. The source part includes the phase-space source, source models and full MC transport through the treatment head. The beam modifier part consists of one module for each beam modifier. To simulate the radiation transport through each individual beam modifier, one out of three full MC transport codes can be selected independently. Additionally, for each beam modifier a simple or an exact geometry can be chosen. Thereby, different complexity levels of radiation transport are applied during the simulation. For the patient dose calculation, two different MC codes are available. A special plug-in in Eclipse providing all necessary information by means of Dicom streams was used to start the developed MC GUI. The implementation of this framework separates the MC transport from the geometry and the modules pass the particles in memory; hence, no files are used as the interface. The implementation is realized for 6 and 15 MV beams of a Varian Clinac 2300 C/D. Several applications demonstrate the usefulness of the framework. Apart from applications dealing with the beam modifiers, two patient cases are shown. Thereby, comparisons are performed between MC calculated dose distributions and those calculated by a pencil beam or the AAA algorithm. Interfacing this flexible and efficient MC environment with Eclipse allows a widespread use for all kinds of investigations from timing and benchmarking studies to clinical patient studies. Additionally, it is possible to add modules keeping the system highly flexible and efficient.

Acute and late toxicity in prostate cancer patients treated by dose escalated intensity modulated radiation therapy and organ tracking

BackgroundTo report acute and late toxicity in prostate cancer patients treated by dose escalated intensity-modulated radiation therapy (IMRT) and organ tracking.MethodsFrom 06/2004 to 12/2005 39 men were treated by 80 Gy IMRT along with organ tracking. Median age was 69 years, risk of recurrence was low 18%, intermediate 21% and high in 61% patients. Hormone therapy (HT) was received by 74% of patients. Toxicity was scored according to the CTC scale version 3.0. Median follow-up (FU) was 29 months.ResultsAcute and maximal late grade 2 gastrointestinal (GI) toxicity was 3% and 8%, late grade 2 GI toxicity dropped to 0% at the end of FU. No acute or late grade 3 GI toxicity was observed. Grade 2 and 3 pre-treatment genitourinary (GU) morbidity (PGUM) was 20% and 5%. Acute and maximal late grade 2 GU toxicity was 56% and 28% and late grade 2 GU toxicity decreased to 15% of patients at the end of FU. Acute and maximal late grade 3 GU toxicity was 8% and 3%, respectively. Decreased late ≥ grade 2 GU toxicity free survival was associated with higher age (P = .025), absence of HT (P = .016) and higher PGUM (P < .001).DiscussionGI toxicity rates after IMRT and organ tracking are excellent, GU toxicity rates are strongly related to PGUM.

Monte Carlo simulation of a dynamic MLC based on a multiple source model

Detailed knowledge of the characteristics of the radiation field shaped by a multileaf collimator (MLC) is essential in intensity modulated radiotherapy (IMRT). A previously developed multiple source model (MSM) for a 6 MV beam was extended to a 15 MV beam and supplemented with an accurate model of an 80-leaf dynamic MLC. Using the supplemented MSM and the MC code GEANT, lateral dose distributions were calculated in a water phantom and a portal water phantom. A field which is normally used for the validation of the step and shoot technique and a field from a realistic IMRT treatment plan delivered with dynamic MLC are investigated. To assess possible spectral changes caused by the modulation of beam intensity by an MLC, the energy spectra in five portal planes were calculated for moving slits of different widths. The extension of the MSM to 15 MV was validated by analysing energy fluences, depth doses and dose profiles. In addition, the MC-calculated primary energy spectrum was verified with an energy spectrum which was reconstructed from transmission measurements. MC-calculated dose profiles using the MSM for the step and shoot case and for the dynamic MLC case are in very good agreement with the measured data from film dosimetry. The investigation of a 13 cm wide field shows an increase in mean photon energy of up to 16% for the 0.25 cm slit compared to the open beam for 6 MV and of up to 6% for 15 MV, respectively. In conclusion, the MSM supplemented with the dynamic MLC has proven to be a powerful tool for investigational and benchmarking purposes or even for dose calculations in IMRT.

