Pre-treatment quality assurance (QA) in modern radiotherapy attempts to perform independent validation of patient-specific treatment plans. Stereotactic radiotherapy, in particular, provides a high dose conformal to the lesion to be treated. An ideal three-dimensional dosimeter, suited for patient-specific QA, quantifies with high resolution the degree of agreement between prescribed volumetric dose to the target and the actual output of the linear accelerator. Polymer gel dosimeters retain the information on absorbed dose as a function of radiation-induced polymerization, and since they are not affected by diffusion, this response is stable in time. In this thesis work, it was found that polymerization inhibitors, controlling the dosimeter's chemical response, can extend the dose range of polymer gel dosimeter's response in compliance with clinical requirements. Due to this, PAGAT gel dosimeter inhibited with p-nitrophenol has been characterized at different concentrations, measuring the response in terms of optical absorbance and transverse relaxation rate. Data were obtained through optical analysis and magnetic resonance imaging, exploiting a previously optimized pulse sequence. The inhibitors addition determines a bi-exponential dosimetric response with an increased dose range extended from 4 Gy up to 40 Gy without effects on the dosimetric performance, in terms of accuracy, precision, dose resolution, and dose rate dependences. Investigation on high dose spatial resolution, evaluated on a phantom irradiated with a step dose gradient, adopts as acceptance criterion, the so-called gamma index. It compares the dose distributions prescribed by the treatment planning system (TPS) with the absorbed dose measured at each point in the dosimeter. Since the results are promising the dosimetric system can be tested in relative validation of more complex stereotactic treatment plans.
La garanzia di qualità di pre-trattamento nella radioterapia moderna è finalizzata alla validazione di trattamenti specifici pianificati per ogni singolo paziente. La radioterapia stereotassica, in particolare, fornisce una dose elevata, geometricamente conforme alla lesione da trattare. Un dosimetro tridimensionale ideale, adatto alla valutazione pre-trattamento, quantifica con alta risoluzione il grado di accordo tra la distribuzione volumetrica della dose prescritta e l'effettivo irraggiamento erogato. I dosimetri a gel polimerici conservano le informazioni sulla dose assorbita in funzione della polimerizzazione indotta in seguito ad irraggiamento, e poiché non sono influenzati dalla diffusione, la risposta risulta stabile nel tempo. In questo progetto di tesi, è stato studiato come gli inibitori di polimerizzazione, controllando la risposta del dosimetro, possano essere utilizzati per estendere l'intervallo di risposta in dose in accordo con i requisiti delle moderne tecniche radioterapiche. Il dosimetro PAGAT inibito con p-nitrofenolo, a diverse concentrazioni, è stato caratterizzato misurando la risposta in termini di assorbanza ottica e rateo di rilassamento trasversale. I dati sono stati ottenuti attraverso analisi ottiche e risonanza magnetica basata su una sequenza di impulsi precedentemente ottimizzata. L'aggiunta degli inibitori determina nel dosimetro una risposta bi-esponenziale in un intervallo di dose aumentato da 4 Gy fino a 40 Gy e non influisce sulle prestazioni dosimetriche di accuratezza, precisione, risoluzione in dose e dipendenza dal rateo di dose. La risoluzione spaziale, valutata su un fantoccio irraggiato con elevato gradiente di dose, adotta come criterio di accettazione il cosiddetto indice gamma, che confronta la distribuzione di dose prescritta dal sistema di pianificazione (TPS) e la dose misurata in ogni punto del dosimetro. Poiché i risultati sono promettenti, il sistema dosimetrico può essere testato nella validazione relativa di piani di trattamento stereotassici complessi.
Innovative polymer gel dosimeter for 3D dose mapping in radiotherapy
Caprioli, Marco
2020/2021
Abstract
Pre-treatment quality assurance (QA) in modern radiotherapy attempts to perform independent validation of patient-specific treatment plans. Stereotactic radiotherapy, in particular, provides a high dose conformal to the lesion to be treated. An ideal three-dimensional dosimeter, suited for patient-specific QA, quantifies with high resolution the degree of agreement between prescribed volumetric dose to the target and the actual output of the linear accelerator. Polymer gel dosimeters retain the information on absorbed dose as a function of radiation-induced polymerization, and since they are not affected by diffusion, this response is stable in time. In this thesis work, it was found that polymerization inhibitors, controlling the dosimeter's chemical response, can extend the dose range of polymer gel dosimeter's response in compliance with clinical requirements. Due to this, PAGAT gel dosimeter inhibited with p-nitrophenol has been characterized at different concentrations, measuring the response in terms of optical absorbance and transverse relaxation rate. Data were obtained through optical analysis and magnetic resonance imaging, exploiting a previously optimized pulse sequence. The inhibitors addition determines a bi-exponential dosimetric response with an increased dose range extended from 4 Gy up to 40 Gy without effects on the dosimetric performance, in terms of accuracy, precision, dose resolution, and dose rate dependences. Investigation on high dose spatial resolution, evaluated on a phantom irradiated with a step dose gradient, adopts as acceptance criterion, the so-called gamma index. It compares the dose distributions prescribed by the treatment planning system (TPS) with the absorbed dose measured at each point in the dosimeter. Since the results are promising the dosimetric system can be tested in relative validation of more complex stereotactic treatment plans.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/173968