Patient specific quality control for Stereotactic Ablative Body Radiotherapy (SABR): it takes more than one phantom

Study of PRESAGE® dosimeter for end-to-end 3D radiotherapy verification using an anthropomorphic phantom with bespoke dosimeter insert

Modern radiotherapy techniques have advanced and become more sophisticated. End-to-end 3D verification of the complex radiotherapy dose distribution in an anthropomorphic phantom can ensure the accuracy of the treatment delivery. The phantoms commonly used for dosimetry are homogeneous solid water phantom which lacks the capability to measure the 3D dose distribution for heterogeneous tissues necessary for advanced radiotherapy techniques. Therefore, we developed an end-to-end 3D radiotherapy dose verification system based on MAX-HD anthropomorphic phantom (Integrated Medical Technologies Inc., Troy, New York) with bespoke intracranial insert for PRESAGE® dosimeter. In this study, several advanced radiotherapy treatment techniques of various levels of complexity; 3D-CRT, IMRT and VMAT treatment, were planned for a 20 mm diameter of a spherical target in the brain region and delivered to the phantom. The dosimeters were read out using an in-house developed optical computed tomography (OCT) imaging system known as 3DmicroHD-OCT. It was found that the measured dose distribution of the PRESAGE® when compared with the measured dose distribution of EBT film and Monaco TPS has a maximum difference of less than 3% for 3D-CRT, IMRT and VMAT treatment plans. The gamma analysis results of PRESAGE® in comparison to EBT film and Monaco TPS show pass rates of more than 95% for the criteria of 3% dose difference and 3 mm distance-to-agreement. This study proves the capability of PRESAGE® and bespoke MAX-HD phantom in conjunction with the 3DmicroHD-OCT system to measure 3D dose distribution for end-to-end dosimetry verification.

A low-cost phantom design for evaluating spine SABR calculations in the presence of prosthetic vertebral stabilization

Dose-perturbation characteristics are important to consider during the calculation of radiation therapy protocols for patients who are going to receive high doses that would reach the tolerance limits of the spinal cord [1]. Several studies have investigated dose perturbations introduced by metal implants in close proximity to spine SABR treatments [2-7]. However, there is a lack of work assessing this effect using the RayStation TPS [8]. We present an initial design for a low-cost phantom to evaluate spine stereotactic ablative radiotherapy (SABR) in the presence of prosthetic vertebral stabilization. The phantom is modular, allowing the prosthetic at the centre of the phantom to be removed by exchanging the central block. It also includes space to insert ion chamber and film. The agreement of the RayStation TPS (v8.0B) collapsed cone convolution (CCC) calculation and measurement was determined for phantom versions with and without prosthetic. There was little to no change in the agreement between the measured and calculated dose when introducing metallic hardware. This suggests that our Raystation-based SABR planning approach for patients with spinal hardware meets clinical expectations. Departments without access to anthropomorphic phantoms may find this design useful but should test their phantom design in typical clinical settings to ensure it is robust to real world situations.

3D-printed patient-specific pelvis phantom for dosimetry measurements for prostate stereotactic radiotherapy with dominant intraprostatic lesion boost

AIM

Developing and assessing the feasibility of using a three-dimensional (3D) printed patient-specific anthropomorphic pelvis phantom for dose calculation and verification for stereotactic ablative radiation therapy (SABR) with dose escalation to the dominant intraprostatic lesions.

MATERIAL AND METHODS

A 3D-printed pelvis phantom, including bone-mimicking material, was fabricated based on the computed tomography (CT) images of a prostate cancer patient. To compare the extent to which patient and phantom body and bones overlapped, the similarity Dice coefficient was calculated. Modular cylindrical inserts were created to encapsulate radiochromic films and ionization chamber for absolute dosimetry measurements at the location of prostate and at the boost region. Gamma analysis evaluation with 2%/2mm criteria was performed to compare treatment planning system calculations and measured dose when delivering a 10 flattening filter free (FFF) SABR plan and a 10FFF boost SABR plan.

RESULTS

Dice coefficients of 0.98 and 0.91 were measured for body and bones, respectively, demonstrating agreement between patient and phantom outlines. For the boost plans the gamma analysis yielded 97.0% of pixels passing 2%/2mm criteria and these results were supported by the chamber average dose difference of 0.47 ± 0.03%. These results were further improved when overriding the bone relative electron density: 97.3% for the 2%/2mm gamma analysis, and 0.05 ± 0.03% for the ionization chamber average dose difference.

CONCLUSIONS

The modular patient-specific 3D-printed pelvis phantom has proven to be a highly attractive and versatile tool to validate prostate SABR boost plans using multiple detectors.

Investigation of the effects of spinal surgical implants on radiotherapy dosimetry: A study of 3D printed phantoms

PURPOSE

The use of three-dimensional (3D) printing to develop custom phantoms for dosimetric studies in radiotherapy is increasing. The process allows production of phantoms designed to evaluated specific geometries, patients, or patient groups with a defining feature. The ability to print bone-equivalent phantoms has, however, proved challenging. The purpose of this work was to 3D print a series of three similar spine phantoms containing no surgical implants, implants made of titanium, and implants made of carbon fiber, for future dosimetric and imaging studies. Phantoms were evaluated for (a) tissue and bone equivalence, (b) geometric accuracy compared to design, and (c) similarity to one another.

