Phantom investigations on CT seed imaging for interstitial brachytherapy

GEC-ESTRO ACROP prostate brachytherapy guidelines

This is an evidence-based guideline for prostate brachytherapy. Throughout levels of evidence quoted are those from the Oxford Centre for Evidence based Medicine (https://www.cebm.ox.ac.uk/resources/levels-of-evidence/oxford-centre-for-evidence-based-medicine-levels-of-evidence-march-2009). Prostate interstitial brachytherapy using either permanent or temporary implantation is an established and evolving treatment technique for non-metastatic prostate cancer. Permanent brachytherapy uses Low Dose Rate (LDR) sources, most commonly I-125, emitting photon radiation over months. Temporary brachytherapy involves first placing catheters within the prostate and, on confirmation of accurate positioning, temporarily introducing the radioactive source, generally High Dose Rate (HDR) radioactive sources of Ir-192 or less commonly Co-60. Pulsed dose rate (PDR) brachytherapy has also been used for prostate cancer [1] but few centres have adopted this approach. Previous GEC ESTRO recommendations have considered LDR and HDR separately [2-4] but as there is considerable overlap, this paper provides updated guidance for both treatment techniques. Prostate brachytherapy allows safe radiation dose escalation beyond that achieved using external beam radiotherapy alone as it has greater conformity around the prostate, sparing surrounding rectum, bladder, and penile bulb. In addition there are fewer issues with changes in prostate position during treatment delivery. Systematic review and randomised trials using both techniques as boost treatments demonstrate improved PSA control when compared to external beam radiotherapy alone [5-7].

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.

Endorectal digital prostate tomosynthesis (endoDPT): a proof-of-concept study

In this study we present endorectal digital prostate tomosynthesis (endoDPT), a proposed method of high resolution prostate imaging using an endorectal x-ray sensor and an external x-ray source. endoDPT may be useful for visualizing the fine detail of small structures such as low dose rate brachytherapy (LDRBT) seeds that are difficult to visualize with current methods. The resolution of endoDPT was characterized through measurement of the modulation transfer function (MTF) and artifact spread function (ASF) in computational and physical phantoms. The qualitative resolution of endoDPT was assessed relative to computed tomography (CT) through imaging of LDRBT seeds implanted inex vivocanine prostates. The x-ray sensor MTF reached 10% at 11.50 mm^-1, the reconstruction algorithm MTF reached a maximum at 7.08 mm^-1, and the ASF was 2.5 mm (full-width at half-maximum). Fine structures in LDRBT seeds like the 0.05 mm thick shell were visible with endoDPT but not CT. All endoDPT images exhibited an overshoot artifact. The measured MTFs were consistent with other studies using similar x-ray sensors and demonstrated improved resolution compared to digital breast tomosynthesis; this result was due to the smaller endoDPT x-ray sensor detection element size and quantitatively demonstrates the high resolution of endoDPT. The ASF demonstrated worse depth resolution compared to in-plane resolution, due to partial angular sampling; partial angular sampling also caused the observed overshoot artifact in the endoDPT images. However, endoDPT still was able to visualize fine structures such as the LDRBT seed shell to a much higher degree than CT. This high-resolution visualization may be useful for improvements in patient specific LDRBT dosimetry. Overall, these results indicate endoDPT is capable of high in-plane spatial resolution and is thus well poised for optimization and studies assessing clinical utility.

Development and characterization of an anthropomorphic breast phantom for permanent breast seed implant brachytherapy credentialing

PURPOSE

To develop an anthropomorphic breast phantom for use in credentialing of permanent breast seed implant brachytherapy.

METHODS AND MATERIALS

A representative external contour and target volume was used as the basis of mold manufacturing for anthropomorphic breast phantom development. Both target and normal tissue were composed of gel-like materials that provide suitable computed tomography and ultrasound contrast for brachytherapy delivery. The phantoms were evaluated for consistency in construction (target location) and Hounsfield unit (computed tomography contrast). For both target and normal tissue, the speed of sound was measured and compared to the image reconstruction algorithm's expectation value. Five phantoms were imaged preimplant and postimplant to assess interphantom similarity as well as to evaluate the uncertainty in quantifying seed position.

RESULTS

The average Hounsfield units of the target and normal tissue gels is -146 ± 5 and 23 ± 1, respectively. The average speed of sound of the target and normal tissue gels is 1485 ± 7 m/s and 1558 ± 9 m/s, respectively, resulting in an estimated 0.4 mm uncertainty in image guidance. The registration/deformation uncertainty was determined to be 0.8 mm. The standard combined uncertainty in assessing seed position spatial accuracy, also including a 0.9 mm estimate based on literature for seed localization, is estimated to be 1.3 mm.

