Dosimetric Verification of Intensity Modulated Radiotherapy Treatment Plans with Radiographic Film

"Intensity modulated radiation therapy (IMRT) is a fairly new modality in the treatment of cancer with radiotherapy. The original idea is Swedish and has existed for more than 20 years. But it is only in recent years that clinics have implemented the IMRT technique. IMRT uses many segments (small subfields) that build up radiation fields with nonuniform energy fluence. With this technique, better target conformity can be achieved and the absorbed dose to healthy tissues in the vicinity of the tumour can be limited. The purpose of this work is to find a reliable method to verify IMRT delivery, which can lead to commissioning of the IMRT technique at Lund University Hospital. The dosimetric equipment chosen to verify the IMRT delivery has been radiographic film, complemented with ion chamber measurements for absolute dose determination. The verification has been performed on a phantom case, which is part of the QUASIMODO project used as an external audit of our dosimetric system. Additionally, a clinically patient case was also verified. Both treatment plans were optimized in the Oncentra Treatment Planning (OTP) system and forward calculated in Helax-TMS. The EDR2 film from Kodak is a rather new type of radiographic film that has been used throughout this study. It was shown that the EDR2 film could accurately be used, within 2%, for relative as well as absolute dose measurements, down to a depth in the patient/phantom of at least 20 cm, when ignoring the build-up region. Absolute dose measurements with a cylindrical ion chamber (NE 2571 0.6 cm3 Farmer type) showed, on the other hand, discrepancies up to 8% when compared to calculations by the treatment planning system (TPS). The reason for this might be due to partial irradiation of the ion chamber in IMRT. Thermoluminescent dosimeters (TLD) were used as an alternative detector for comparison and the result was within 2% of the TPS calculations. With proper care the film dosimetry can be used to verify IMRT delivery. But before that, the large discrepancies with ion chamber measurements have to be further investigated. Many more cases have to be verified to get a better statistical base.Idag behandlas cancer med bl.a. strålbehandling, tyvärr ger denna många gånger biverkningar. Detta beror på att strålningen inte bara skadar cancerceller utan även friska celler, eftersom strålningen måste passera frisk vävnad för att nå in till cancertumören. Det har nyligen utvecklats en ny strålbehandlingsteknik som kan minska strålningen till frisk vävnad samtidigt som tumören ges en hög stråldos. Tekniken går ut på att man med många små strålfält kan koncentrera stråldosen, dvs. den absorberade energin, bättre till tumören och således undvika för hög stråldos till frisk vävnad. Tekniken kallas IMRT som står för Intensitetsmodulerad Radioterapi och är en komplicerad krävande teknik. Tekniken går ut på att man bestämmer önskad stråldos till tumören och högsta tillåtna stråldos till omkringliggande frisk vävnad. Ett datorprogram räknar utifrån dessa satta kriterier ut bästa fältsammansättningen, oftast bestående av tiotals till över hundra små strålfält. IMRT är en teknik som kan medföra stora förbättringar om man lyckas koncentrera stråldosen till tumören. En liten felpositionering av patienten eller felberäkning av programmet kan dock medföra att stråldosen istället koncentreras till organen runt omkring tumören, dvs. friska organ som man vill undvika att bestråla. Efter att man räknat fram en plan för patientbehandling krävs därför en grundlig undersökning av stråldosfördelningen innan patienten behandlas. Svårigheten ligger i att man inte kan mäta direkt i patienten hur dosfördelningen ser ut, eller med andra ord, hur hög stråldosen är på olika områden i patienten. Istället flyttar man över behandlingsplanen för patienten (med exakt samma fältsammansättning och bestrålningstid) på ett fantom som är tillverkat av vävnadsekvivalent material som ska motsvara människokroppen. Detta fantom är tillverkat så att man kan stoppa in olika typer av detektorer, exempelvis strålkänslig film. Filmerna mäter dosfördelningen som sedan kan jämföras med den som är beräknad i datorprogrammet. Om dessa stämmer överens inom rimliga gränser väntar man sig att det även gör det inuti patienten. Då har man lyckats med sin behandlingsplan och patienten kan börja behandlas. Syftet med detta arbete har varit att finna ett pålitligt system för att verifiera IMRTbehandlingar, så att patienter kan börja behandlas med den nya tekniken vid universitetssjukhuset i Lund. I arbetet har huvudsakligen film använts och detta med lovande resultat som kan leda fram till ett tillförlitligt verifieringsystem. Arbetet har även kompletterats med mätningar av andra detektorer som jonkammare och termoluminiscensdetektorer (TLD)