Effects of dose reduction on the detectability of standardized radiolucent lesions in digital panoramic radiography

Dose reduction in digital panoramic radiography was studied. Intentional underexposure was performed with the Orthophos DS while six different human mandibles were radiographed. Exposure settings were 69 kV/15 mA (standard), 64 kV/16 mA, and 60 kV/16 mA. Standardized spherical defects, each either 1 or 1.25 mm in diameter, were simulated in 288 of 432 images, and seven observers decided whether defects were present or not. Areas under the receiver operating characteristics curves were calculated. They showed no significant differences in the detectability of the 1-mm defect at 69, 64, or 60 kV. For the 1.25-mm defect, no difference was found between the 69 and 60 kV images, but a statistically significant different detectability was found for 64 kV images in comparison with both 69 and 60 kV images. A dose reduction of up to 43% was ascertained with a Pedo-RT-Humanoid phantom when panoramic radiography was performed at 60 kV/16 mA. The conclusion is that with the Orthophos DS, it seems possible to reduce the dose rate of x-rays without loss of diagnostic quality in the case of radiolucent changes.

High-Dose (80 Gy) Intensity-Modulated Radiation Therapy with Daily Image-Guidance as Primary treatment for Localized Prostate Cancer

AbstractPurpose:To report acute and late toxicity in prostate cancer patients treated by high-dose intensity-modulated radiation therapy (IMRT) with daily image-guidance.Patients and Methods:From 06/2004–03/2008, 102 men were treated with 80 Gy IMRT with daily image-guidance. The risk groups were as follows: low, intermediate, and high risk in 21%, 27%, and 52% of patients, respectively. Hormone therapy was given to 65% of patients. Toxicity was scored according to the CTC scale version 3.0.Results:Median age was 69 years and median follow-up was 39 months (range, 16–61 months). Acute and late grade 2 gastrointestinal (GI) toxicity occurred in 2% and 5% of patients, respectively, while acute and late grade 3 GI toxicity was absent. Grade 2 and 3 pretreatment genitourinary (GU) morbidity (PGUM) were 15% and 2%, respectively. Acute grade 2 and 3 GU toxicity were 43% and 5% and late grade 2 and 3 GU toxicity were 21% and 1%, respectively. After multiple Cox regression analysis, PGUM was an independent predictor of decreased late ≥ grade 2 GU toxicity-free survival (hazard ratio = 9.4 (95% confidence interval: 4.1, 22.0), p < 0.001). At the end of follow-up, the incidence of late grade 2 and 3 GU toxicity decreased to 7% and 1%, respectively.Conclusion:GI toxicity rates after IMRT with daily image-guidance were excellent. GU toxicity rates were acceptable and strongly related to PGUM.ZusammenfassungZiel:Beschreibung der Akut- und Spätnebenwirkungen bei Patienten mit Prostatakarzinom, die mit intensitätsmodulierter Hochdosis- Radiotherapie (IMRT) und täglicher Image Guidance behandelt wurden.Patienten und Methodik:Von 06/2004 bis 03/2008 wurden 102 Männer mit 80 Gy IMRT behandelt. Die Zuordnung zu Risikogruppen ergab: niedrig bei 21%, mittelhoch bei 27% und hoch bei 52% der Patienten. Hormontherapie erhielten 65% der Patienten. Die Toxizität wurde anhand der CTC Kriterien Version 3.0 bestimmt.Ergebnisse:Das mediane Alter betrug 69 Jahre und die mediane Nachbeobachtungszeit war 39 Monate (Spannbreite, 16–61 Monate). Akute und späte Grad-2-gastrointestinale (GI) Nebenwirkungen traten in 2% und 5% der Fälle auf; akute oder späte Grad-3-GI Nebenwirkungen wurden nicht beobachtet. Die Grad-2- und -3-urogenitale (GU) Morbidität, welche bereits vor IMRT bestand (PGUM), betrug 15% und 2%. Akute Grad-2- und -3-GU Nebenwirkungen lagen bei 43% und 5% und späte Grad-2- und -3-GU Nebenwirkungen bei 21% und 1%. In der multiplen Cox Regressionsanalyse erwies sich PGUM als unabhängiger Prädiktor für ein verkürztes (spät) ≥ Grad-2-GU nebenwirkungsfreies Überleben (hazard ratio = 9.4 [95% confidence interval: 4.1, 22.0], p < 0.001). Am Ende des Nachbeobachtungszeitraumes sank die Inzidenz von späten Grad-2 und -3-GU Nebenwirkungen auf 7% und 1%.Schlussfolgerung:Die GI Nebenwirkungsrate nach IMRT mit täglicher Image Guidance war exzellent. GU Nebenwirkungsraten waren akzeptabel und standen in engem Zusammenhang den urogenitalen Beschwerden vor Radiotherapie.