CONCLUSIONS

The development of the anthropomorphic breast phantom and evaluation of both the consistency as well as overall seed position uncertainty illustrates the suitability of this phantom for use in brachytherapy end-to-end delivery and implant accuracy evaluation. When evaluating a user's implant accuracy, we estimate a standard combined uncertainty of 1.3 mm.

Magnetic resonance imaging basics for the prostate brachytherapist

Magnetic resonance imaging (MRI) is increasingly being used in radiation therapy, and integration of MRI into brachytherapy in particular is becoming more common. We present here a systematic review of the basic physics and technical aspects of incorporating MRI into prostate brachytherapy. Terminology and MRI system components are reviewed along with typical work flows in prostate high-dose-rate and low-dose-rate brachytherapy. In general, the brachytherapy workflow consists of five key components: diagnosis, implantation, treatment planning (scan + plan), implant verification, and delivery. MRI integration is discussed for diagnosis; treatment planning; and MRI-guided brachytherapy implants, in which MRI is used to guide the physical insertion of the brachytherapy applicator or needles. Considerations and challenges for establishing an MRI brachytherapy program are also discussed.

Establishing implantation uncertainties for focal brachytherapy with I-125 seeds for the treatment of localized prostate cancer

BACKGROUND

The efficacy of focal continuous low dose-rate brachytherapy (CLDR-BT) for prostate cancer requires that appropriate margins are applied to ensure robust target coverage. In this study we propose a method to establish such margins by emulating a focal treatment in patients treated with CLDR-BT to the entire gland.

MATERIAL AND METHODS

In 15 patients with localized prostate cancer, prostate volumes and dominant intra-prostatic lesions were delineated on pre-treatment magnetic resonance imaging (MRI). Delineations and MRI were registered to trans-rectal ultrasound images in the operating theater. The patients received CLDR-BT treatment to the total prostate volume. The implantation consisted of two parts: an experimental focal plan covering the dominant intra-prostatic lesion (F-GTV), followed by a plan containing additional seeds to achieve entire prostate coverage. Isodose surfaces were reconstructed using follow-up computed tomography (CT). The focal dose was emulated by reconstructing seeds from the focal plan only. The distance to agreement between planned and delivered isodose surfaces and F-GTV coverage was determined to calculate the margin required for robust treatment.

RESULTS

If patients had been treated only focally, the target volume would have been reduced from an average of 40.9 cm3 for the entire prostate to 5.8 cm3 for the focal plan. The D90 for the F-GTV in the focal plan was 195±60 Gy, the V100 was 94% [range 71-100%]. The maximum distance (cd95) between the planned and delivered isodose contours was 0.48 cm.

CONCLUSIONS

This study provides an estimate of 0.5 cm for the margin required for robust coverage of a focal target volume prior to actually implementing a focal treatment protocol.

Review of clinical brachytherapy uncertainties: analysis guidelines of GEC-ESTRO and the AAPM

Background and purpose

A substantial reduction of uncertainties in clinical brachytherapy should result in improved outcome in terms of increased local control and reduced side effects. Types of uncertainties have to be identified, grouped, and quantified.

Methods

A detailed literature review was performed to identify uncertainty components and their relative importance to the combined overall uncertainty.

Results

Very few components (e.g., source strength and afterloader timer) are independent of clinical disease site and location of administered dose. While the influence of medium on dose calculation can be substantial for low energy sources or non-deeply seated implants, the influence of medium is of minor importance for high-energy sources in the pelvic region. The level of uncertainties due to target, organ, applicator, and/or source movement in relation to the geometry assumed for treatment planning is highly dependent on fractionation and the level of image guided adaptive treatment. Most studies to date report the results in a manner that allows no direct reproduction and further comparison with other studies. Often, no distinction is made between variations, uncertainties, and errors or mistakes. The literature review facilitated the drafting of recommendations for uniform uncertainty reporting in clinical BT, which are also provided. The recommended comprehensive uncertainty investigations are key to obtain a general impression of uncertainties, and may help to identify elements of the brachytherapy treatment process that need improvement in terms of diminishing their dosimetric uncertainties. It is recommended to present data on the analyzed parameters (distance shifts, volume changes, source or applicator position, etc.), and also their influence on absorbed dose for clinically-relevant dose parameters (e.g., target parameters such as D₉₀ or OAR doses). Publications on brachytherapy should include a statement of total dose uncertainty for the entire treatment course, taking into account the fractionation schedule and level of image guidance for adaptation.

