Preliminary Evaluation of a Novel Thermoplastic Mask System with Intra-fraction Motion Monitoring for Future Use with Image-Guided Gamma Knife

Design and manufacturing of a head mask for fixation in stereotactic radiosurgery by the Gamma Knife® Icon™

Background

This study evaluates an alternative to the classical method of head fixation during Gamma Knife radiosurgery using a Leksell head frame. In the Gamma Knife^® Icon™ model, a new method of head fixation is used by utilizing a thermal molded polymer mask that takes the shape of the patient's head before fixing the head to the table. However, this mask is for single use and quite expensive.

Methods

We describe a new, very economical method to fix the head of the patient during radiosurgery. We used commercial, quite cheap material [polylactic acid (PLA)] plastic and made a 3D printing model for the patient's face, taking special measurements to put this mask and fix it on the Gamma Knife. The actual material cost is only $4 (100 times less than the original mask cost).

Results

The new mask efficiency was tested using the movement checker software, the same one used to measure the efficiency of the original mask.

Conclusion

The newly designed and manufactured mask is quite effective for use with the Gamma Knife^® Icon™, with a much lower cost, and it can be manufactured locally.

Assessment of the dosimetric impact of intra-fraction motion during frameless treatment delivery on GammaKnife® Icon™

This study investigated the impact of patient motion on the dosimetric quality of treatment plans for metastatic patients undergoing frameless GammaKnife^® Icon™ treatments. By quantifying dosimetric robustness at increasing high definition motion management (HDMM) gating tolerances, this study investigated the possibility of increasing the HDMM threshold for patients treated at our centre from our current standard of 1 mm.

Methods

Motion was retrospectively simulated by shifting the stereotactic co-ordinates of shots in treatment plans using three motion models. Dosimetric quality indicators of original and shifted plans were compared. Influence of target location and size was determined.

Results

Motion models showed median (p-value) absolute changes in target coverage of up to -0.133% (<0.0001), -0.267% (<0.0001) and -0.667% (<0.0001) for HDMM tolerances of 1mm, 1.5mm and 3mm. The greatest median (p-value) absolute changes in Paddick Conformity Index (PCI) and Gradient Index (GI) were -0.008 (0.0032) and 0.017 (0.6893). A reduction in target size correlated weakly with greater changes in target coverage for all models and HDMM tolerances (r² =0.040-0.309). No location dependence was observed.

Conclusion

HDMM tolerances up to and including 3mm all resulted in negligible changes in PCI and GI. Target coverage exhibited greater sensitivity to motion, but only at 3mm was the target coverage reduced below local planning aims. Our HDMM tolerance could therefore potentially be increased to 1.5mm, with likely benefits to treatment delivery efficiency.

Mask-based immobilization in Gamma Knife stereotactic radiosurgery

The Gamma Knife Icon (Elekta AB, Stockholm) is a cobalt-based stereotactic radiosurgery (SRS) unit to support the use of a thermoplastic mask in lieu of a rigid frame, using an onboard cone-beam CT (CBCT) and an intrafraction motion management system (IFMM). We retrospectively reviewed 124 patients treated with Gamma Knife SRS from January 2018 to December 2019 at our institution using a mask-based immobilization system. Patient and treatment characteristics were collected and summarized as well as interfraction shifts and treatment-related outcomes. This dataset includes 124 patients with an associated 358 intracranial tumors. Twenty-four patients presented with primary brain tumors, which included 14 meningiomas and 10 other histologies, with 100 patients having brain metastases. Sixty tumors were post-operative, while 298 were intact. The median dose for primary tumors was 25 Gy in 5 fractions. Median doses to metastases were 20 Gy in 1 fraction, 27 Gy in 3 fractions, and 25 Gy in 5 fractions. Median interfraction CBCT shifts were submillimeter. Median patient follow-up was 6.28 months. 91% of patients with metastases maintained local control. Our early clinical experience has demonstrated limited toxicity profiles and high patient tolerance, which suggests that mask-based Gamma Knife SRS provides a safe alternative option for frameless SRS. Patients with large target volumes where fractionation is preferred or with small target volumes in non-eloquent areas can be considered for this approach. Response rates are encouraging, and continued follow-up is necessary to investigate long-term control and survival.

