In vivo dosimetry in gynecological applications-A feasibility study

The clinical application of in vivo dosimetry for gynaecological brachytherapy: A scoping review

Brachytherapy is a key treatment for gynaecological malignancies, delivering high doses to the tumour volume whilst sparing nearby normal tissues due to its steep dose gradient. Accuracy is imperative as small shifts can lead to clinically significant under- or over-dosing of the target volume or organs at risk (OARs), respectively. Independent verification of dose delivered during brachytherapy is not routinely performed but it is important to identify gross errors and define action thresholds to guide inter-fraction treatment decisions. In vivo dosimetry (IVD) is one strategy for improving accuracy and identifying potential errors. Despite promising phantom work, clinical application of IVD is lacking. A literature search was performed using Medline and EMBASE without date limits and based on the PICO framework to evaluate the clinical application of IVD in gynaecological brachytherapy. After screening of titles and abstracts, full text papers were reviewed and 28 studies were identified. Several dosimeters were utilised and measurements were typically taken from the rectum, bladder, vagina and within interstitial catheters. Significant differences between calculated and measured dose were attributed to geometric shifts. The studies reviewed demonstrated the feasibility of IVD in brachytherapy for dose verification but further work is required before IVD can be used to optimise treatment. The purpose of this scoping review is to investigate the clinical application of IVD in gynaecological brachytherapy, understand its challenges and identify the steps required to facilitate integration into everyday clinical practice.

In vivo dosimetry with an inorganic scintillation detector during multi-channel vaginal cylinder pulsed dose-rate brachytherapy: Dosimetry for pulsed dose-rate brachytherapy

Background and purpose

In vivo dosimetry is not standard in brachytherapy and some errors go undetected. The aim of this study was to evaluate the accuracy of multi-channel vaginal cylinder pulsed dose-rate brachytherapy using in vivo dosimetry.

Materials and methods

In vivo dosimetry data was collected during the years 2019-2022 for 22 patients (32 fractions) receiving multi-channel cylinder pulsed dose-rate brachytherapy. An inorganic scintillation detector was inserted in a cylinder channel. Each fraction was analysed as independent data sets. In vivo dosimetry-based source-tracking was used to determine the relative source-to-detector position. Measured dose was compared to planned and re-calculated source-tracking based doses. Assuming no change in organ and applicator geometry throughout treatment, the planned and source-tracking based dose distributions were compared in select volumes via γ-index analysis and dose-volume-histograms.

Results

The mean ± SD planned vs. measured dose deviations in the first pulse were 0.8 ± 5.9 %. In 31/32 fractions the deviation was within the combined in vivo dosimetry uncertainty (averaging 9.7 %, k = 2) and planning dose calculation uncertainty (1.6 %, k = 2). The dwell-position offsets were < 2 mm for 88 % of channels, with the largest being 5.1 mm (4.0 mm uncertainty, k = 2). 3 %/2 mm γ pass-rates averaged 97.0 % (clinical target volume (CTV)), 100.0 % (rectum), 99.9 % (bladder). The mean ± SD deviation was -1. 1  ± 2.9 % for CTV D98, and -0.2 ± 0.9 % and -1.2 ± 2.5 %, for bladder and rectum D2cm3 respectively, indicating good agreement between intended and delivered dose.

Conclusions

In vivo dosimetry verified accurate and stable dose delivery in multi-channel vaginal cylinder based pulsed dose-rate brachytherapy.

Feasibility of internal-source tracking with C-arm CT/SPECT imaging with limited-angle projection data for online in vivo dose verification in brachytherapy: A Monte Carlo simulation study

PURPOSE

The current protocol for use of the image-guided adaptive brachytherapy (IGABT) procedure entails transport of a patient between the treatment room and the 3-D tomographic imaging room after implantation of the applicators in the body, which movement can cause position displacement of the applicator. Moreover, it is not possible to track 3-D radioactive source movement inside the body, even though there can be significant inter- and intra-fractional patient-setup changes. In this paper, therefore, we propose an online single-photon emission computed tomography (SPECT) imaging technique with a combined C-arm fluoroscopy X-ray system and attachable parallel-hole collimator for internal radioactive source tracking of every source position in the applicator.

METHODS AND MATERIALS

In the present study, using Geant4 Monte Carlo (MC) simulation, the feasibility of high-energy gamma detection with a flat-panel detector for X-ray imaging was assessed. Further, a parallel-hole collimator geometry was designed based on an evaluation of projection image quality for a 192Ir point source, and 3-D limited-angle SPECT-image-based source-tracking performances were evaluated for various source intensities and positions.

