Karin Poljanc

Application of commercial MOSFET detectors for in vivo dosimetry in the therapeutic x-ray range from 80 kV to 250 kV

The purpose of this study was to investigate the dosimetric characteristics (energy dependence, linearity, fading, reproducibility, etc) of MOSFET detectors for in vivo dosimetry in the kV x-ray range. The experience of MOSFET in vivo dosimetry in a pre-clinical study using the Alderson phantom and in clinical practice is also reported. All measurements were performed with a Gulmay D3300 kV unit and TN-502RDI MOSFET detectors. For the determination of correction factors different solid phantoms and a calibrated Farmer-type chamber were used. The MOSFET signal was linear with applied dose in the range from 0.2 to 2 Gy for all energies. Due to fading it is recommended to read the MOSFET signal during the first 15 min after irradiation. For long time intervals between irradiation and readout the fading can vary largely with the detector. The temperature dependence of the detector signal was small (0.3% degrees C(-1)) in the temperature range between 22 and 40 degrees C. The variation of the measuring signal with beam incidence amounts to +/-5% and should be considered in clinical applications. Finally, for entrance dose measurements energy-dependent calibration factors, correction factors for field size and irradiated cable length were applied. The overall accuracy, for all measurements, was dominated by reproducibility as a function of applied dose. During the pre-clinical in vivo study, the agreement between MOSFET and TLD measurements was well within 3%. The results of MOSFET measurements, to determine the dosimetric characteristics as well as clinical applications, showed that MOSFET detectors are suitable for in vivo dosimetry in the kV range. However, some energy-dependent dosimetry effects need to be considered and corrected for. Due to reproducibility effects at low dose levels accurate in vivo measurements are only possible if the applied dose is equal to or larger than 2 Gy.

Investigating the accuracy of the FLUKA code for transport of therapeutic ion beams in matter

In-beam positron emission tomography (PET) is currently used for monitoring the dose delivery at the heavy ion therapy facility at GSI Darmstadt. The method is based on the fact that carbon ions produce positron emitting isotopes in fragmentation reactions with the atomic nuclei of the tissue. The relation between dose and beta(+)-activity is not straightforward. Hence it is not possible to infer the delivered dose directly from the PET distribution. To overcome this problem and enable therapy monitoring, beta(+)-distributions are simulated on the basis of the treatment plan and compared with the measured ones. Following the positive clinical impact, it is planned to apply the method at future ion therapy facilities, where beams from protons up to oxygen nuclei will be available. A simulation code capable of handling all these ions and predicting the irradiation-induced beta(+)-activity distributions is desirable. An established and general purpose radiation transport code is preferred. FLUKA is a candidate for such a code. For application to in-beam PET therapy monitoring, the code has to model with high accuracy both the electromagnetic and nuclear interactions responsible for dose deposition and beta(+)-activity production, respectively. In this work, the electromagnetic interaction in FLUKA was adjusted to reproduce the same particle range as from the experimentally validated treatment planning software TRiP, used at GSI. Furthermore, projectile fragmentation spectra in water targets have been studied in comparison to available experimental data. Finally, cross sections for the production of the most abundant fragments have been calculated and compared to values found in the literature.

Optimized Conformal Paraaortic Lymph Node Irradiation is not Associated with Enhanced Renal Toxicity

Background and Purpose:For patients with gynecologic carcinomas, irradiation of paraaortic lymph nodes (PLNs) is a routine treatment concept. Planning target volumes (PTVs) individually optimized by radiation field delineations along the big vessels permit the inclusion of at least 97% of potentially involved PLNs. However, this novel treatment technique might increase radiation-induced nephrotoxicity. Therefore, the actual incidence of kidney damage after PLN irradiation has to be assessed in order to validate the safety of this treatment concept.Patients and Methods:19 patients were treated with irradiation alone (50.4 Gy; 5 × 1.8 Gy/week) and monitored for up to 90 months. Functional renal parameters, namely renal plasma flow (RPF) and glomerular filtration rate (GFR), were assessed by dynamic renal scintigraphy. Additionally, patients were clinically observed (i.e., hypertension, proteinuria) and calculations of normal-tissue complication probability (NTCP) values for nonuniform kidney irradiation were performed using the Lyman-Wolbarst algorithm.Results:Two patients with anticipated moderate NTCP values (12.6% and 8.7%) showed slightly impaired RPF rates at 12, 24, and after 48 months of follow-up. Only one patient in the subgroup showing NTCP values > 50% (n = 9) developed a notable impairment of renal RPF. However, all patients including those with elevated complication probabilities exhibited neither impaired GFR nor clinically apparent symptoms related to a loss of functioning renal tissue from 12 to > 48 months post irradiation.Conclusion:Conformal irradiation of retroperitoneal lymph nodes with individual PTV delineation appears not to be associated with clinically relevant functional impairment of the kidneys.Hintergrund und Ziel:Die Behandlung von Patientinnen mit gynäkologischen Tumoren beinhaltet häufig die Bestrahlung der paraaortalen Lymphknoten (PLNs). Durch eine individuelle Anpassung des Planungszielvolumens (PTV) an den Verlauf der großen abdominalen Gefäße können mindestens 97% aller potentiell befallenen PLNs behandelt werden. Die dadurch z.T. vergrößerten PTVs könnten aber mit einer gesteigerten Inzidenz für eine radiogene Nephropathie einhergehen. Um die Sicherheit dieser neuen Bestrahlungstechnik zu überprüfen, wurde das tatsächliche Auftreten von radiogenen Nephropathien nach solchen PLN-Bestrahlungen untersucht.Patienten und Methodik:19 Patientinnen mit gynäkologischen Tumoren, die eine Bestrahlung der PLNs (50,4 Gy; 5 × 1,8 Gy/Woche) ohne Chemotherapie in der Klinik der Autoren erhielten, wurden bis zu 90 Monate nachbeobachtet. Mittels seitengetrennter Nierenclearence wurden renaler Plasmafluss (RPF) und glomeruläre Filtrationsrate (GFR) bestimmt und die Patientinnen regelmäßig klinisch untersucht (u.a. Blutdruck, Proteinurie). Außerdem wurden für jede Patientin aus den dreidimensionalen Bestrahlungsplänen für beide Nieren NTCP-Werte (Wahrscheinlichkeit von Normalgewebskomplikationen) nach dem Lyman-Wolbarst-Algorithmus berechnet.Ergebnisse:Zwei Patientinnen mit moderaten NTCP-Werten (12,6% und 8,7%) wiesen nach 12, 24 und 48 Monaten eine leichte Störung des RPF auf. Nur eine Patientin aus der Gruppe mit NTCP-Werten > 50% (n = 9) entwickelte eine ausgeprägtere Störung des RPF. Keine Patientin zeigte eine Störung der GFR oder klinische Symptome einer Nierenschädigung in der Zeit von 12 bis > 48 Monate nach der Bestrahlung.Schlussfolgerung:Die konformale Bestrahlung der PLNs mit einem individuellen, dem Verlauf der großen Gefäße angepassten PTV führte im eigenen Patientenkollektiv in keinem Fall zu einer klinisch relevanten radiogenen Nephropathie.

