Frameless Gamma Knife Radiosurgery with Leksell ICON: Initial Experience.

Gamma knife radiosurgery saw the light of the day when the Swedish physician "Lars Leksell" postulated the salient first principles of stereotactic radiosurgery. Prior to being realized in its new 'avatar' "The ICON", Leksell Gamma Knife (LGK) "Perfexion" has been the most practiced model and is still in practice in most of the centers in India. The Gamma Knife ICON (the sixth generation model) utilizes the concept of the Cone-Beam Computed Tomography (CBCT) module, thus allowing non-invasive immobilization of the skull employing frameless treatments without jeopardizing accuracy to sub-millimeters. The LGK ICON however has the same stereotactic delivery and patient positioning system as Perfexion and mesmerizes the care givers with the added technically sound feature of the CBCT imaging arm, that is, CBCT and an intra-fraction motion management system. The experience with ICON on both the sub-sets of patients has been intriguing and awe-inspiring. Despite its challenges of being detected with significant intra-fraction errors, we realized that the non-invasive thermoplastic mask fixation system has its own set of specific characteristics: fairly simple dosimetry; short radiation delivery times; and calm, composed, co-operative patients. We have been successful in conducting frameless gamma knife surgeries in ~25% of patients planned for gamma knife surgery. We look forward to witness this avant-garde pioneering scientific automation being practiced in a higher number of patients.

Optimal mask fixation method for frameless radiosurgery with Leksell Gamma Knife IconTM.

The Leksell Gamma Knife (LGK) IconTM is used for mask-based and frame-based fixation. The mask fixation provides a noninvasive method. However, an optimal mask fixation method is yet to be established. We evaluated the characteristics of three mask fixation methods (Plain, Folded, and Wide) for the LGK IconTM . Force-sensitive resistor sensors were attached to the forehead, supraorbital, zygoma, mandible, and occipital bone of the phantom, and digital humidity and temperature sensors were attached to both temporal lobes. Cone-beam computed tomography (CBCT) and high-definition motion management (HDMM) for each mask fixation method were used to evaluate the phantom motion during the initial application. Subsequently, the mask was removed and reapplied on the second (1st reapplication) and third days (2nd reapplication). In the initial application, forces acting on most portions of the phantom were stabilized within 1.5 h. The largest force acted on the occipital bone for the Plain and Wide methods and on the mandible for the Folded method. The temperature rapidly approaches the initial temperature, whereas the humidity gradually approached the initial humidity in all fixation methods. The Folded method exhibited a significantly lower translation along the Y-axis of the Leksell coordinate system, and rotations along all axes were under 0.5°. The HDMM values remained at 0.1 mm for all fixation methods. In the reapplications, the force acting on the occipital bone was significantly greater than that during the initial application for all mask fixation methods; the temperature and humidity remained unchanged. All mask fixation methods in the 1st reapplication were not significantly different from those in the 2nd reapplication. The Folded method is recommended as an optimal mask fixation for patients who require tight fixation; the Wide method can be considered if patient comfort is a priority.

Task synergies in neurovascular surgery: Image-guided navigation using a thermoplastic facial mask for the resection of a symptomatic occult to MRI brain microarteriovenous malformation.

Due to the size of microarteriovenous malformations (mAVM), its precise angioarchitecture description often requires a supraselective DSA and detecting the nidus during microsurgical resection is challenging. An accurate intraoperative navigation system is desirable but available softwares which can combine DSA and MRI are not always available. The authors present here a technical note describing the use of a stereotactic thermoplastic mask with a fiducial box to guide the resection of a mAVM.

Development of a device that remotely removes a mask in the head and neck immobilization system: a prototype and demonstration experiment.

In this study, a prototype device was developed to quickly remove the mask used to immobilize the head and neck by remotely releasing the quick fasteners. As a first step in investigating the usefulness of this prototype, we performed repeated removal tests and examined the accuracy of dose calculation. The results showed that the quick-release fasteners of a Type-S system (CIVCO Medical Solutions, Iowa, USA) could be removed remotely and accurately (success rate: 100%). Additionally, the dose errors in treatment planning were negligible (< 1.0%), and the gamma pass rate was equivalent (99.9%). Therefore, this prototype device with a remote system would help manage patient safety in emergencies, such as a disaster or a sudden change in the patient's condition. However, age-related deterioration with long-term clinical use or its ability to link with beam-off still requires further exploration.