Applicability and dosimetric impact of ultrasound-based preplanning in high-dose-rate brachytherapy of prostate cancer

Background and Purpose:Analyses of permanent brachytherapy seed implants of the prostate have demonstrated that the use of a preplan may lead to a considerable decrease of dosimetric implant quality. The authors aimed to determine whether the same drawbacks of preplanning also apply to high-dose-rate (HDR) brachytherapy.Patients and Methods:15 patients who underwent two separate HDR brachytherapy implants in addition to external-beam radiation therapy for advanced prostate cancer were analyzed. A pretherapeutic transrectal ultrasound was performed in all patients to generate a preplan for the first brachytherapy implant. For the second brachytherapy, a subset of patients were treated by preplans based on the ultrasound from the first brachytherapy implant. Preplans were compared with the respective postplans assessing the following parameters: coverage index, minimum target dose, homogeneity index, and dose exposure of organs at risk. The prostate geometries (volume, width, height, length) were compared as well.Results:At the first brachytherapy, the matching between the preplan and actual implant geometry was sufficient in 47% of the patients, and the preplan could be applied. The dosimetric implant quality decreased considerably: the mean coverage differed by –0.11, the mean minimum target dose by –0.15, the mean homogeneity index by –0.09. The exposure of organs at risk was not substantially altered. At the second brachytherapy, all patients could be treated by the preplan; the differences between the implant quality parameters were less pronounced. The changes of prostate geometry between preplans and postplans were considerable, the differences in volume ranging from –8.0 to 13.8 cm3 and in dimensions (width, height, length) from –1.1 to 1.0 cm.Conclusion:Preplanning in HDR brachytherapy of the prostate is associated with a substantial decrease of dosimetric implant quality, when the preplan is based on a pretherapeutic ultrasound. The implant quality is less impaired in subsequent implants of fractionated brachytherapy.Hintergrund und Ziel:Die Anwendung eines Preplan bei der Prostata-Brachytherapie mit radioaktiven Seeds geht häufig mit einer relevanten Abnahme der dosimetrischen Qualität des Implantats einher. Es wird untersucht, ob bei der High-Dose-Rate- (HDR-)Brachytherapie dieselben Nachteile des Preplannings auftreten.Patienten und Methodik:Die Studie stützt sich auf 15 Patienten mit einem fortgeschrittenen Prostatakarzinom, die mit externer Strahlentherapie und zwei HDR-Brachytherapie-Sitzungen behandelt wurden. Alle Patienten wurden mit prätherapeutischem transrektalem Ultraschall als Basis zur Erstellung eines Preplan für die erste Brachytherapie untersucht. Die zweite Brachytherapie wurde in einer Subgruppe (n = 6) mittels Preplan appliziert, der auf dem Ultraschall der ersten Brachytherapie basierte (Abbildung 1). Die Preplans wurden mit den entsprechenden Postplans auf folgende Parameter hin verglichen: Coverage-Index, minimale Target-Dosis, Homogenitätsindex und Dosisbelastung der Risikoorgane. Ebenso wurden die geometrischen Maße der Prostata verglichen (Volumen, Breite, Höhe, Länge).Ergebnisse:Bei 47% der Patienten wurde die Übereinstimmung der Implantatgeometrie von Preplan und erster Brachytherapiesitzung als ausreichend erachtet, und der Preplan konnte nachfolgend angewendet werden. Die Implantatqualität nahm deutlich ab (Tabelle 1): Der mittlere Coverage-Index sank um –0,11, die mittlere Target-Dosis um –0,15, der mittlere Homogenitätsindex um –0,09. Die Strahlenbelastung der Risikoorgane erhöhte sich nicht wesentlich (Tabelle 2). Die Unterschiede der geometrischen Parameter waren beträchtlich (–8,0 bis 13,8 cm3 betreffend Volumen; –1,1 bis 1,0 cm betreffend Breite, Höhe und Länge; Abbildung 2 und Tabelle 3).Schlussfolgerung:Die Verwendung eines Preplan in der HDR-Brachytherapie der Prostata resultiert in einer wesentlichen Abnahme der Implantatqualität, wenn der Preplan auf einem prätherapeutischen Ultraschall basiert. Diese Abnahme ist bei fraktionierter Behandlung in nachfolgenden Brachytherapiesitzungen weniger ausgeprägt.

