AAPM Medical Physics Practice Guideline 14.a: Yttrium-90 microsphere radioembolization

Radioembolization using Yttrium-90 (90 Y) microspheres is widely used to treat primary and metastatic liver tumors. The present work provides minimum practice guidelines for establishing and supporting such a program. Medical physicists play a key role in patient and staff safety during these procedures. Products currently available are identified and their properties and suppliers summarized. Appropriateness for use is the domain of the treating physician. Patient work up starts with pre-treatment imaging. First, a mapping study using Technetium-99m (Tc-99m ) is carried out to quantify the lung shunt fraction (LSF) and to characterize the vascular supply of the liver. An MRI, CT, or a PET-CT scan is used to obtain information on the tumor burden. The tumor volume, LSF, tumor histology, and other pertinent patient characteristics are used to decide the type and quantity of 90 Y to be ordered. On the day of treatment, the appropriate dose is assayed using a dose calibrator with a calibration traceable to a national standard. In the treatment suite, the care team led by an interventional radiologist delivers the dose using real-time image guidance. The treatment suite is posted as a radioactive area during the procedure and staff wear radiation dosimeters. The treatment room, patient, and staff are surveyed post-procedure. The dose delivered to the patient is determined from the ratio of pre-treatment and residual waste exposure rate measurements. Establishing such a treatment modality is a major undertaking requiring an institutional radioactive materials license amendment complying with appropriate federal and state radiation regulations and appropriate staff training commensurate with their respective role and function in the planning and delivery of the procedure. Training, documentation, and areas for potential failure modes are identified and guidance is provided to ameliorate them.

Retrospective SPECT/CT dosimetry following transarterial radioembolization

Transarterial radioembolization (TARE) effectively treats unresectable primary and metastatic liver tumors through intra‐arterial injection of Yttrium‐90 (⁹⁰Y) beta particle emitting microspheres which implant around the tumor. Current dosimetry models are highly simplistic and there is a large need for an image‐based dosimetry post‐TARE, which would improve treatment safety and efficacy. Current post‐TARE imaging is ⁹⁰Y bremsstrahlung SPECT/CT and we study the use of these images for dosimetry. Retrospective image review of ten patients having a Philips Healthcare^TM SPECT/CT following TARE SIR‐Spheres® implantation. Emission series with attenuation correction were resampled to 3 mm resolution and used to create image‐based dose distributions. Dose distributions and analysis were performed in MIM Software SurePlan^TM utilizing SurePlan^TM Local Deposition Method (LDM) and a dose convolution method (WFBH). We sought to implement a patient‐specific background subtraction prior to dose calculation to make these noisy bremsstrahlung SPECT images suitable for post‐TARE dosimetry. On average the percentage of mean background counts to maximum count in the image across all patients was 9.4 ± 4.9% (maximum = 7.6%, minimum = 2.3%). Absolute dose increased and profile line width decreased as background subtraction value increased. The average value of the LDM and WFBH dose methods was statistically the same. As background subtraction value increased, the DVH curves become unrealistic and distorted. Background subtraction on bremsstrahlung SPECT image has a large effect on post‐TARE dosimetry. The background contour defined provides a systematic estimate to the activity background that accounts for the scanner and patient conditions at the time of the image study and is easily implemented using commercially available software. Using the mean count in the background contour as a subtraction across the entire image gave the most realistic dose distributions. This methodology is independent of microsphere and software manufacturer allowing for use with any available products or tools.

Emerging Indications for Fractionated Gamma Knife Radiosurgery

BACKGROUND

Gamma Knife radiosurgery (GKRS) allows for the treatment of intracranial tumors with a high degree of dose conformality and precision. There are, however, certain situations wherein the dose conformality of GKRS is desired, but single session treatment is contraindicated. In these situations, a traditional pin-based GKRS head frame cannot be used, as it precludes fractionated treatment.

OBJECTIVE

To report our experience in treating patients with fractionated GKRS using a relocatable, noninvasive immobilization system.

