Optimization of treatment isocenter location in single-isocenter LINAC-based stereotactic radiosurgery for management of multiple brain metastases

Single-Isocenter, Multiple-Target Abdominal Cone-Beam Computed Tomography (CBCT)-Guided Online Adaptive Stereotactic Body Radiotherapy (SBRT)

Stereotactic body radiotherapy (SBRT) is increasingly being prescribed for treating patients with multiple metastases, especially in the setting of oligometastatic disease. Treating multiple targets presents unique challenges in radiotherapy planning and delivery, including practical considerations relating to treatment time, resource allocation, and treatment planning complexity. Treating targets in a common isocenter reduces the time required for treatment and simplifies planning, but historically, it has often not been feasible due to inter- and intra-fractional variation in relative target positions. With online adaptation, individual targets can be re-contoured on each treatment fraction to obviate inter-fractional variation, and with appropriate margin selection intra-fractional motion can be managed. In this case report, we describe single-isocenter, multiple-target treatment via online adaptation of a 93-year-old man with a history of metastatic hepatocellular carcinoma. He initially presented with a 9.1 cm liver mass, suspicious lung lesions, and an enlarged porta hepatis lymph node, which were biopsy proven to be hepatocellular carcinoma. Following 18 months of systemic immunotherapy, he demonstrated a favorable response, including a reduction in primary liver mass to 5.1 cm and resolution of pulmonary lesions; however, recent serial imaging demonstrated oligoprogression of two peripancreatic lymph node conglomerates that were biopsy proven to be poorly differentiated carcinoma. The patient was offered adaptive SBRT to a dose of 35-40 Gy in five fractions as a consolidative approach for treating both the primary liver mass and oligoprogressive lymph nodes. He tolerated treatment without any grade 2 or higher acute toxicity and had stable disease on three-month post-treatment imaging. By leveraging online adaptation, especially for the daily re-definition of target volumes, we were able to treat three targets in the abdomen accurately in a common isocenter. Treating in this manner vastly shortened and simplified the patient's radiation course. Quantitative evaluation of re-contoured targets and post-treatment imaging highlighted the value of online adaption with careful margin specification and alignment instructions.

A multi-centre stereotactic radiosurgery planning study of multiple brain metastases using isocentric linear accelerators with 5 and 2.5 mm width multi-leaf collimators, CyberKnife and Gamma Knife

Objectives

This study compared plans of high definition (HD), 2.5 mm width multi-leaf collimator (MLC), to standard, 5 mm width, isocentric linear accelerator (linacs), CyberKnife (CK), and Gamma Knife (GK) for stereotactic radiosurgery (SRS) techniques on multiple brain metastases.

Methods

Eleven patients undergoing SRS for multiple brain metastases were chosen. Targets and organs at risk (OARs) were delineated and optimized SRS plans were generated and compared.

Results

The linacs delivered similar conformity index (CI) values, but the gradient index (GI) for HD MLCs was significantly lower (P-value <.001). Half the OARs received significantly lower dose using HD MLCs. CK delivered a significantly lower CI than HD MLC linac (P-value <.001), but a significantly higher GI (P-value <.001). CI was significantly improved with the HD MLC linac compared to GK (P-value = 4.591 × 10-3), however, GK delivered a significantly lower GI (P-value <.001). OAR dose sparing was similar for the HD MLC TL, CK, and GK.

Conclusions

Comparing linacs for SRS, the preferred choice is HD MLCs. Similar results were achieved with the HD MLC linac, CK, or GK, with each delivering significant improvements in different aspects of plan quality.

Advances in knowledge

This article is the first to compare HD and standard width MLC linac plans using a combination of single isocentre volumetric modulated arc therapy and multi-isocentric dynamic conformal arc plans as required, which is a more clinically relevant assessment. Furthermore, it compares these plans with CK and GK, assessing the relative merits of each technique.