RESULTS

The detector module attached to the collimator could discriminate the 192Ir point source with about 3.4% detection efficiency when including the total counts in the entire deposited energy region. As the result of collimator optimization, hole size, thickness, and length were determined to be 0.5, 0.2, and 45 mm, respectively. Accordingly, the source intensities and positions also were successfully tracked with the 3-D SPECT imaging system when the C-arm was rotated within 110° in 2 seconds.

CONCLUSIONS

We expect that this system can be effectively implemented for online IGABT and in vivo patient dose verification.

In vivo dosimetry in pelvic brachytherapy

ADVANCES IN KNOWLEDGE

This paper describes the potential role for in vivo dosimetry in the reduction of uncertainties in pelvic brachytherapy, the pertinent factors for consideration in clinical practice, and the future potential for in vivo dosimetry in the personalisation of brachytherapy.

In Vivo Verification of Treatment Source Dwell Times in Brachytherapy of Postoperative Endometrial Carcinoma: A Feasibility Study

(1) Background: In brachytherapy, there are still many manual procedures that can cause adverse events which can be detected with in vivo dosimetry systems. Plastic scintillator dosimeters (PSD) have interesting properties to achieve this objective such as real-time reading, linearity, repeatability, and small size to fit inside brachytherapy catheters. The purpose of this study was to evaluate the performance of a PSD in postoperative endometrial brachytherapy in terms of source dwell time accuracy. (2) Methods: Measurements were carried out in a PMMA phantom to characterise the PSD. Patient measurements in 121 dwell positions were analysed to obtain the differences between planned and measured dwell times. (3) Results: The repeatability test showed a relative standard deviation below 1% for the measured dwell times. The relative standard deviation of the PSD sensitivity with accumulated absorbed dose was lower than 1.2%. The equipment operated linearly in total counts with respect to absorbed dose and also in count rate versus absorbed dose rate. The mean (standard deviation) of the absolute differences between planned and measured dwell times in patient treatments was 0.0 (0.2) seconds. (4) Conclusions: The PSD system is useful as a quality assurance tool for brachytherapy treatments.

Quality Assurance in Modern Gynecological HDR-Brachytherapy (Interventional Radiotherapy): Clinical Considerations and Comments

Simple Summary

This is a focused review discussing quality assurance during interventional brachytherapy in gynecological cancers. This topic is very large and is usually addressed from the technical and physical sides, therefore, we decided to select “hot-spots” under this large title and discuss them from the point of view of clinicians. We hope that this concise and focused review will help clinicians in improving their quality assurance protocols and draw attention to the discussed issues.

Abstract

The use of brachytherapy (interventional radiotherapy) in the treatment of gynecological cancers is a crucial element in both definitive and adjuvant settings. The recent developments in high-dose rate remote afterloaders, modern applicators, treatment-planning software, image guidance, and dose monitoring systems have led to improvement in the local control rates and in some cases improved the survival rates. The development of these highly advanced and complicated treatment modalities has been accompanied by challenges, which have made the existence of quality assurance protocols a must to ensure the integrity of the treatment process. Quality assurance aims at standardizing the technical and clinical procedures involved in the treatment of patients, which could eventually decrease the source of uncertainties whether technical (source/equipment related) or clinical. This commentary review sheds light (from a clinical point of view) on some potential sources of uncertainties associated with the use of modern brachytherapy in the treatment of gynecological cancers.

A review of brachytherapy physical phantoms developed over the last 20 years: clinical purpose and future requirements

Within the brachytherapy community, many phantoms are constructed in-house, and less commercial development is observed as compared to the field of external beam. Computational or virtual phantom design has seen considerable growth; however, physical phantoms are beneficial for brachytherapy, in which quality is dependent on physical processes, such as accuracy of source placement. Focusing on the design of physical phantoms, this review paper presents a summary of brachytherapy specific phantoms in published journal articles over the last twenty years (January 1, 2000 - December 31, 2019). The papers were analyzed and tabulated by their primary clinical purpose, which was deduced from their associated publications. A substantial body of work has been published on phantom designs from the brachytherapy community, but a standardized method of reporting technical aspects of the phantoms is lacking. In-house phantom development demonstrates an increasing interest in magnetic resonance (MR) tissue mimicking materials, which is not yet reflected in commercial phantoms available for brachytherapy. The evaluation of phantom design provides insight into the way, in which brachytherapy practice has changed over time, and demonstrates the customised and broad nature of treatments offered.