METHODS

Patients were considered candidates for fractionated GKRS if they had one or more of the following indications: a benign tumor >10 cc in volume or abutting the optic pathway, a vestibular schwannoma with the intent of hearing preservation, or a tumor previously irradiated with single fraction GKRS. The immobilization device used for all patients was the Extend system (Leksell Gamma Knife Perfexion, Elekta, Kungstensgatan, Stockholm).

RESULTS

We identified 34 patients treated with fractionated GKRS between August 2013 and February 2015. There were a total of 37 tumors treated including 15 meningiomas, 11 pituitary adenomas, 6 brain metastases, 4 vestibular schwannomas, and 1 hemangioma. At last follow-up, all 21 patients treated for perioptic tumors had stable or improved vision and all 4 patients treated for vestibular schwannoma maintained serviceable hearing. No severe adverse events were reported.

CONCLUSION

Fractionated GKRS was well-tolerated in the treatment of large meningiomas, perioptic tumors, vestibular schwannomas with intent of hearing preservation, and in reirradiation of previously treated tumors.

Guidelines by the AAPM and GEC-ESTRO on the use of innovative brachytherapy devices and applications: Report of Task Group 167

Although a multicenter, Phase III, prospective, randomized trial is the gold standard for evidence-based medicine, it is rarely used in the evaluation of innovative devices because of many practical and ethical reasons. It is usually sufficient to compare the dose distributions and dose rates for determining the equivalence of the innovative treatment modality to an existing one. Thus, quantitative evaluation of the dosimetric characteristics of innovative radiotherapy devices or applications is a critical part in which physicists should be actively involved. The physicist's role, along with physician colleagues, in this process is highlighted for innovative brachytherapy devices and applications and includes evaluation of (1) dosimetric considerations for clinical implementation (including calibrations, dose calculations, and radiobiological aspects) to comply with existing societal dosimetric prerequisites for sources in routine clinical use, (2) risks and benefits from a regulatory and safety perspective, and (3) resource assessment and preparedness. Further, it is suggested that any developed calibration methods be traceable to a primary standards dosimetry laboratory (PSDL) such as the National Institute of Standards and Technology in the U.S. or to other PSDLs located elsewhere such as in Europe. Clinical users should follow standards as approved by their country's regulatory agencies that approved such a brachytherapy device. Integration of this system into the medical source calibration infrastructure of secondary standard dosimetry laboratories such as the Accredited Dosimetry Calibration Laboratories in the U.S. is encouraged before a source is introduced into widespread routine clinical use. The American Association of Physicists in Medicine and the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology (GEC-ESTRO) have developed guidelines for the safe and consistent application of brachytherapy using innovative devices and applications. The current report covers regulatory approvals, calibration, dose calculations, radiobiological issues, and overall safety concerns that should be addressed during the commissioning stage preceding clinical use. These guidelines are based on review of requirements of the U.S. Nuclear Regulatory Commission, U.S. Department of Transportation, International Electrotechnical Commission Medical Electrical Equipment Standard 60601, U.S. Food and Drug Administration, European Commission for CE Marking (Conformité Européenne), and institutional review boards and radiation safety committees.

Computer modeling of controlled microsphere release and targeting in a representative hepatic artery system

Combating liver tumors via yttrium-90 ((90)Y) radioembolization is a viable treatment option of nonresectable liver tumors. Employing clinical (90)Y microparticles (i.e., SIR-Spheres and TheraSpheres) in a computational model of a representative hepatic artery system, laminar transient 3D particle-hemodynamics were simulated. Specifically, optimal particle release positions in the right hepatic (parent) artery as well as the best temporal release window were determined for the microspheres to exit specific outlet daughter vessels, potentially connected to liver tumors. The results illustrate the influence of a curved geometry on the velocity field and the particle trajectory dependence on the spatial and temporal particle injection conditions. The differing physical particle characteristics of the SIR-Spheres and the TheraSpheres had a subtle impact on particle trajectories in the decelerating portion of the arterial pulse, i.e., when the inertial forces on the particles are weaker. Conversely, particle characteristics and inelastic wall collisions had little effect on particles released during the accelerating phase of the arterial pulse, i.e., both types of microspheres followed organized paths to predetermined outlets. Such results begin paving the way towards directing 100% of the released microspheres to specific daughter vessels (e.g., those connected to tumors) under transient flow conditions in realistic geometries via a novel drug-particle targeting methodology.

