Development of a System for Comprehensive in-vivo validation of Transrectal ultrasound based high Dose rate prostate brachytherapy Treatments

High dose rate (HDR) brachytherapy is a treatment modality commonly used for the treatment of prostate cancer. In this technique, radiation is emitted from within the prostate, allowing for high doses to be delivered to the prostate whilst still sparing the surrounding organs at risk such as the urethra, bladder, and rectum. However, due to the close proximity of these organs at risk to the prostate, and therefore the radiation source, there is potential that small errors in placement of the source may lead to substantial changes in the delivered dose distribution. Consequently, these errors may lead to significant post-treatment gastrointestinal or genitourinary complications as well as suboptimal biochemical progression free survival. Due to the risk of HDR prostate brachytherapy (pBT) mistreatment resulting in significant post-treatment complications, it is essential that all potential risks in the treatment planning and delivery process are identified, and that these risks are minimised. One method of minimising these risks is through the implementation of routine in-vivo treatment verification (IVTV), through either in-vivo dosimetry or in-vivo source tracking

The characterisation of planar silicon pin diodes for use in proton therapy

Many new techniques in the delivery of radiation therapy are being developed for the treatment of cancer. One of these new techniques, proton therapy is becoming increasingly popular due to the presence of the characteristic proton Bragg peak, which allows for better conformation of the dose to the tumour volume. The production of high LET secondary particles in the beam line and within the patient however could result in a significant contribution to the integral dose and diminish this potential advantage. Measured secondary particle doses from clinical proton facilities vary greatly; this is partly due to the differences in beam delivery methods at different centres and due also to the different methods used to measure this secondary particle dose. The potential of quantification of this mixed particle dose can be achieved through practical and simple measurements of non-ionizing energy losses (NIEL) and ionizing energy losses (IEL). The suitable sensor for NIEL measurements is a silicon PIN diode, through the development of the silicon PIN diodes the possibility of the quantification and therefore significance of the dose delivered by the primary and secondary particles can be realised. This thesis investigates the characterisation of the response of silicon PIN diodes for use in proton therapy, of particular interest is the in field forward bias response of the PIN diodes as well as their dependence on temperature and light. Monte Carlo calculations are performed using the Geant4 platform to characterise the response of the silicon PIN diodes when placed in field during proton therapy. The forward bias response of the detector when placed in field was found to be dependent on protons only, with the neutron component of the response being negligible. This allows for the possibility of characterisation of the PIN diodes on the central axis of the beam. The relative sensitivity of the PIN diodes to protons was found by dividing the forward bias response by the theoretical depth dose and it was found that the relative sensitivity of the diode is independent of the phantom material and depends only on the initial energy of the primary proton beam. Experiments are performed in order to characterise the response of the PIN diodes under various conditions. The effect on the forward bias response of the PIN diodes when exposed to visible light was examined by taking forward voltage measurements on each of the diodes both exposed and not exposed to visible light. It was found that the difference in forward bias voltage measurements with light incident on the diode and when light is blocked is no greater than the uncertainty involved in the measurement, using the dedicated forward bias voltage read – out system. In order to investigate any change in forward bias voltage across the temperature range of interest each of the PIN diodes forward bias voltage is measured over a range from 25 - 35°C. The average temperature coefficient was found to be 0.75mV/°C at 1mA and 1.8mV/°C at 20mA. Considering that the variation in room temperature was expected to be minimal (\u3c 1°C) the temperature of the diodes should differ very little over the several hours between pre and post irradiation read out. The linearity of the current source in the read-out system is tested at current values of 1, 10, 15 and 20 mA over a wide range of resistances to ensure that the current remained constant over these values. A change in the linear relationship between voltage and resistance was observed in the 10, 15 and 20 mA characteristics. This change was attributed to the fact that the current source is no longer putting out a constant value, highlighting the limit of power in the forward bias voltage read-out device

Robustness of IPSA optimized high-dose-rate prostate brachytherapy treatment plans to catheter displacements

Purpose: Inverse planning simulated annealing (IPSA) optimized brachytherapy treatment plans are characterized with large isolated dwell times at the first or last dwell position of each catheter. The potential of catheter shifts relative to the target and organs at risk in these plans may lead to a more significant change in delivered dose to the volumes of interest relative to plans with more uniform dwell times. Material and methods: This study aims to determine if the Nucletron Oncentra dwell time deviation constraint (DTDC) parameter can be optimized to improve the robustness of high-dose-rate (HDR) prostate brachytherapy plans to catheter displacements. A set of 10 clinically acceptable prostate plans were re-optimized with a DTDC parameter of 0 and 0.4. For each plan, catheter displacements of 3, 7, and 14 mm were retrospectively applied and the change in dose volume histogram (DVH) indices and conformity indices analyzed. Results: The robustness of clinically acceptable prostate plans to catheter displacements in the caudal direction was found to be dependent on the DTDC parameter. A DTDC value of 0 improves the robustness of planning target volume (PTV) coverage to catheter displacements, whereas a DTDC value of 0.4 improves the robustness of the plans to changes in hotspots. Conclusions: The results indicate that if used in conjunction with a pre-treatment catheter displacement correction protocol and a tolerance of 3 mm, a DTDC value of 0.4 may produce clinically superior plans. However, the effect of the DTDC parameter in plan robustness was not observed to be as strong as initially suspected