Evaluation of adaptation strengths of CARE Dose 4D in pediatric CT

The motivation of this study is the general lack of knowledge regarding the efficiency and the appropriate use of the adaptation strengths of Siemens automatic exposure control system CARE Dose 4D. The purpose was to evaluate the effect on radiation absorbed dose using different adaptation strengths of CARE Dose 4D in three routine pediatric CT protocols. A pediatric anthropomorphic whole body phantom was used to simulate a 4 year old patient. CT scans were performed with a Siemens SOMATOM Definition Flash using three different pediatric protocols: neck, thorax, and abdomen. The characteristic of the tube current modulation was similar for all adaptation strengths. The difference is the extent of decrease in tube current. The degree of dose reduction using CARE Dose 4D and CARE kV compared using a fix effective mAs was 34-57%, 51-88%, and 56-91% for neck, thorax, and abdomen protocol, respectively. Accordingly, there is a large difference in radiation dose dependent on the adaptation strength: a factor of 1.5, 4.5, and 4.6 for neck, thorax, and abdomen protocol, respectively. The adaptation strengths can be used to obtain user-specified modifications of image quality or radiation dose to the patient. Radiologists and medical physicists need to be aware of the large differences between the adaptation strengths, and such differences are useful when attempting strategies to optimize CT radiation dose

Evaluación del fallo de presas de relaves con el método del árbol de fallos

Uno de los elementos que más peligros generan a la salud personal y al medio ambiente en la industria minera son las presas de residuos. Por este motivo al analizar este tipo de situaciones se ha dedicado especial atención a utilizar los procedimientos cuantitativos dentro de los herramientas del análisis de riesgos, que son necesarios para caracterizar las principales condiciones de incertidumbre que afectan tanto a las presa de nueva construcción como a las abandonadas en situación de peligro. Para ello se marcó como objetivo principal la definición de una metodología con el Árbol de Fallos (Fault Tree Analysis-FTA) que, permite determinar las causas que condicionan la rotura de las presas de relaves y su liberación catastrófica al medio ambiente. Para lograr este objetivo se elaboró una base de datos donde se recopilan 407 fallos de las presas de relaves a nivel mundial. Con esa base de datos se han definido las causas o mecanismos de fallo más representativos: 1) deslizamiento del talud o inestabilidad de la pendiente, 2) desbordamiento o sobrepaso, 3) sismo o terremoto, 4) filtración, 5) erosión interna (piping) y 6) licuefacción estática.. Partiendo de la información generada en la base de datos se ha elaborado un Análisis Preliminar de Riesgo (PHA) a las presas de relaves, ya que el PHA constituye una base de apoyo para identificar los peligros y las consecuencias potenciales y, sobre todo, definir el “top-event” o máximo daño como consecuencia del fallo de una presa de relaves. El paso siguiente ha consistido en elaborar el Árbol de Fallos correspondiente al “top-event” ya definido, apoyándose en la matriz de caracterización internacional sobre las roturas de presas más importantes. Esta matriz parte de diversas fuentes y ha sido filtrada y ordenada para ese fin. El análisis se ha realizado simplificando en un esquema genérico las relaciones entre contribuidores al fallo principal, dejando para mayores empeños y tiempo el detalle de alguna de sus ramas. De esta manera se llega a la determinación de una probabilidad del efecto más grave (al menos pérdida de una vida humana) a nivel mundial del orden de un fallo o rotura de una presa a nivel catastrófico cada año. El análisis se completa con una discusión sobre otras posibilidades, de sus aproximaciones y las recomendaciones para próximos trabajos. ABSTRACT One of the elements which generate more hazards to the environment and human beings in the mining industry is a tailings dam. For this reason, when analyzing this type of situation, special attention has been devoted to using quantitative procedures within the tools of risk analysis, which are necessary to characterize the main uncertainty conditions that affect both new construction dams and dams abandoned in danger. The main goal was to define a methodology with the use of the Fault Tree Analysis (FTA), which allows determining the causes that determine the failure causes of tailings dams and their catastrophic release to the environment. To achieve this goal, a database was developed to collect 407 tailings dam failures worldwide. With this database, the most representative causes or failure mechanisms have been defined: 1) slope slip or slope instability, 2) overflow or surpass, 3) earthquake, 4) leakage, 5) internal erosion (piping) and 6) static liquefaction. Based on the information generated in the database, a Preliminary Hazard Analysis (PHA) has been developed for tailings dams, as the PHA provides a base of support to identify potential hazards and consequences and, defining the “top-event” or maximum damage as a consequence of tailings dam failure. The next step has been to elaborate with the failure tree corresponding to the “top-event” already defined, based on the international characterization matrix on the most important dam destruction. This matrix comes from various sources and has been filtered and sorted for that purpose. The analysis has been made simplifying in a generic relations scheme between contributors to the main fault, leaving for greater efforts and time the detail of some of its branches. In this way, we arrive at the determination of a probability of the most important effect (at least one loss of human life) worldwide of the order of a catastrophic failure every year. The analysis is completed with a discussion of other possibilities, their approaches and recommendations for upcoming work