Computer modeling of yttrium-90-microsphere transport in the hepatic arterial tree to improve clinical outcomes

PURPOSE

Radioembolization (RE) via yttrium-90 ((90)Y) microspheres is an effective and safe treatment for unresectable liver malignancies. However, no data are available regarding the impact of local blood flow dynamics on (90)Y-microsphere transport and distribution in the human hepatic arterial system.

METHODS AND MATERIALS

A three-dimensional (3-D) computer model was developed to analyze and simulate blood-microsphere flow dynamics in the hepatic arterial system with tumor. Supplemental geometric and flow data sets from patients undergoing RE were also available to validate the accuracy of the computer simulation model. Specifically, vessel diameters, curvatures, and branching patterns, as well as blood flow velocities/pressures and microsphere characteristics (i.e., diameter and specific gravity), were measured. Three-dimensional computer-aided design software was used to create the vessel geometries. Initial trials, with 10,000 noninteracting microspheres released into the hepatic artery, used resin spheres 32-microm in diameter with a density twice that of blood.

RESULTS

Simulations of blood flow subject to different branch-outlet pressures as well as blood-microsphere transport were successfully carried out, allowing testing of two types of microsphere release distributions in the inlet plane of the main hepatic artery. If the inlet distribution of microspheres was uniform (evenly spaced particles), a greater percentage would exit into the vessel branch feeding the tumor. Conversely, a parabolic inlet distribution of microspheres (more particles around the vessel center) showed a high percentage of microspheres exiting the branch vessel leading to the normal liver.

CONCLUSIONS

Computer simulations of both blood flow patterns and microsphere dynamics have the potential to provide valuable insight on how to optimize (90)Y-microsphere implantation into hepatic tumors while sparing normal tissue.

A first report of radioembolization for hepatic metastases from ocular melanoma

BACKGROUND

Ocular melanoma (OM) metastasizes to the liver and is rapidly fatal despite aggressive therapy. Yttrium-90 microspheres (radioembolization) delivered via the hepatic artery is an established and effective approach for primary and metastatic hepatic tumors, although (90)Y use in OM has not been reported previously.

METHODS

A retrospective review was performed for all patients with OM who received radioembolization at 5 centers.

RESULTS

11 patients received 12 treatments with a median activity of 1.55 GBq delivered per treatment. Toxicity was minimal, with PET/CT at 3 months posttreatment showing a response in all patients; 1 patient had a complete response.

CONCLUSIONS

Radioembolization can control hepatic metastases of OM with very few side effects.

Treatment parameters and outcome in 680 treatments of internal radiation with resin 90Y-microspheres for unresectable hepatic tumors

PURPOSE

Radioembolization (RE) using (90)Y-microspheres is an effective and safe treatment for patients with unresectable liver malignancies. Radiation-induced liver disease (RILD) is rare after RE; however, greater understanding of radiation-related factors leading to serious liver toxicity is needed.

METHODS AND MATERIALS

Retrospective review of radiation parameters was performed. All data pertaining to demographics, tumor, radiation, and outcomes were analyzed for significance and dependencies to develop a predictive model for RILD. Toxicity was scored using the National Cancer Institute Common Toxicity Criteria Adverse Events Version 3.0 scale.

RESULTS

A total of 515 patients (287 men; 228 women) from 14 US and 2 EU centers underwent 680 separate RE treatments with resin (90)Y-microspheres in 2003-2006. Multifactorial analyses identified factors related to toxicity, including activity (GBq) Selective Internal Radiation Therapy delivered (p < 0.0001), prescribed (GBq) activity (p < 0.0001), percentage of empiric activity (GBq) delivered (p < 0.0001), number of prior liver treatments (p < 0.0008), and medical center (p < 0.0001). The RILD was diagnosed in 28 of 680 treatments (4%), with 21 of 28 cases (75%) from one center, which used the empiric method.

CONCLUSIONS

There was an association between the empiric method, percentage of calculated activity delivered to the patient, and the most severe toxicity, RILD. A predictive model for RILD is not yet possible given the large variance in these data.

