Hassan Tarif Zubair

This study aims to quantify the dosimetric impact of afterloader-specific source dynamics in high-dose-rate (HDR) interstitial prostate brachytherapy using time-resolved (4D) Monte Carlo (MC) simulations. While HDR brachytherapy is known for its ability to deliver conformal dose distributions via afterloader-driven radioactive sources, most treatment planning systems neglect dose contributions during source transit, i.e. when the source enters the body, moves between dwell positions, and returns to its safe position. To address this limitation, we modeled three widely used commercial HDR afterloaders (Varian GammaMedPlus, BEBIG MultiSource, and Elekta microSelectron), each with distinct source acceleration profiles, using detailed MC simulations implemented in TOPAS. Simulated treatment scenarios included varying needle and dwell point configurations, and transit dose contributions were evaluated for the tumor, prostate body, bladder, and rectum. The results indicate that increased needle and dwell counts lead to higher transit doses to both the target and organs at risk (OAR). Furthermore, systems with lower source acceleration produced significantly greater transit doses due to longer source travel times. Notably, the prostate tissue surrounding the tumor, referred to here as the prostate body, was shown to receive up to 14.2% of the prescribed tumor dose, with approximately 72% of that amount attributable to transit dose. This corresponds to a transit dose contribution of around 10.2% of the total prescription dose, underscoring the clinical relevance of transit effects even in non-target tissues. These findings demonstrate that neglecting source motion can result in clinically meaningful underestimation of OAR doses, particularly in treatment areas with limited separation between the target and sensitive normal tissues, and highlight the value of incorporating source transit effects into treatment planning for improved dose estimation in clinical practice.

Radiation therapy plays a pivotal role in modern cancer treatment, demanding precise and accurate dose delivery to tumor sites while minimizing harm to surrounding healthy tissues. Monte Carlo simulations have emerged as indispensable tools for achieving this precision, offering detailed insights into radiation transport and interaction at the subatomic level. As the use of scintillation and luminescence dosimetry becomes increasingly prevalent in radiation therapy, there arises a need for validated Monte Carlo tools tailored to optical photon transport applications. In this paper, an evaluation process of the TOPAS (TOol for PArticle Simulation) Monte Carlo tool for Cerenkov light generation, optical photon transport and radioluminescence based dosimetry is presented. Three distinct sources of validation data are utilized: one from a published set of experimental results and two others from simulations performed with the Geant4 code. The methodology employed for evaluation includes the selection of benchmark experiments, making use of opt3 and opt4 Geant4 physics models and simulation setup, with observed slight discrepancies within the calculation uncertainties. Additionally, the complexities and challenges associated with modeling optical photons generation through luminescence or Cerenkov radiation and their transport are discussed. The results of our evaluation suggests that TOPAS can be used to reliably predict Cerenkov generation, luminescence phenomenon and the behavior of optical photons in common dosimetry scenarios.

The quality of treatment delivery as prescribed in radiotherapy is exceptionally important. One element that helps provide quality assurance is the ability to carry out time-resolved radiotherapy dose measurements. Reports on doped silica optical fibers scintillators using radioluminescence (RL) based radiotherapy dosimetry have indicated merits, especially regarding robustness, versatility, wide dynamic range, and high spatial resolution. Topping the list is the ability to provide time-resolved measurements, alluding to pulse-by-pulse dosimetry. For effective time-resolved dose measurements, high temporal resolution is enabled by high-speed electronics and scintillator material offering sufficiently fast rise and decay time. In the present work, we examine the influence of Ge doping on the RL response of Ge-doped silica optical fiber scintillators. We particularly look at the size of the Ge-doped core relative to the fiber diameter, and its associated effects as it is adjusted from single-mode fiber geometry to a large core-to-cladding ratio structure. The primary objective is to produce a structure that facilitates short decay times with a sufficiently large yield for time-resolved dosimetry. RL characterization was carried out using a high-energy clinical X-ray beam (6 MV), delivered by an Elekta Synergy linear accelerator located at the Advanced Medical and Dental Institute, Universiti Sains Malaysia (USM). The Ge-doped silica optical fiber scintillator samples, fabricated using chemical vapor deposition methods, comprised of large core and small core optical fiber scintillators with high and low core-to-cladding ratios, respectively. Accordingly, these samples having different Ge-dopant contents offer distinct numbers of defects in the amorphous silica network. Responses were recorded for six dose-rates (between 35 MU/min and 590 MU/min), using a photomultiplier tube setup with the photon-counting circuit capable of gating time as small as 1 μs. The samples showed linear RL response, with differing memory and afterglow effects depending on its geometry. Samples with a large core-to-cladding ratio showed a relatively short decay time (2 matrix, thereby reducing phosphorescence effects. This is a desirable feature of scintillating glass materials that enables avoiding the pulse pile-up effect, especially in high dose-rate applications. These results demonstrate the potential of Ge-doped optical-fiber scintillators, with a large core-to-cladding ratio for use in time-resolved radiation dosimetry.

In this paper, we review the highly promising silica glass, fabricated as doped and undoped optical fiber for intended use in radiation dosimetry. The dosimetry techniques reviewed here, underpinned by intrinsic and extrinsic defects in silica glass, focus on Thermoluminescence (TL), Optically Stimulated Luminescence (OSL) and Radioluminescence (RL), with occasional references to the much more established Radiation Induced Attenuation (RIA). The other focus in this review is on the various materials that have been reported earlier as dopants and modifiers used in silica glass optical fiber radiation dosimeters. This article also elaborates on recently reported optical fiber structures, namely, cylindrical fibers, photonic crystal fibers and flat fibers, as well as dimensions and shapes used for optimization of dosimeter performance. The various types of optical fiber radiation dosimeters are subsequently reviewed for various applications ranging from medical dosimetry such as in external beam radiotherapy, brachytherapy and diagnostic imaging, as well as in industrial processing and space dosimetry covering a dynamic dose range from μGy to kGy. Investigated dosimetric characteristics include reproducibility, fading, dose response, reciprocity between luminescence yield to dose-rate and energy dependence. The review is completed by a brief discussion on limitations and future developments in optical fiber radiation dosimetry.