Two-dimensional real-time quality assurance dosimetry system using μ-Al2O3:C,Mg radioluminescence films

BACKGROUND AND PURPOSE

There is a continual need for more accurate and effective dosimetric systems for quality assurance (QA) as radiotherapy evolves in complexity. The purpose of this project was to introduce a new system that minimally perturbs the main beam, while assessing its real time 2D dose-rate and field shapes. The system combined reusability, linear dose-rate response, and high spatial and time resolution in a single radiation detection technology that can be applied to surface dose estimation and QA.

MATERIALS AND METHODS

We developed a 2D prototype system consisting of a camera, focusing lenses and short pass filter, placed on the head of a linear accelerator, facing an Al2O3:C,Mg radioluminescent film. To check the appropriateness of multi-leaf collimator, stability/reproducibility QA tests were prepared using the treatment planning system: including the routinely used alternating leaves, chair and pyramid checks.

RESULTS

The Al2O3:C,Mg film did not perturb the dose vs. depth dose curves determined with a point detector (-0.5% difference). Our results showed a dose-rate linear film response (R2 = 0.999), from 5 to 600 MU/min. Measured output factors agreed with reference data within ~1%, indicating a potential for small field dosimetry. Both chair and pyramid measured profiles were comparable with those obtained with the treatment planning system within 1%. The alternating leaves test showed an average discrepancy in the valleys of 14%.

CONCLUSIONS

The prototype demonstrated promising results. It obviated the need for corrections regarding the relative position of the camera, confirming accurate dose-rate delivery and detection of radiation fields.

In vivo dosimetry in brachytherapy: Requirements and future directions for research, development, and clinical practice

Brachytherapy can deliver high doses to the target while sparing healthy tissues due to its steep dose gradient leading to excellent clinical outcome. Treatment accuracy depends on several manual steps making brachytherapy susceptible to operational mistakes. Currently, treatment delivery verification is not routinely available and has led, in some cases, to systematic errors going unnoticed for years. The brachytherapy community promoted developments in in vivo dosimetry (IVD) through research groups and small companies. Although very few of the systems have been used clinically, it was demonstrated that the likelihood of detecting deviations from the treatment plan increases significantly with time-resolved methods. Time–resolved methods could interrupt a treatment avoiding gross errors which is not possible with time-integrated dosimetry. In addition, lower experimental uncertainties can be achieved by using source-tracking instead of direct dose measurements. However, the detector position in relation to the patient anatomy remains a main source of uncertainty. The next steps towards clinical implementation will require clinical trials and systematic reporting of errors and near-misses. It is of utmost importance for each IVD system that its sensitivity to different types of errors is well understood, so that end-users can select the most suitable method for their needs. This report aims to formulate requirements for the stakeholders (clinics, vendors, and researchers) to facilitate increased clinical use of IVD in brachytherapy. The report focuses on high dose-rate IVD in brachytherapy providing an overview and outlining the need for further development and research.

Review of strategies for MRI based reconstruction of endocavitary and interstitial applicators in brachytherapy of cervical cancer

Brachytherapy plays an essential role in the curative intent management of locally advanced cervical cancer. The introduction of the magnetic resonance (MR) as a preferred image modality and the development of new type of applicators with interstitial components have further improved its benefits. The aim of this work is to review the current status of one important aspect in the cervix cancer brachytherapy procedure, namely catheter reconstruction. MR compatible intracavitary and interstitial applicators are described. Considerations about the use of MR imaging (MRI) regarding appropriate strategies for applicator reconstruction, technical requirements, MR sequences, patient preparation and applicator commissioning are included. It is recommendable to perform the reconstruction process in the same image study employed by the physician for contouring, that is, T2 weighted (T2W) sequences. Nevertheless, a clear identification of the source path inside the catheters and the applicators is a challenge when using exclusively T2W sequences. For the intracavitary component of the implant, sometimes the catheters may be filled with some substance that produces a high intensity signal on MRI. However, this strategy is not feasible for plastic tubes or titanium needles, which, moreover, induce magnetic susceptibility artifacts. In these situations, the use of applicator libraries available in the treatment planning system (TPS) is useful, since they not only include accurate geometrical models of the intracavitary applicators, but also recent developments have made possible the implementation of the interstitial component. Another strategy to improve the reconstruction process is based on the incorporation of MR markers, such as small pellets, to be used as anchor points. Many institutions employ computed tomography (CT) as a supporting image modality. The registration of CT and MR image sets should be carefully performed, and its uncertainty previously assessed. Besides, an important research work is being carried out regarding the use of ultrasound and electromagnetic tracking technologies.