Innovative margin design and optimized isocenter to minimize the normal tissue in target volumes for single-isocenter multi-target stereotactic radiosurgery

Objective.Treating multiple brain metastases in a single plan is a popular radiosurgery technique. However, targets positioned off-isocenter are subject to rotational uncertainties. This work introduces two new planning target volumes (PTVs) that address this increased uncertainty. The volume of normal tissue included in these PTVs when paired with optimized isocenters are evaluated and compared with conventional methods.Approach.Sets of 1000 random multi-target radiosurgery patients were simulated, each patient with a random number of spherical targets (2-10). Each target had a random volume (0.1-15 cc) and was randomly positioned between 5 and 50 mm or 100 mm from isocenter. Two new PTVs ('LensPTV' and 'SwipePTV') and conventional isotropic PTVs were created using isocenters derived from the center-of-centroids, the center-of-mass, or optimized per PTV type. The total volume of normal tissue in the PTVs for each patient was calculated and compared using 1 mm translations and 0.5°, 1.0°, and 2.0° rotations.Main results.Using the new PTVs and/or using optimized isocenters decreased the total volume of normal tissue in the PTVs per patient. The SwipePTV, in particular, provided the greatest decrease. Compared to the SwipePTV, the LensPTV and the conventional isotropic PTV included an extra 0.68 and 0.73 cc of normal tissue per patient (median), respectively, when using 50 mm max distance to isocenter and 1° max rotation angle. Under these conditions, 25% of patients had extra volume of normal tissue ≥ 0.96 and 1.04 cc. When using 100 mm max distance to isocenter and 2° max rotation angle, 25% of patients had extra volume of normal tissue ≥ 4.35 and 5.75 cc.Significance.PTVs like those presented here, especially when paired with optimized isocenters, can decrease the total volume of included normal tissue and reduce the risk of toxicity without compromising target coverage.

Optimization of isocenter position for multiple brain metastases single-isocenter stereotactic radiosurgery to minimize dosimetric variations due to rotational uncertainty

PURPOSE

This paper studied a novel calculation framework that can determine the optimal value isocenter position of single isocenter SRS treatment plan for multiple brain metastases, in order to minimize the dosimetric variations caused by rotational uncertainty.

MATERIALS AND METHODS

21 patients with 2-4 GTVswho received SRS treatment for multiple brain metastases in our institution were selected for the retrospective study. The PTVwas obtained by expanding GTV 1 mm isotropic margin. We studied a stochastic optimization framework, which determined the optimal value isocenter location by maximizing the average target dose coverageC_target,meanwith a rotation error of no more than 1°. We evaluated the performance of the optimal isocenter by comparing theC_target,meanand average dice similarity coefficient (DSC)with the optimal value and the center of mass (CM) respectively as the treatment isocenter. The extra PTV margin to achieve 100% target dose coverage was calculated by our framework.

RESULTS

Compared to the CM method, the optimal value isocenter method increased the averageC_target,meanof all targets from 97.0% to 97.7%and the average DSC from 0.794to 0.799. Throughout all the cases, the average extra PTV margin to obtain full target dose coverage was 0.7 mmwhen using the optimal value isocenter as the treatment isocenter.

CONCLUSION

We studied a novel computational framework using stochastic optimization to determine the optimal isocenter position of SRS treatment plan for multiple brain metastases. At the same time, our framework gave the extra PTV margin to obtain full target dose coverage.

Comparison of dosimetric parameters and robustness for rotational errors in fractionated stereotactic irradiation using automated noncoplanar volumetric modulated arc therapy for patients with brain metastases: single- versus multi-isocentric technique

To compare the dosimetric parameters of automated noncoplanar volumetric modulated arc therapy plans using single-isocentric (SIC) and multi-isocentric (MIC) techniques for patients with two brain metastases (BMs) in stereotactic irradiation and to evaluate the robustness of rotational errors. The SIC and MIC plans were retrospectively generated (35 Gy/five fractions) for 58 patients. Subsequently, a receiver operating characteristic curve analysis between the tumor surface distance (TSD) and V_25Gy was performed to determine the thresholds for the brain tissue. The SIC and MIC plans were recalculated based on the rotational images to evaluate the dosimetric impact of rotational error. The MIC plans showed better brain tissue sparing for TSD > 6.6 cm. The SIC plans provided a significantly better conformity index for TSD ≤ 6.6 cm, while significantly lower gradient index was obtained (3.22 ± 0.56vs. 3.30 ± 0.57, p < 0.05) in the MIC plans with TSD > 6.6 cm. For organs at risk (OARs) (brainstem, chiasm, lens, optic nerves, and retinas), D_0.1 cc was significantly lower (p < 0.05) in the MIC plans than in the SIC plans. The prescription dose could be delivered (D_99%) to the gross tumor volume (GTV) for patients with TSD ≤ 6.6 cm when the rotational error was < 1°, whereas 31% of the D_99% of GTV fell below the prescription dose with TSD > 6.6 cm. MIC plans can be an optimal approach for reducing doses to OARs and providing robustness against rotational errors in BMs with TSD > 6.6 cm.