Validation of PTV margin for Gamma Knife Icon frameless treatment using a PseudoPatient® Prime anthropomorphic phantom

The Gamma Knife Icon allows the treatment of brain tumors mask-based single-fraction or fractionated treatment schemes. In clinic, uniform axial expansion of 1 mm around the gross tumor volume (GTV) and a 1.5 mm expansion in the superior and inferior directions are used to generate the planning target volume (PTV). The purpose of the study was to validate this margin scheme with two clinical scenarios: (a) the patient's head remaining right below the high-definition motion management (HDMM) threshold, and (b) frequent treatment interruptions followed by plan adaptation induced by large pitch head motion. A remote-controlled head assembly was used to control the motion of a PseudoPatient® Prime head phantom; for dosimetric evaluations, an ionization chamber, EBT3 films, and polymer gels were used. These measurements were compared with those from the Gamma Knife plan. For the absolute dose measurements using an ionization chamber, the percentage differences for both targets were less than 3.0% for all scenarios, which was within the expected tolerance. For the film measurements, the two-dimensional (2D) gamma index with a 2%/2 mm criterion showed the passing rates of ≥87% in all scenarios except the scenario 1. The results of Gel measurements showed that GTV (D₁₀₀ ) was covered by the prescription dose and PTV (D₉₅ ) was well above the planned dose by up to 5.6% and the largest geometric PTV offset was 0.8 mm for all scenarios. In conclusion, the current margin scheme with HDMM setting is adequate for a typical patient's intrafractional motion.

3D-printed headrest for frameless Gamma Knife radiosurgery: Design and validation

Purpose

Frameless Gamma Knife stereotactic radiosurgery (SRS) uses a moldable headrest with a thermoplastic mask for patient immobilization. An efficacious headrest is time consuming and difficult to fabricate due to the expertise required to mold the headrest within machine geometrical limitations. The purpose of this study was to design and validate a three‐dimensional (3D)‐printed headrest for frameless Gamma Knife SRS that can overcome these difficulties.

Materials and methods

A headrest 3D model designed to fit within the frameless adapter was 3D printed. Dosimetric properties of the 3D‐printed headrest and a standard‐of‐care moldable headrest were compared by delivering a Gamma Knife treatment to an anthropomorphic head phantom fitted with an ionization chamber and radiochromic film. Ionization measurements were compared to assess headrest attenuation and a gamma index was calculated to compare the film dose distributions. A volunteer study was conducted to assess the immobilization efficacy of the 3D‐printed headrest compared to the moldable headrest. Five volunteers had their head motion tracked by a surface tracking system while immobilized in each headrest for 20 min. The recorded motion data were used to calculate the average volunteer movement and a paired t‐test was performed.

Results

The ionization chamber readings were within 0.55% for the 3D‐printed and moldable headrests, and the calculated gamma index showed 98.6% of points within dose difference of 2% and 2 mm distance to agreement for the film measurement. These results demonstrate that the headrests were dosimetrically equivalent within the experimental uncertainties. Average motion (±standard deviation) of the volunteers while immobilized was 1.41 ± 0.43 mm and 1.36 ± 0.51 mm for the 3D‐printed and moldable headrests, respectively. The average observed volunteer motion between headrests was not statistically different, based on a P‐value of 0.466.

Conclusions

We designed and validated a 3D‐printed headrest for immobilizing patients undergoing frameless Gamma Knife SRS.

Intracranial motion during frameless Gamma-Knife stereotactic radiosurgery

PURPOSE/OBJECTIVES

The Gamma-Knife radiosurgery (GKRS) (Elekta AB, Stockholm) platform delivers highly conformal and precise radiation; however, intracranial displacement during treatment allows for the potential of a marginal target-miss. Frameless (mask-based) GKRS using the Gamma Knife Icon system monitors nasal tip motion as a surrogate for intracranial motion by tracking an infrared marker using a high-definition motion management (HDMM) system. To date, there is limited data available regarding the incidence and severity of motion and factors that impact intrafraction motion when treating with frameless GKRS.

MATERIALS/METHODS

A retrospective study was performed to evaluate patients with brain tumors who were treated with frameless GKRS using the Gamma Knife Icon between May and December 2018. All patients underwent mask-based immobilization using a thermoplastic mask. Data on patient demographics, mask type, use of bite block, and number of treatments received, use of anxiolytics, treatment time, and whether a physics clearance check was performed prior to treatment were collected. For each treatment session, average displacement (mm), maximum displacement (mm) and total treatment time (min) were recorded and logistic regression analyses were performed.

RESULTS

Data was collected for 89 consecutive treatments (38 patients). Of these, an anxiolytic was used in 61 treatments and a physics clearance check was performed for 45 treatments. The median average and maximum displacement was 0.60 mm and 1.22 mm, respectively. An average displacement greater than 0.60 mm was seen with Eastern Cooperative Oncology Group performance status (ECOG) > 1, male gender, and malignant tumors (p < 0.05). Anxiolytic use prior to treatment was associated with a significant reduction in average displacement (p < 0.05). Significantly greater odds of observing a maximum displacement over 1.22 mm was seen with patients with ECOG > 1, male gender, and increased treatment time (p < 0.05). Age > 65 and anxiolytic use were associated with a significant reduction in maximum displacement (p < 0.05). Performance of clearance checks and use of bite block use did not impact average or maximum patient displacement.