Kilovoltage X-ray beam dosimetry and imaging

Kilovoltage X-ray beams have a wide application in medicine for diagnostic purposes through various imaging modalities as well as therapeutic applications for cancer treatments and other clinical conditions. In addition, these low-energy X-ray units are heavily used for radiobiological research. Kilovoltage X-ray beams are used in CT scanners, mammography X-ray units, diagnostic X-rays, and on-board imagers attached to radiotherapy linear accelerators. The imaging modalities that utilize kilovoltage X-rays include diagnostic X-ray units, CT scanners, and on-board imagers on linear accelerators. The relative dominance of the photoelectric effect and its dependence of the effective atomic number of the different tissues such as bone and soft tissue within this energy range allow clear differentiation between the different tissues

Physics Contributions Robustness of IPSA optimized high-dose-rate prostate brachytherapy treatment plans to catheter displacements

Abstract Purpose: Inverse planning simulated annealing (IPSA) optimized brachytherapy treatment plans are characterized with large isolated dwell times at the first or last dwell position of each catheter. The potential of catheter shifts relative to the target and organs at risk in these plans may lead to a more significant change in delivered dose to the volumes of interest relative to plans with more uniform dwell times. Material and methods: This study aims to determine if the Nucletron Oncentra dwell time deviation constraint (DTDC) parameter can be optimized to improve the robustness of high-dose-rate (HDR) prostate brachytherapy plans to catheter displacements. A set of 10 clinically acceptable prostate plans were re-optimized with a DTDC parameter of 0 and 0.4. For each plan, catheter displacements of 3, 7, and 14 mm were retrospectively applied and the change in dose volume histogram (DVH) indices and conformity indices analyzed. Results: The robustness of clinically acceptable prostate plans to catheter displacements in the caudal direction was found to be dependent on the DTDC parameter. A DTDC value of 0 improves the robustness of planning target volume (PTV) coverage to catheter displacements, whereas a DTDC value of 0.4 improves the robustness of the plans to changes in hotspots. Conclusions: The results indicate that if used in conjunction with a pre-treatment catheter displacement correction protocol and a tolerance of 3 mm, a DTDC value of 0.4 may produce clinically superior plans. However, the effect of the DTDC parameter in plan robustness was not observed to be as strong as initially suspected

Low-dose-rate iodine-125 seed air kerma strength measurement intercomparison

2019 Purpose: The purpose of this study was to investigate the rate of compliance of air kerma strength (AKS) measurements of iodine-125 (I-125) seeds with international recommendations by departments in Australia and determine the potential impact of noncompliance. Methods and Materials: To achieve this aim, we present an intercomparison of AKS measurements for a single I-125 seed performed by 11 radiotherapy departments in Australia. Measurements were performed at two sites, with each participating department traveling to one of the two host sites and measuring the AKS using their own equipment and local protocols. Each of the AKS measurements was compared with each other and the manufacturer-certified AKS. Results: Nine of the 11 participating departments measured AKS fell within ±3% of the manufacturer\u27s calibration certificate value, whereas all participating departments measured AKS within ±5% of the manufacturer\u27s calibration certificate value. The total spread of the measured AKS among the 11 departments was 7.7%. Only two of the 11 participating departments complied with international recommendations and had their well chamber calibrated within the last 2 years. In addition, 2 of the 11 departments used a well chamber calibrated that was calibrated with a different seed model used during the intercomparison, whereas 4 of the 11 departments calibrated their well chamber in-house using a factory-calibrated seed provided by the seed manufacturer. Conclusions: A significant variation in the methods used and frequency of calibration of well chambers were observed among the participating departments. The results of this study support the international recommendations on frequency and methodology of well chamber calibration. Failure to follow these recommendations significantly increases the uncertainty in AKS measurement of I-125 seeds

In vivo dosimetry

In vivo dosimetry is an essential tool in quality-assurance programs. It is important to check the dose delivered to patients undergoing radiation treatment to avoid over or underdosing in areas in which assurance of planning accuracy may be needed. Radiochromic films are uniquely able to measure not only point doses but also two-dimensional maps of applied doses in vivo to a patient. In this section, we will briefly discuss the qualities of an ideal dosimeter and some of the processes and typical applications of radiochromic films to in vivo dosimetry techniques

HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study

Purpose This study aims to assess the accuracy of source position verification during high‐dose rate (HDR) prostate brachytherapy using a novel, in‐house developed two‐dimensional (2D) diode array (the Magic Plate), embedded exactly below the patient within a carbon fiber couch. The effect of tissue inhomogeneities on source localization accuracy is examined. Method Monte Carlo (MC) simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 toolkit. An Ir‐192 Flexisource (Isodose Control, Veenendaal, the Netherlands) was simulated inside a voxelized patient geometry, and the dose deposited in each detector of the Magic Plate evaluated. The dose deposited in each detector was then used to localize the source position using a proprietary reconstruction algorithm. Results The accuracy of source position verification using the Magic Plate embedded in the patient couch was found to be affected by the tissue inhomogeneities within the patient, with an average difference of 2.1 ± 0.8 mm (k = 1) between the Magic Plate predicted and known source positions. Recalculation of the simulations with all voxels assigned a density of water improved this verification accuracy to within 1 mm. Conclusion Source position verification using the Magic Plate during a HDR prostate brachytherapy treatment was examined using MC simulations. In a homogenous geometry (water), the Magic Plate was able to localize the source to within 1 mm, however, the verification accuracy was negatively affected by inhomogeneities; this can be corrected for by using density information obtained from CT, making the proposed tool attractive for use as a real‐time in vivo quality assurance (QA) device in HDR brachytherapy for prostate cancer

Use of deformable image registration techniques to estimate dose to organs at risk following prostate external beam radiation therapy and high-dose-rate brachytherapy

Purpose: The purpose of this investigation was to examine differences in estimates of accumulated rectal dose when using deformable image registration (DIR) compared with rigid image registration (RIR) methods, and parameter addition methods for combined transrectal ultrasound (TRUS)-based high-dose-rate brachytherapy (HDR-BT) and external beam radiation therapy (EBRT) treatments of prostate cancer. Material and methods: In this retrospective study, data from 10 patients who had previously received HDR-BT in one 15 Gy fraction, followed by 46 Gy EBRT in twenty-three fractions were used. To estimate total combined dose to the rectum, dose accumulation using both DIR and RIR methods were compared with parameter addition methods, which assume the same region of rectal anatomy receives the maximum dose from both treatment modalities. For both rigid and deformable image registration techniques, the quality of image registration was evaluated through metrics, including mean distance to agreement and dice similarity coefficient of prostate contours. Total D and D for the rectum was calculated and compared using each method. Results: The parameter addition methods predicted the highest accumulated dose to the rectum. On average, the predicted D dose was higher than that calculated by the DIR method by 6.59 Gy EQD (range, -3.03 to 13.68 Gy EQD )for partial parameter addition (PPA), and 4.88 Gy EQD (range, -3.41 to 11.97 Gy EQD ) for the full parameter addition (FPA) methods. Similarly, RIR predicted higher average doses compared with DIR, with a difference of 3.46 Gy EQD (range, -5.50 to 7.90 Gy EQD ). The results showed a significant difference between DIR and parameter addition methods for dose estimation. Conclusions: This retrospective study demonstrates significant differences in accumulated rectal dose prediction using different image registration methods. Each method has limitations in its application, and when used with realtime HDR-BT dose planning, awareness of these limitations is essential. 1cc 2cc 2cc 2 2 2 2 2

Investigation of in vivo source tracking error thresholds for interstitial and intra-cavitary high-dose-rate cervical brachytherapy

Purpose: The purpose of this study was to determine a comprehensive in vivo source tracking error thresholds in high-dose-rate (HDR) brachytherapy for cervical cancer. Achieving this enables the definition of an action level for imminent in vivo source tracking technologies and treatment monitoring devices, preventing clinically relevant changes to the applied dose. Material and methods: Retrospective HDR interstitial (n = 10) and intra-cavitary (n = 20) cervical brachytherapy patients were randomly selected to determine the feasibility of implementing in vivo source tracking error thresholds. A script was developed to displace all dwell positions in each treatment plan, along all major axes from their original position. Dose-volume histogram (DVH) indices were calculated without re-optimization of modified plans to determine the appropriate in vivo source tracking error thresholds in each direction. Results: In vivo source tracking error thresholds were directionally dependent; the smallest were found to be 2 mm in the anterior and posterior directions for both interstitial and intra-cavitary treatments. High-risk clinical treatment volume (HR-CTV) coverage was significantly impacted by displacements of 4 to 5 mm along each axis. Critically, there was a large variation in DVH metrics with displacement due to change in dwell weightings and patient anatomy. Conclusions: Determining the dosimetric impact of dwell position displacement provides a clinical benchmark for the development of pre-treatment verification devices and an action level for real-time treatment monitoring. It was established that an in vivo source tracking error threshold needs to be patient-specific. In vivo source tracking error thresholds should be determined for each patient, and can be conducted with extension of the method established in this work