Manuel Todorovic

Dosimetric evaluation of a 2D pixel ionization chamber for implementation in clinical routine

In this paper we present the results of a dosimetric evaluation of a 2D ionization chamber array with the objective of its implementation for quality assurance in clinical routine. The pixel ionization chamber MatriXX (Scanditronix Wellhofer, Germany) consists of 32x32 chambers with a distance of 7.6 mm between chamber centres. The effective depth of measurement under the surface of the detector was determined. The dose and energy dependence, the behaviour of the device during its initial phase and its time stability as well as the lateral response of a single chamber of the detector in cross-plane and diagonal directions were analysed. It could be shown, that the detectors response is linear with dose and energy independent. Taking the lateral response into account, two different dose profiles, for a pyramidal and an IMRT dose distribution, were applied to compare the data generated by a treatment planning system with measurements. From these investigations it can be concluded that the detector is a suitable device for quality assurance and 2D dose verifications.

Evaluation of GafChromic EBT prototype B for external beam dose verification

The capability of the new GafChromic EBT prototype B for external beam dose verification is investigated in this paper. First the general characteristics of this film (dose response, postirradiation coloration, influence of calibration field size) were derived using a flat-bed scanner. In the dose range from 0.1 to 8 Gy, the sensitivity of the EBT prototype B film is ten times higher than the response of the GafChromic HS, which so far was the GafChromic film with the highest sensitivity. Compared with the Kodak EDR2 film, the response of the EBT is higher by a factor of 3 in the dose range from 0.1 to 8 Gy. The GafChromic EBT almost does not show a temporal growth of the optical density and there is no influence of the chosen calibration field size on the dose response curve obtained from this data. A MatLab program was written to evaluate the two-dimensional dose distributions from treatment planning systems and GafChromic EBT film measurements. Verification of external beam therapy (SRT, IMRT) using the above-mentioned approach resulted in very small differences between the planned and the applied dose. The GafChromic EBT prototype B together with the flat-bed scanner and MatLab is a successful approach for making the advantages of the GafChromic films applicable for verification of external beam therapy.

Experiences with a New High-Dose-Rate Brachytherapy (HDR-BT) Boost Technique for T3b Prostate Cancer

Purpose:Experiences with a new high-dose-rate brachytherapy (HDR-BT) boost technique in 41 patients with stage T3b prostate cancer are presented.Patients and Methods:The patients received 18 Gy of HDR-BT (9 Gy on days 1 + 8) plus 50.4 Gy of EBRT. 20 patients (group A) had BT applicators placed into the prostate alone resulting in 18 Gy to prostate and 0 Gy (tip) to 12 Gy (base) to seminal vesicles (SV). The cumulative EQD2 (equivalent dose in 2-Gy fractions, α/β 1.5 Gy) to the SV was 47.5-73.3 Gy. 21 patients (group B) had BT applicators placed into both prostate and SV resulting in 18 Gy to prostate and to > 80% (but not 100%) of the SV (cumulative EQD2 81.5-101.5 Gy). Both groups were compared for acute and late toxicity and for biochemical relapse-free survival (bRFS).Results:The 3-year bRFS was 57% for group A and 79% for group B patients (p = 0.29). A grade 3 acute toxicity (CTC 2.0) was not observed. Grade 2 acute toxicity (proctitis, cystitis, skin toxicity) was comparable in both groups. A grade 3 late toxicity did not occur. Impotence rates were 35% in group A and 24% in group B, respectively (p = 0.73).Conclusion:The new HDR-BT technique (group B) was associated only with minor acute and late toxicity and appears to result in better bRFS than the conventional HDR-BT technique (group A). The results must be confirmed in a prospective trial.Ziel:Erfahrungen mit einer neuen High-Dose-Rate-Brachytherapie-(HDR-BT-)Boost-Technik bei 41 Patienten mit T3b-Prostatakarzinom (Tabelle 1) werden präsentiert.Patienten und Methodik:Die Patienten erhielten 18 Gy HDR-BT (9 Gy an Tag 1 + 8) plus 50,4 Gy perkutan. Bei 20 Patienten (Gruppe A) wurden BT-Applikatoren nur in der Prostata platziert. Die Dosis betrug 18 Gy in der Prostata sowie 0 Gy (Spitze) bis 12 Gy (Basis) in den Samenblasen (SB). Die kumulative EQD2 („equivalent dose in 2-Gy fractions“, α/β 1,5 Gy) betrug in den SB 47,5-73,3 Gy (Abbildung 2). Bei 21 Patienten (Gruppe B) wurden BT-Applikatoren in der Prostata und den SB platziert (Abbildung 1). Die Dosis in Prostata und > 80% (aber nicht 100%) der SB betrug 18 Gy (kumulative EQD2 81,5-101,5 Gy; Abbildung 2). Die Gruppen wurden hinsichtlich Akut- und Spättoxizität sowie des biochemischen rezidivfreien Überlebens (bRFS) verglichen.Ergebnisse:Das bRFS nach 3 Jahren betrug 57% in Gruppe A und 79% in Gruppe (p = 0,29; Abbildung 3). Eine Grad-3-Akuttoxizität (CTC 2.0) wurde in beiden Gruppen nicht beobachtet. Die Grad-2-Akuttoxizität (Proktitis, Zystitis, Hauttoxizität) war in beiden Gruppen vergleichbar (Abbildung 4). Eine Grad-3-Spättoxizität trat bei keinem Patienten auf. Die Impotenzraten betrugen 35% in Gruppe A und 24% in Gruppe B (p = 0,73).Schlussfolgerung:Die neue HDR-BT-Technik (Gruppe B) ist gut verträglich und scheint mit einem besseren bRFS einherzugehen als die konventionelle HDR-BT-Technik (Gruppe A). Die Ergebnisse müssen in einer prospektiven Studie überprüft werden.