Conclusions

This report on brachytherapy clinical uncertainties represents a working project developed by the Brachytherapy Physics Quality Assurances System (BRAPHYQS) subcommittee to the Physics Committee within GEC-ESTRO. Further, this report has been reviewed and approved by the American Association of Physicists in Medicine.

CT- and MRI-based seed localization in postimplant evaluation after prostate brachytherapy

PURPOSE

To compare the uncertainties in CT- and MRI-based seed reconstruction in postimplant evaluation after prostate seed brachytherapy in terms of interobserver variability and quantify the impact of seed detection variability on a selection of dosimetric parameters for three postplan techniques: (1) CT, (2) MRI-T1 weighted fused with MRI-T2 weighted, and (3) CT fused with MRI-T2 weighted.

METHODS AND MATERIALS

Seven physicists reconstructed the seed positions on postimplant CT and MRI-T1 images of three patients. For each patient and imaging modality, the interobserver variability was calculated with respect to a reference seed set. The effect of this variability on dosimetry was calculated for CT and CT + MRI-T2 (CT-based seed reconstruction), as well as for MRI-T1 + MRI-T2 (MRI-T1-based seed reconstruction), using fixed CT and MRI-T2 prostate contours.

RESULTS

Averaged over three patients, the interobserver variability in CT-based seed reconstruction was 1.1 mm (1 SDref, i.e., standard deviation with respect to the reference value). The D90 (dose delivered to 90% of the target) variability was 1.5% and 1.3% (1 SDref) for CT and CT + MRI-T2, respectively. The mean interobserver variability in MRI-based seed reconstruction was 3.0 mm (1 SDref), and the impact of this variability on D90 was 6.6% for MRI-T1 + MRI-T2.

CONCLUSIONS

Seed reconstruction on MRI-T1-weighted images was less accurate than on CT. This difference in uncertainties should be weighted against uncertainties due to contouring and image fusion when comparing the overall reliability of postplan techniques.

Prostate post-implant dosimetry: interobserver variability in seed localisation, contouring and fusion

AIM

Reliable post-implant evaluation of prostate seed implants requires optimal seed identification and accurate delineation of anatomical structures. In this study the GEC-ESTRO groups BRAPHYQS and PROBATE investigated the interobserver variability in post-implant prostate contouring, seed reconstruction and image fusion and its impact on the dose-volume parameters.

MATERIALS

Post-implant T2-TSE, T1-GE and CT images were acquired for three patients, in order to evaluate four post-plan techniques: (a) CT, (b) T1+T2, (c) CT+T2, (d) CT+T1(int)+T2. Three interobserver studies were set up. (1) Contouring: the CTV-prostate was delineated on CT and T2 by eight physicians. Additionally one reference contour was defined on both image modalities for each patient. (2) Seed reconstruction: seven physicists localised the seeds on T1 and CT, manually and with CT seed finder tools. A reference seed geometry was defined on CT and T1. (3) Fusion: six physicists registered the image sets for technique (b)-(d), using seeds (if visible) and anatomical landmarks. A reference fusion was determined for each combined technique.

RESULTS

(1) The SD(ref) for contouring (1 SD with respect to the reference volume) was largest for CT (23%), but also surprisingly large for MRI (17%). This resulted in large SD(ref) values for D90 for all techniques (17-23%). The surprisingly large SD(ref) for MRI was partly due to variations in interpretation of what to include in the prostate contour. (2) The SD(ref) in D90 for seed reconstruction was small (2%) for all techniques, except for T1+T2 (7%). (3) The SD(ref) in D90 due to image fusion was quite large, especially for direct fusion of CT+T2 (16%) where clearly corresponding landmarks were missing (seeds hardly visible on T2). In general, we observed large differences in D90 depending on the technique used.

CONCLUSIONS

The dosimetric parameters for prostate post-implant evaluation showed large technique-dependent interobserver variabilities. Contouring and image fusion are the 'weak links' in the procedure. Guidelines and training in contouring together with incorporation of automated fusion software need to be implemented.