CONCLUSIONS

This is the first study to evaluate patient and treatment-related factors that influence intrafraction motion during GKRS with mask-based immobilization through HDMM tracking. Increased intracranial displacement during frameless GKRS was associated with higher ECOG, male gender, increased treatment time and malignant tumors, while anxiolytics were shown to mitigate excessive motion. Radiosurgery teams should consider these patient factors when treating patients with mask immobilization.

Whole-brain radiation therapy without a thermoplastic mask

The aim of the study was to investigate the clinical feasibility of whole-brain radiation therapy without a thermoplastic mask. Positioning and intra-fractional motion monitoring were performed using optical surface scanning. The motion threshold was 3 mm/3 degrees. The group mean vector deviation was 1.1 mm. The roll was larger compared to pitch and rotation. Two patients out of 30 were not able to lie still. All other patients completed their treatment successfully without a mask. With a probability of success of 93%, we concluded that irradiation without a mask is a clinically feasible method.

Characterization and validation of an intra-fraction motion management system for masked-based radiosurgery

Purpose

Characterize the intra‐fraction motion management (IFMM) system found on the Gamma Knife Icon (GKI), including spatial accuracy, latency, temporal performance, and overall effect on delivered dose.

Methods

A phantom was constructed, consisting of a three‐axis translation mount, a remote motorized flipper, and a thermoplastic sphere surrounding a radiation detector. An infrared marker was placed on the translation mount secured to the flipper. The spatial accuracy of the IFMM was measured via the translation mount in all Cartesian planes. The detector was centered at the radiation focal point. A remote signal was used to move the marker out of the IFMM tolerance and pause the beam. A two‐channel electrometer was used to record the signals from the detector and the flipper when motion was signaled. These signals determined the latency and temporal performance of the GKI.

Results

The spatial accuracy of the IFMM was found to be <0.1 mm. The measured latency was <200 ms. The dose difference with five interruptions was <0.5%.

Conclusion

This work provides a quantitative characterization of the GKI IFMM system as required by the Nuclear Regulatory Commission. This provides a methodology for GKI users to satisfy these requirements using common laboratory equipment in lieu of a commercial solution.

Targeting inaccuracy caused by mechanical distortion of the Leksell stereotactic frame during fixation

Background

The stereotactic frame represents the mainstay of accuracy for targeting in stereotactic procedures. Any distortion of the frame may induce a significant source of error for the stereotactic coordinates.

Objective

To analyze the sources of distortion of the Leksell frame G induced by fixation to the patient's head and to evaluate the clinical impact of frame distortion on the accuracy of targeting in stereotactic procedures.

Methods

We analyzed the torques exerted on the fixation screws after frame placement in a series of patients treated stereotactically by an experienced team. We studied the risk for frame bending in an experimental model of stereotactic frame fixation, with increasing torque of fixation screws in a homogeneous and heterogeneous distribution of torques between the four screws. We assessed the impact of expanding dimensions of bending of the Leksell frame both on surgeries utilizing the stereotactic frame, and on radiosurgical procedures with the Gamma Knife.

Results

Frames were fixed clinically at a range of torques of 0.147–0.522 Nm (mean = 0.348 Nm). The torques did not vary significantly with time. Heterogeneity between the two opposite pairs of screws is often limited, but can reach 96.3%. Distortion of the frame may occur even at minimal levels of torque. Heterogeneity between the two opposite pairs of screws will significantly raise the amount of frame distortion. We found a direct correlation between measures of the frame distortion and extend of the deviation from the stereotactic target in clinical models of stereotactic procedures.

Conclusion

Stereotactic frames were subjected to distortion due to the torque used for frame fixation. The risk of distortion increased with the torque used and the heterogeneity between the torques of the fixation screws. Distortion of the frame was a significant source of inaccuracy of targeting for stereotactic procedures in clinical practice.

Frameless Fractionated Gamma Knife Radiosurgery with ICON™ for Large Metastatic Brain Tumors

BACKGROUND

Recently, a new generation of gamma knife radiosurgery (GKRS) equipped with a frameless immobilization system has encouraged the use of fractionated GKRS as an increasingly favorable treatment option. We investigated the preliminary outcome of efficacy and toxicity associated with frameless fractionated gamma knife radiosurgery (FF GKRS) for the treatment of large metastatic brain tumors.

METHODS

Fifteen patients with 17 lesions were treated using FF GKRS and included in this study, because of the large tumor size of more than 10 cm³. FF GKRS was performed based on a thermoplastic mask system for 3 to 5 consecutive days.

RESULTS

The mean duration of clinical follow-up was 12 months (range, 4-24), and the local control rate was 100%. Tumor volume decreased in 13 lesions (76.5%), and remained stable in 4 lesions (23.5%). One patient was classified as new lesion development because of the occurrence of leptomeningeal seeding regardless of the tumor volume change. Compared with the initial volume at the time of FF GKRS, tumor volume change at the last follow-up was 62.32% ± 29.80%. Cumulative survival rate at 12 months was 93.3% ± 6.4%. One patient died during the follow-up period because of the progression of the primary disease. No patient showed radiation necrosis on the follow-up images.