METHODS

Sample blocks of PLA, HIPS, and StoneFil PLA-concrete with different infill densities were printed to evaluate tissue and bone equivalence. The samples were used to develop CT to physical (PD) and effective relative electron density (RED_eff ) conversion curves and define the settings for printing the phantoms. CT scans of the printed phantoms were obtained to assess the geometry and densities achieved. Mean distance to agreement (MDA) and DICE coefficient (DSC) values were calculated between contours defining the different materials, obtained from design and like phantom modules. HU values were used to determine PD and RED_eff and subsequently evaluate tissue and bone equivalence.

RESULTS

Sample objects showed linear relationships between HU and both PD and RED_eff for both PLA and StoneFil. The PD and RED_eff of the objects calculated using clinical CT conversion curves were not accurate and custom conversion curves were required. PLA printed with 90% infill density was found to have a PD of 1.11 ± 0.03 g.cm^-3 and RED_eff of 1.04 ± 0.02 and selected for tissue- equivalent phantom elements. StoneFil printed with 100% infill density showed a PD of 1.35 ± 0.03 g.cm^-3 and RED_eff of 1.24 ± 0.04 and was selected for bone-equivalent elements. Upon evaluation of the final phantoms, the PLA elements displayed PD in the range of 1.10 ± 0.03 g.cm^-3 -1.13 ± 0.03 g.cm^-3 and RED_eff in the range of 1.02 ± 0.03-1.06 ± 0.03. The StoneFil elements showed PD in the range of 1.43 ± 0.04 g.cm^-3 -1.46 ± 0.04 g.cm^-3 and RED_eff in the range of 1.31 ± 0.04-1.33 ± 0.04. The PLA phantom elements were shown to have MDA of ≤1.00 mm and DSC of ≥0.95 compared to design, and ≤0.48 mm and ≥0.91 compared like modules. The StoneFil elements displayed MDA values of ≤0.44 mm and DSC of ≥0.98 compared to design and ≤0.43 mm and ≥0.92 compared like modules.

CONCLUSIONS

Phantoms which were radiologically equivalent to tissue and bone were produced with a high level of similarity to design and even higher level of similarity of one another. When used in conjunction with the derived CT to PD or RED_eff conversion curves they are suitable for evaluating the effects of spinal surgical implants of varying material of construction.

Credentialing of vertebral stereotactic ablative body radiotherapy in a multi-centre trial

PURPOSE/OBJECTIVE

Stereotactic ablative body radiotherapy (SABR) in multi-centre trials requires rigorous quality assurance to ensure safe and consistent treatment for all trial participants. We report results of vertebral SABR dosimetry credentialing for the ALTG/TROG NIVORAD trial.

MATERIAL/METHODS

Centres with a previous SABR site visit performed axial film measurement of the benchmarking vertebral plan in a local phantom and submitted radiochromic film images for analysis. Remaining centres had on-site review of SABR processes and axial film measurement of the vertebral benchmarking plan. Films were analysed for dosimetric and positional accuracy: gamma analysis (>90% passing 2%/2mm/10% threshold) and ≤ 1 mm positional accuracy at target-cord interface was required.

RESULTS

19 centres were credentialed; 11 had on-site measurement. Delivery devices included linear accelerator, TomoTherapy and CyberKnife systems. Five centres did not achieve 90% gamma passing rate. Of these, three were out of tolerance (OOT) in low (<5Gy) dose regions and > 80% passing rate and deemed acceptable. Two were OOT over the full dose range: one elected not to remeasure; the other also had positional discrepancy greater than 1 mm and repeat measurement with a new plan was in tolerance. The original OOT was attributed to inappropriate MLC constraints. All centres delivered planned target-cord dose gradient within 1 mm.

CONCLUSION

Credentialing measurements for vertebral SABR in a multi-centre trial showed although the majority of centres delivered accurate vertebral SABR, there is high value in independent audit measurements. One centre with inappropriate MLC settings was detected, which may have resulted in delivery of clinically unacceptable vertebral SABR plans.

A Systematic Review on 3D-Printed Imaging and Dosimetry Phantoms in Radiation Therapy

INTRODUCTION

Additive manufacturing or 3-dimensional printing has become a widespread technology with many applications in medicine. We have conducted a systematic review of its application in radiation oncology with a particular emphasis on the creation of phantoms for image quality assessment and radiation dosimetry. Traditionally used phantoms for quality assurance in radiotherapy are often constraint by simplified geometry and homogenous nature to perform imaging analysis or pretreatment dosimetric verification. Such phantoms are limited due to their ability in only representing the average human body, not only in proportion and radiation properties but also do not accommodate pathological features. These limiting factors restrict the patient-specific quality assurance process to verify image-guided positioning accuracy and/or dose accuracy in "water-like" condition.

METHODS AND RESULTS

English speaking manuscripts published since 2008 were searched in 5 databases (Google Scholar, Scopus, PubMed, IEEE Xplore, and Web of Science). A significant increase in publications over the 10 years was observed with imaging and dosimetry phantoms about the same total number (52 vs 50). Key features of additive manufacturing are the customization with creation of realistic pathology as well as the ability to vary density and as such contrast. Commonly used printing materials, such as polylactic acid, acrylonitrile butadiene styrene, high-impact polystyrene and many more, are utilized to achieve a wide range of achievable X-ray attenuation values from -1000 HU to 500 HU and higher. Not surprisingly, multimaterial printing using the polymer jetting technology is emerging as an important printing process with its ability to create heterogeneous phantoms for dosimetry in radiotherapy.

CONCLUSION

Given the flexibility and increasing availability and low cost of additive manufacturing, it can be expected that its applications for radiation medicine will continue to increase.