Optimization of isocenter position for multiple targets with nonuniform-margin expansion

PURPOSE

The single isocenter for multiple-target (SIMT) technique has become a popular treatment technique for multiple brain metastases. We have implemented a method to obtain a nonuniform margin for SIMT technique. In this study, we further propose a method to determine the isocenter position so that the total expanded margin volume is minimal.

MATERIALS AND METHOD

Based on a statistical model, the relationship between nonuniform margin and the distance d (from isocenter to target point), setup uncertainties, and significance level was established. Due to the existence of rotational error, there is a nonlinear relationship between the margin volume and the isocenter position. Using numerical simulation, we study the relationship between optimal isocenter position and translational error, rotational error, and target size. In order to find the optimal isocenter position quickly, adaptive simulated annealing (ASA) algorithm was used. This method was implemented in the Pinnacle3 treatment planning system and compared with isocenter at center-of-geometric (COG), center-of-volume (COV), and center-of-surface (COS). Ten patients with multiple brain metastasis targets treated with the SIMT technique was selected for evaluation.

RESULTS

When the size of tumors is equal, the optimal isocenter obtained by ASA and numerical simulation coincides with COG, COV, and COS. When the size of tumors is different, the optimal isocenter is close to the large tumor. The position of COS point is closer to the optimal point than the COV point for nearly all cases. Moreover, in some cases the COS point can be approximately selected as the optimal point. The ASA algorithm can reduce the calculating time from several hours to tens of seconds for three or more tumors. Using multiple brain metastases targets, a series of volume difference and calculating time were obtained for various tumor number, tumor size, and separation distances. Compared with the margin volume with isocenter at COG, the margin volume for optimal point can be reduced by up to 27.7%.

CONCLUSION

Optimal treatment isocenter selection of multiple targets with large differences could reduce the total margin volume. ASA algorithm can significantly improve the speed of finding the optimal isocenter. This method can be used for clinical isocenter selection and is useful for the protection of normal tissue nearby.

A customizable, open-source Winston-Lutz system for multi-target, single isocentre radiotherapy

OBJECTIVE

To present and share an open-source system (phantom and software) for verifying the targeting accuracy of linac-based, single-isocenter, multi-target radiotherapy. This quality assurance test extends the traditional Winston-Lutz test, which considers a single target located at isocentre.

APPROACH

Plans for a 3D-printed phantom are provided, which can be customized to accommodate various target (BB) positions. Given BB positions and gantry/collimator/couch combinations, the software generates multi-leaf collimator positions to facilitate multi-target Winston-Lutz (MTWL) plan creation. The software determines deviations between detected and expected BB positions on MV images resulting from MTWL plan delivery. BBs are located using a Hough circle detection algorithm, which is modified to favour the detection of circles: (1) having reasonable size, (2) that are contained within the radiation field, and (3) having reasonable pixel intensities. Validation was performed in two ways: (1) using synthetic data with zero targeting errors and (2) by measuring real linac targeting errors and comparing against results obtained using a commercial system.

MAIN RESULTS

Validation using the synthetic data yielded a mean (maximum) absolute discrepancy of 0.11 mm (0.21 mm), which is comparable to the synthetic phantom resolution (0.2 mm). The mean (maximum) absolute discrepancy compared to the commercial system is 0.13 mm (0.43 mm). These values are similar to results obtained with repeated deliveries of the same MTWL plan with the same phantom setup. Both validation tests yield reasonable results and are therefore considered successful. The MTWL test was performed independently by three physicists on two linacs to investigate repeatability, resulting in a mean (maximum) absolute discrepancy of 0.14 mm (0.51 mm) among the various attempts.

SIGNIFICANCE

Successful completion of this quality assurance test, using our customizable and open-source system, provides confidence that multi-target, single isocentre radiotherapy treatments can be delivered with sufficient geometric accuracy according to the chosen tolerance level.