Resin Y90 microsphere activity measurements for liver brachytherapy

The measurement of the radioactivity administered to the patient is one of the major components of 90Y microsphere liver brachytherapy. The activity of 90Y microspheres in a glass delivery vial was measured in a dose calibrator. The calibration value to use for 90Y in the dose calibrator was verified using an activity calibration standard provided by the microsphere manufacturer. This method allowed for the determination of a consistent, reproducible local activity standard. Additional measurements were made to determine some of the factors that could affect activity measurement. The axial response of the dose calibrator was determined by the ratio of activity measurements at the bottom and center of the dose calibrator. The axial response was 0.964 for a glass shipping vial, 1.001 for a glass V-vial, and 0.988 for a polycarbonate V-vial. Comparisons between activity measurements in the dose calibrator and those using a radiation survey meter were found to agree within 10%. It was determined that the dose calibrator method was superior to the survey meter method because the former allowed better defined measurement geometry and traceability of the activity standard back to the manufacturer. Part of the preparation of resin 9()Y microspheres for patient delivery is to draw out a predetermined activity from a shipping vial and place it into a V-vial for delivery to the patient. If the drawn activity was placed in a glass V-vial, the activity measured in the dose calibrator with a glass V-vial was 4% higher than the drawn activity from the shipping vial standard. If the drawn activity was placed in a polycarbonate V-vial, the activity measured in the dose calibrator with a polycarbonate V-vial activity was 20% higher than the drawn activity from the shipping vial standard. Careful characterization of the local activity measurement standard is recommended instead of simply accepting the calibration value of the dose calibrator manufacturer.

Safety and efficacy of Y-90 microsphere treatment in patients with primary and metastatic liver cancer: the tumor selectivity of the treatment as a function of tumor to liver flow ratio

BACKGROUND

Treatment records and follow-up data on 40 patients with primary and metastatic liver malignancies who underwent a single whole-liver treatment with Y-90 resin microspheres (SIR-Spheres Sirtex Medical, Lake Forest, IL) were retrospectively reviewed. The objective of the study was to evaluate the anatomic and physiologic determinants of radiation dose distribution, and the dose response of tumor and liver toxicity in patients with liver malignancies who underwent hepatic arterial Y-90 resin microsphere treatment.

METHODS

Liver and tumor volume calculations were performed on pre-treatment CT scans. Fractional tumor and liver flow characteristics and lung shunt fractions were determined using hepatic arterial Tc-99m MAA imaging. Absorbed dose calculations were performed using the MIRD equations. Liver toxicity was assessed clinically and by liver function tests. Tumor response to therapy was assessed by CT and/or tumor markers.

RESULTS

Of the 40 patients, 5 had hepatocellular cancer (HCC), and 35 had metastatic liver tumors (15 colorectal cancer, 10 neuroendocrine tumors, 4 breast cancer, 2 lung cancer, 1 ovarian cancer, 1 endometrial cancer, and 2 unknown primary adenocarcinoma). All patients were treated in a salvage setting with a 3 to 80 week follow-up (mean: 19 weeks). Tumor volumes ranged from 15.0 to 984.2 cc (mean: 294.9 cc) and tumor to normal liver uptake ratios ranged from 2.8 to 15.4 (mean: 5.4). Average administered activity was 1.2 GBq (0.4 to 2.4 GBq). Liver absorbed doses ranged from 0.7 to 99.5 Gy (mean: 17.2 Gy). Tumor absorbed doses ranged from 40.1 to 494.8 Gy (mean: 121.5 Gy). None of the patients had clinical venoocclusive disease or therapy-induced liver failure. Seven patients (17.5 %) had transient and 7 patients (17.5 %) had persistent LFT abnormalities. There were 27 (67.5%) responders (complete response, partial response, and stable disease). Tumor response correlated with higher tumor flow ratio as measured by Tc-99m MAA imaging.

CONCLUSION

Doses up to 99.5 Gy to uninvolved liver are tolerated with no clinical venoocclusive disease or liver failure. The lowest tumor dose producing a detectable response is 40.1 Gy. The utilization of MAA-based imaging techniques to determine tumor and liver blood flow for clinical treatment planning and the calculation of administered activity may improve clinical outcomes.