Use of Gold Markers for Setup in Image-Guided Fractionated High-Dose-Rate Brachytherapy as a Monotherapy for Prostate Cancer

Background and Purpose:In order to use a single implant with one treatment plan in fractionated high-dose-rate brachytherapy (HDR-B), applicator position shifts must be corrected prior to each fraction. The authors investigated the use of gold markers for X-ray-based setup and position control between the single fractions.Patients and Methods:Caudad-cephalad movement of the applicators prior to each HDR-B fraction was determined on radiographs using two to three gold markers, which had been inserted into the prostate as intraprostatic reference, and one to two radiopaque-labeled reference applicators. 35 prostate cancer patients, treated by HDR-B as a monotherapy between 10/2003 and 06/2006 with four fractions of 9.5 Gy each, were analyzed. Toxicity was scored according to the CTCAE Score, version 3.0. Median follow-up was 3 years.Results:The mean change of applicators positions compared to baseline varied substantially between HDR-B fractions, being 1.4 mm before fraction 1 (range, –4 to 2 mm), –13.1 mm before fraction 2 (range, –36 to 0 mm), –4.1 mm before fraction 3 (range, –21 to 9 mm), and –2.6 mm at fraction 4 (range, –16 to 9 mm). The original position of the applicators could be readjusted easily prior to each fraction in every patient. In 18 patients (51%), the applicators were at least once readjusted > 10 mm, however, acute or late grade ≥ 2 genitourinary toxicity was not increased (p = 1.0) in these patients.Conclusion:Caudad position shifts up to 36 mm were observed. Gold markers represent a valuable tool to ensure setup accuracy and precise dose delivery in fractionated HDR-B monotherapy of prostate cancer.Hintergrund und Ziel:Um ein einziges Implantat mit einem Bestrahlungsplan fur die fraktionierte High-Dose-Rate-Brachytherapie (HDR-B) nutzen zu konnen, mussen Positionsverschiebungen der Katheter vor jeder Fraktion erkannt und korrigiert werden. Die Autoren untersuchten den Nutzen von Goldmarkern fur rontgenbildbasierte Konfiguration und Positionskontrolle zwischen den Einzelfraktionen.Patienten und Methodik:Die kraniokaudalen Verschiebungen der Applikatoren wurden vor jeder HDR-B-Fraktion anhand von zwei bis drei Goldmarkern als intraprostatische Referenz und ein bis zwei rontgendicht markierten Referenzapplikatoren mittels Rontgenbild bestimmt. 35 Patienten mit Prostatakarzinom, welche zwischen 10/2003 and 06/2006 eine HDR-B als Monotherapie mit vier Fraktionen von jeweils 9,5 Gy erhielten, wurden untersucht. Die Behandlungstoxizitat wurde mit dem CTCAE-Score, Version 3.0, erfasst. Die mediane Nachbeobachtungszeit lag bei 3 Jahren.Ergebnisse:Die mittlere Positionsabweichung der Applikatoren von der Sollposition variierte erheblich zwischen den HDR-B-Fraktionen und betrug 1,4 mm vor der ersten Fraktion (Spannweite: –4 bis 2 mm), –13,1 mm vor der zweiten Fraktion (Spannweite: –36 bis 0 mm), –4,1 mm vor der dritten Fraktion (Spannweite: –21 bis 9 mm) und –2,6 mm vor der vierten Fraktion (Spannweite: –16 bis 9 mm). Die ursprungliche Position der Applikatoren konnte bei jedem Patienten problemlos vor jeder Fraktion wiederhergestellt werden. Bei 18 Patienten (51%) wurden die Applikatoren wenigstens einmal > 10 mm verschoben; dennoch war die genitourethrale Akut- oder Spattoxizitat Grad ≥ 2 bei diesen Patienten nicht erhoht (p = 1,0).Schlussfolgerung:Positionsverschiebungen von bis zu 36 mm nach kaudal wurden beobachtet. Goldmarker sind bei der fraktionierten HDR-B-Monotherapie des Prostatakarzinoms von Nutzen, um die akkurate Konfiguration und die prazise Verabreichung der Strahlendosis zu gewahrleisten.