Image guided, adaptive, accelerated, high dose brachytherapy as model for advanced small volume radiotherapy

Brachytherapy has consistently provided a very conformal radiation therapy modality. Over the last two decades this has been associated with significant improvements in imaging for brachytherapy applications (prostate, gynecology), resulting in many positive advances in treatment planning, application techniques and clinical outcome. This is emphasized by the increased use of brachytherapy in Europe with gynecology as continuous basis and prostate and breast as more recently growing fields. Image guidance enables exact knowledge of the applicator together with improved visualization of tumor and target volumes as well as of organs at risk providing the basis for very individualized 3D and 4D treatment planning. In this commentary the most important recent developments in prostate, gynecological and breast brachytherapy are reviewed, with a focus on European recent and current research aiming at the definition of areas for important future research. Moreover the positive impact of GEC-ESTRO recommendations and the highlights of brachytherapy physics are discussed what altogether presents a full overview of modern image guided brachytherapy. An overview is finally provided on past and current international brachytherapy publications focusing on "Radiotherapy and Oncology". These data show tremendous increase in almost all research areas over the last three decades strongly influenced recently by translational research in regard to imaging and technology. In order to provide high level clinical evidence for future brachytherapy practice the strong need for comprehensive prospective clinical research addressing brachytherapy issues is high-lighted.

Objective automated assessment of time trends in prostate edema after (125)I implantation

PURPOSE

To present an objective automated method to determine time trends in prostatic edema resulting from iodine-125 brachytherapy.

METHODS AND MATERIALS

We followed 20 patients, implanted with stranded seeds, with seven consecutive CT scans to establish a time trend in prostate edema. Seed positions were obtained automatically from the CT series. The change in seed positions was used as surrogate for edema. Two approaches were applied to model changes in volume. (1) A cylindrical model: seeds from the compared distribution were linked to the reference distribution of Day 28. After alignment, the compared distribution was scaled in cylindrical coordinates, leading to the changes in radial and craniocaudal directions. The volume changes were calculated using these scaling factors. (2) A spherical model: distances of seeds to the center of gravity of all seeds were used as a measure to model volume changes.

RESULTS

With Day 28 as reference, the observed volume changes were smaller than 18% ± 6% (1 standard deviation) for the cylindrical model and 12% ± 7% for the spherical model. One day after implantation, the implanted prostate was less than 10% larger than in the reference scan for both models. Apart from Day 0, both models showed similar volume changes.

CONCLUSIONS

We present an objective automated method to determine changes in the implanted prostate volume, eliminating the influence of an observer in the assessment of the prostate size. The implanted volume change was less than 18% ± 7% for the studied group of 20 patients. Edema was 9% ± 5% from 1 day after implantation onward.

AAPM recommendations on dose prescription and reporting methods for permanent interstitial brachytherapy for prostate cancer: report of Task Group 137

During the past decade, permanent radioactive source implantation of the prostate has become the standard of care for selected prostate cancer patients, and the techniques for implantation have evolved in many different forms. Although most implants use 125I or 103Pd sources, clinical use of 131Cs sources has also recently been introduced. These sources produce different dose distributions and irradiate the tumors at different dose rates. Ultrasound was used originally to guide the planning and implantation of sources in the tumor. More recently, CT and/or MR are used routinely in many clinics for dose evaluation and planning. Several investigators reported that the tumor volumes and target volumes delineated from ultrasound, CT, and MR can vary substantially because of the inherent differences in these imaging modalities. It has also been reported that these volumes depend critically on the time of imaging after the implant. Many clinics, in particular those using intraoperative implantation, perform imaging only on the day of the implant. Because the effects of edema caused by surgical trauma can vary from one patient to another and resolve at different rates, the timing of imaging for dosimetry evaluation can have a profound effect on the dose reported (to have been delivered), i.e., for the same implant (same dose delivered), CT at different timing can yield different doses reported. Also, many different loading patterns and margins around the tumor volumes have been used, and these may lead to variations in the dose delivered. In this report, the current literature on these issues is reviewed, and the impact of these issues on the radiobiological response is estimated. The radiobiological models for the biological equivalent dose (BED) are reviewed. Starting with the BED model for acute single doses, the models for fractionated doses, continuous low-dose-rate irradiation, and both homogeneous and inhomogeneous dose distributions, as well as tumor cure probability models, are reviewed. Based on these developments in literature, the AAPM recommends guidelines for dose prescription from a physics perspective for routine patient treatment, clinical trials, and for treatment planning software developers. The authors continue to follow the current recommendations on using D90 and V100 as the primary quantitles, with more specific guidelines on the use of the imaging modalities and the timing of the imaging. The AAPM recommends that the postimplant evaluation should be performed at the optimum time for specific radionuclides. In addition, they encourage the use of a radiobiological model with a specific set of parameters to facilitate relative comparisons of treatment plans reported by different institutions using different loading patterns or radionuclides.