Commissioning of a double-focused micro multileaf collimator (μMLC)

Double‐focused μMLCs are able to create fields with steeper dose gradients at the field edges and are, therefore, an advancement in delivering stereotactic treatments. A double‐focused μMLC has been installed at a Siemens Primus linear accelerator (linac) as a first research installation in Europe. The basic dosimetric parameters, such as leakage, output factors, depth‐dose curves and penumbra, have been measured in 6 and 15 MV‐mode by use of radiochromic films (GafChromic EBT), ionization chambers and our solid water QA‐phantom (Easy Cube). The leakage between the leaves is minimal and lower than that of most commercially available MLCs. Therefore, the field size of the linac can be kept constant while the leaves of the μMLC are creating different aperture shapes. Percentage depth doses (PDDs) generated by the double‐focused μMLC are equal to depth‐dose curves of the original linac. That means the μMLC affects only the off‐axis ratio (OAR). Based on the fact that the μMLC is double‐focused and the source‐to‐collimator distance is larger, the penumbra is sharper than that for fields defined by the original linac MLC. The mechanical and dosimetric investigations show the benefit of the double‐focused μMLC attached to a Siemens Primus linear accelerator. PACS number: 87.10.+e

Beta dosimetry with microMOSFETs for endovascular brachytherapy

The aim of this study was to investigate if microMOSFETs are suitable for the dosimetry and quality assurance of beta sources. The microMOSFET dosimeters have been tested for their angular dependence in a 6 MeV electron beam. The dose rate dependence was measured with an iridium-192 afterloading source. By varying the source-to-surface distance (SSD) in a 12 MeV electron beam the dose rate dependence in an electron beam was also investigated. To measure a depth dose curve the dose rate at 2, 5, 8 and 12 mm distance from the beta source train axis was determined with the OPTIDOS and the microMOSFET detector. A comparison between the two detector types shows that the microMOSFET is suitable for quality assurance of beta sources for endovascular brachytherapy (EVBT). The homogeneity of the source is checked by measurements at five points (for the 60 mm source at 10, 20, 30, 40 and 50 mm) along the source train. The microMOSFET was then used to evaluate the influence of a common stent type (single layer stainless steel) on the dose distribution in water. The stent led to a dose inhomogeneity of +/-8.5%. Additionally the percentage depth dose curves with and without a stent were compared. The depth dose curves show good agreement which means that the stent does not change the beta spectrum significantly.

SU‐FF‐T‐241: Evaluation of EBT GafChromic Film for External Beam Dose Verification

Purpose: Radiochromic Films are a widely accepted method to measure dose distributions for brachytherapy. Compared to radiographicfilms they offer a variety of advantages. They are self developing, almost tissue‐equivalent and insensitive to daylight. The films mainly used so far had the disadvantage of a low sensitivity to radiation. The new GafChromic EBT film offers a solution to this problem. The purpose of this paper is to prove the application of this new film for verification measurements concerning IMRT and stereotactic radiotherapy (SRT), Method and Materials: In order to establish a precise and reliable dosimetric measuring device, it was necessary to investigate the relevant features of the film. After this the films could be applied for dosimetric measurements with 6 MV x‐rays using a Siemens Primus linear accelerator. The verification measurements (IMRT and SRT) were done using a μMLC manufactured by 3D Line in combination with the Primus. The films were placed in our multi‐purpose Phantom (EasyCube). After irradiation they were scanned using a flat‐bed scanner (MicroTek) and compared with the data provided by the treatment planning system Ergo++ using selfwritten MatLab routines. Results: The general properties of the film were determined (eg. response, post‐irradiation growth of the OD, energy dependence) and compared with the GafChromic HS and radiographic Kodak EDR‐2 films. The response of the EBT is 10‐times higher than the response of the HS, which so far was the GafChromic film with the highest sensitivity, and 3‐times higher than the response of the EDR‐2. The measured dose distributions (IMRT and SRT) were in good agreement with the data from the planning system. Conclusion: The results of our investigations prove that the EBT is a successful approach for making the advantages of the GafChromic films applicable for external beam radiotherapy.