Targeted radionuclide therapy: theoretical study of the relationship between tumour control probability and tumour radius for a 32P/33P radionuclide cocktail

As revealed by previous theoretical studies, targeted radionuclide therapy (TRT) that relies on a single beta-emitting radioisotope is likely to be inappropriate for clinical scenarios such as disseminated malignancy. For a patient with a vast number of tumours and metastases of largely differing sizes a high level of therapeutical efficiency might be achieved only for a restricted range of tumour sizes. This is due to the limited range of beta-electrons in human tissue, essentially causing the therapeutical impact to vary tremendously with tumour size. The dependence of curability on the tumour dimension is expected to be significantly altered if a radionuclide cocktail, consisting of a long-range and a short-range beta-emitter, such as (32)P and (33)P, is involved in the treatment. In this study, a radiation transport simulation was performed, using the MCNP4c2 Monte Carlo code, in order to investigate the relationship between tumour control probability (TCP) and tumour size, associated with concurrent use of (32)P and (33)P. Two different models of intratumoural distribution of cumulated activity were taken into account. One simulated an ideal radionuclide uptake in tumour tissue and the other referred to a limited radiotracer penetration. The results were examined in comparison to tumours targeted with pure (32)P, (33)P and (131)I. For both uptake scenarios a considerable reduction of the overall variation of TCP and thus an increasing chance of achieving tumour cure was observed for tumour sizes ranging from microscopic dimensions up to macroscopic diameters, if the targeted radionuclide treatment relies on a (32)P/(33)P cocktail. It was revealed that particular attention has to be given to the ratio of the (32)P and (33)P specific cumulated activities (SCA) in the tumour, since this is a significant determinant of the resulting behaviour of tumour control probability as the tumour diameter varies. This study suggests that a 32P/33P approach is more applicable to diseases that involve a variety of tumours and metastases differing in size.

Comparison of PDR brachytherapy and external beam radiation therapy in the case of breast cancer

Pulsed dose rate brachytherapy (PDR) was compared to external beam radiation therapy (EBRT) in the case of breast cancer. The benefits were figured out by evaluation of dosimetric parameters and calculating the normal tissue complication probability (NTCP). PDR plans were set up for five randomly chosen left-sided breast cancer patients delivering a total dose of 50.4 Gy to the target (dose rate 0.8 Gy h(-1)). For EBRT five left-sided breast cancer patients were planned using 3D-conformal tangential photon beams with a prescribed total dose of 50 Gy (2 Gy/fraction) to the total breast volume. For plan ranking and NTCP calculation the physical dose was first converted into the biologically effective dose (BED) and then into the normalized total dose (NTD) using the linear quadratic model with an alpha/beta ratio of 3 Gy. In PDR the relative effectiveness (RE) was calculated for each dose bin of the differential dose volume histogram to get the BED. NTCPs were calculated for the ipsilateral lung and the heart as contoured on CT slices based on the Lyman model and the Kutcher reduction scheme. Dosimetric parameters as V(th) (percentage of the total volume exceeding a threshold dose) and Jacksons f(dam) (fraction of the organ damaged) were also used to figure out the benefits. The comparison of calculated NTCPs in PDR and EBRT showed no difference between these two modalities. All values were below 0.01%. f(dam) derived from EBRT was always higher (mean value 8.95% versus 1.21% for the lung). The mean V(10) and V(20) of the lung related to BED were 6.32% and 1.72% for PDR versus 11.72% and 9.59% for EBRT. When using dosimetric parameters as V(th) and f(dam), PDR was mostly superior to EBRT in respect of sparing normal tissues. NTCP calculation as a single method of modality ranking showed a lack of information, especially when normal tissue was exposed to low radiation doses.

On the Commissioning of CT‐simulation in Radiotherapy Treatment Planning: A Phantom Study

The development of CT‐simulation leads to major improvements in radiation therapy treatment planning. However it is not clear whether this new technology has evolved to the point of completely eliminating the “physical simulator”. At present there is an ongoing debate about the significance of virtual simulation in the treatment planning process and its limitations for different clinical purposes. The purpose of this study is to measure the accuracy in the determination of the isocenter in a virtual simulation process and to report about parameters which influence on the precision of its determination.