CONCLUSION

Daily FF GKRS by gamma knife ICON™ revealed satisfactory tumor control rate and low morbidity, despite the short follow-up period. Further prospective studies and a longer follow-up of a large cohort of patients diagnosed with brain metastases are required to elucidate the effect of FF GKRS in brain metastases.

Dosimetric validation of the Gamma Knife® IconTM plan adaptation and high-definition motion management system with a motorized anthropomorphic head phantom

PURPOSE

To perform dosimetric validation of the plan adaptation and high-definition motion management (HDMM) system of Gamma Knife® Icon^TM in various clinical scenarios.

METHODS AND MATERIALS

We built an assembly for a pitch-adjustable anthropomorphic head phantom. We then used films to measure dosimetric and positional accuracy in 13 clinical scenarios, including movement near HDMM thresholds, multiple plan adaptations, frequent coughing, and initial setup error.

RESULTS

The dose for the superiorly located 4-mm shot was decreased up to 7-13% near 2- to 3-mm HDMM thresholds in the chin-down position. Dosimetric deviation was within ±3.5% for initial pitch angles of up to 20°. Multiple treatment interruption and frequent coughing did not cause substantial dosimetric deviation (<2%).

CONCLUSION

Our results indicated that dosimetric accuracy of the Gamma Knife® Icon^TM system is reliable even in extreme treatment conditions. However, the user should exercise caution for superiorly located small lesions with an HDMM threshold ≥2 mm or in the scenario of large initial setup error.

An Evaluation of the ICON® Mask Fixation: Curing Characteristics of the Thermoplastic Fixation and Implications for Patient Workflow

Background

Thermoplastic mask immobilization is used to perform hypo-fractionated treatments with the Gamma Knife ICON®.

Materials and methods

We evaluated the curing characteristics of the ICON® Nanor mask using force sensing resistors coupled with a data logging tool designed by us.

Results

For patients being treated with masks made the same day as the treatment, often in the same sitting with no removal and replacement of the patient from the treatment cradle, based on the curves 80% of the force of fixation is reached at 30 minutes.

Conclusions

Allowing for curing over 10-15 minutes and the subsequent localizing and delivery Cone beam CT (CBCT)s as well as the plan evaluation this is a reasonable time to start of therapy. For more exacting targets that are still requiring hypo-fractionation a cure period of 15 hours or greater will ensure that maximum rigidity of fixation is achieved.

Acute skin allergy to thermoplastic mask used for patient immobilization during radiation therapy: a case report

Background

Radiosurgical treatments of brain tumors, vascular malformations, and functional disorders are more and more frequently used. Gamma Knife irradiation with the Icon system necessitates the use of a thermoplastic mask for head immobilization during treatment. Acute cutaneous allergy to thermoplastic masks has never been reported.

Case presentation

A 71-year-old Caucasian woman treated radiosurgically for a sphenoidal meningioma using the Icon Gamma Knife system developed an acute allergic skin reaction to the thermoplastic mask used for head immobilization. Corticoids and antihistamine drugs were needed to continue the radiosurgical procedure to its end.

Conclusion

Allergic reaction of the skin during radiosurgery with a thermoplastic mask for head fixation can develop due to cutaneous contact of the face with the mask.

Evaluation of stability of stereotactic space defined by cone-beam CT for the Leksell Gamma Knife Icon

The Gamma Knife Icon comes with an integrated cone‐beam CT (CBCT) for image‐guided stereotactic treatment deliveries. The CBCT can be used for defining the Leksell stereotactic space using imaging without the need for the traditional invasive frame system, and this allows also for frameless thermoplastic mask stereotactic treatments (single or fractionated) with the Gamma Knife unit. In this study, we used an in‐house built marker tool to evaluate the stability of the CBCT‐based stereotactic space and its agreement with the standard frame‐based stereotactic space. We imaged the tool with a CT indicator box using our CT‐simulator at the beginning, middle, and end of the study period (6 weeks) for determining the frame‐based stereotactic space. The tool was also scanned with the Icon's CBCT on a daily basis throughout the study period, and the CBCT images were used for determining the CBCT‐based stereotactic space. The coordinates of each marker were determined in each CT and CBCT scan using the Leksell GammaPlan treatment planning software. The magnitudes of vector difference between the means of each marker in frame‐based and CBCT‐based stereotactic space ranged from 0.21 to 0.33 mm, indicating good agreement of CBCT‐based and frame‐based stereotactic space definition. Scanning 4‐month later showed good prolonged stability of the CBCT‐based stereotactic space definition.