Comparison of different evaluation programs for the verification of patient irradiations with tomotherapy

Three different evaluation programs used to verify patient irradiations with tomotherapy were checked. All programs compare a planned dose distribution with the measured dose distribution using radiochromic film. The evaluation programs differ in quality, time effort and user friendliness.

Untersuchungen zur β-Dosimetrie an zwei unterschiedlichen Quellen für die kardiovaskuläre Brachytherapie

Zusammenfassung Am Universitatsklinikum Hamburg-Eppendorf wird die endovaskulare Brachytherapie zur Behandlung von In-Stent-Restenosen verwendet. Die hierbei benutzten β-Strahler ( 32 P und 90 Sr/ 90 Y) wurden mit zwei unterschiedlichen radiochromen Filmen dosimetrisch verifiziert. Im ersten Schritt wurden diese Filmtypen bezuglich ihrer Eignung fur die Dosimetrie untersucht. Beide Filme zeigten im untersuchten Dosisbereich (MD-55-2: 0 bis 33Gy, HD-810: 0 bis 105Gy) einen dosislinearen Verlauf der optischen Dichte. Das Nachdunkeln der radiochromen Filme wurde durch Verwendung einer farbstabilisierenden Methode (RCS-Methode) verhindert. Hierdurch lies sich zusatzlich die Dauer der Auswertung der Filme von 24 Stunden (Wartezeit bis der Film einen quasi farbstabilen Zustand erreicht hat) auf 2,5 Stunden nach Bestrahlung verkurzen. Farbstabilisierte Filme konnen uber einen langen Zeitraum nach der Bestrahlung mit annahernd gleichen Ergebnissen ausgewertet werden. Fur die beiden verwendeten Filmtypen ergab sich innerhalb der Messfehler ein energieunabhangiges Ansprechvermogen. Die Untersuchungen der beiden Quellentypen ergaben Unterschiede von 13,5% ( 32 P) sowie 21,8% (fur die 90 Sr/ 90 Y) innerhalb der Dosisprofile. Dies deckt sich mit den von den Herstellern angegebenen Fertigungstoleranzen.

SU‐FF‐T‐397: Characteristics of a Double‐Focused μMLC

Purpose: Micro multi‐leaf collimators (μMLC) are widely used as add‐on to linear accelerators for the generation of small irregular shaped beams. The purpose of this paper is to measure the dosimetric characteristics of a double‐focused μMLC. The dose field generated by a double‐focused μMLC has a steep dose gradient at all field edges and is therefore able to spare the healthy tissue better than a single‐focused μMLC Method and Materials: At our department we have mounted a double‐focused μMLC on a Siemens Primus linear accelerator. Base on the separate gantry sensor, this μMLC is independent from the type or manufacturer of the linac. The basic dosimetric properties, such as transmission or leakage, depth‐dose curves and penumbra, have been measured by use of radiographic and radiochromic films, pinpoint ionization chambers and our multi‐purpose QA‐Phantom (Easy Cube). Results: Transmission and leakage between the leaves are due to their special focusing design very low. The depth doses distributions for different field sizes generated by the μMLC are identical to the depth doses for the same field sizes generated by the MLC integrated in the linac. Therefore the μMLC affects only the off‐axis ratio. Due to the double‐focusing characteristics the penumbra is small and identical for the field length and width. The penumbra gradient does not differ significantly with different field sizes.Conclusion: The mechanical investigations show that the μMLC can securely be used with a Siemens Primus linear accelerator. The measureddosimetric properties show a small penumbra that is identical in both directions (inplane and crossplane). This is important if the μMLC is applied for stereotactic and intensity modulated radiotherapy.

Emergency localization of radioactive seeds lost during intracoronary brachytherapy.

Recently, it has been reported that brachytherapy catheters ruptured in vivo. Localization of lost β‐radiation‐emitting seeds is a problem because no appropriate technique is available that is rapid and precise. We developed a technique to localize β‐emitting seeds utilizing the effect that β‐radiation induces bremsstrahlung. The loss of a single radioactive source was simulated in an Alderson Phantom representing a human body. The β‐induced bremsstrahlung could be detected selectively by a γ‐camera. The position of the radioactive seed could be located within 5 min with an accuracy of ± 0.5 cm. The result of this study suggests that in an emergency case of loss of a brachytherapy source, a commercially available γ‐camera can be a valuable tool to detect lost β‐radiation‐emitting seeds rapidly and precisely. In addition, the technique minimizes the patients as well as the surgeons exposure to radiation and reduces the extent of surgical trauma. Catheter Cardiovasc Interv 2004;62:482–484.