Development of a Real-Time Thermoplastic Mask Compression Force Monitoring System Using Capacitive Force Sensor.

Purpose: Thermoplastic masks keep patients in an appropriate position to ensure accurate radiation delivery. For a thermoplastic mask to maintain clinical efficacy, the mask should wrap the patient's surface properly and provide uniform pressure to all areas. However, to our best knowledge, no explicit method for achieving such a goal currently exists. Therefore, in this study, we intended to develop a real-time thermoplastic mask compression force (TMCF) monitoring system to measure compression force quantitatively. A prototype system was fabricated, and the feasibility of the proposed method was evaluated. Methods: The real-time TMCF monitoring system basically consists of four force sensor units, a microcontroller board (Arduino Bluno Mega 2560), a control PC, and an in-house software program. To evaluate the reproducibility of the TMCF monitoring system, both a reproducibility test using a micrometer and a setup reproducibility test using a head phantom were performed. Additionally, the reproducibility tests of mask setup and motion detection tests were carried out with a cohort of six volunteers. Results: The system provided stable pressure readings in all 10 trials during the sensor unit reproducibility test. The largest standard deviation (SD) among trials was about 36 gf/cm2 (∼2.4% of the full-scale range). For five repeated mask setups on the phantom, the compression force variation of the mask was less than 39 gf/cm2 (2.6% of the full-scale range). We were successful in making masks together with the monitoring system connected and demonstrated feasible utilization of the system. Compression force variations were observed among the volunteers and according to the location of the sensor (among forehead, both cheekbones, and chin). The TMCF monitoring system provided the information in real time on whether the mask was properly pressing the human subject as an immobilization tool. Conclusion: With the developed system, it is possible to monitor the effectiveness of the mask in real time by continuously measuring the compression force between the mask and patient during the treatment. The graphical user interface (GUI) of the monitoring system developed provides a warning signal when the compression force of the mask is insufficient. Although the number of volunteers participated in the study was small, the obtained preliminary results suggest that the system could ostensibly improve the setup accuracy of a thermoplastic mask.

Thermoplastic Mask-Induced Contact Dermatitis: A Case Report.

Thermoplastic masks are commonly used in radiation therapy to immobilize a patient's head and neck during treatment. They are primarily composed of non-toxic polyester compounds that can be manipulated with heat to mold the shape of a patient's head and neck. There is little previously reported evidence of these masks causing allergic contact dermatitis. We present a case of a 44-year-old female with a history of squamous cell carcinoma of the right tonsil with multiple enlarged lymph nodes following surgical excision of the right tonsillar mass and ipsilateral neck dissection elected to undergo adjuvant radiation therapy with volumetric modulated arc therapy (VMAT) technique without concurrent chemotherapy. A thermoplastic mask was issued prior to radiation therapy. Following the mask fitting, the patient developed an allergic contact dermatitis reaction of the head and neck in areas covered by the mask. Her symptoms worsened with continued use of the thermoplastic mask and radiation therapy. As the patient continued and eventually finished the radiation treatment regimen, the dermatologic symptoms failed to respond to topical facial moisturizer and steroid treatment. The contact dermatitis reaction did not completely dissipate until about three months following completion of radiation therapy and contact with the thermoplastic mask. Thermoplastic masks are not known to cause an allergic contact dermatitis reaction. There is only one other reported case documented in the literature. Such reactions can alter the course of radiation therapy if symptoms are severe enough to disrupt treatment or if they cause worsening of the radiation dermatitis. Allergic contact dermatitis to thermoplastic masks should be well documented in the future to better understand the cause and possible risk factors related to the reaction.

A Multi-Center Study of CT-Based Neck Nodal Radiomics for Predicting an Adaptive Radiotherapy Trigger of Ill-Fitted Thermoplastic Masks in Patients with Nasopharyngeal Carcinoma.

Significant lymph node shrinkage is common in patients with nasopharyngeal carcinoma (NPC) throughout radiotherapy (RT) treatment, causing ill-fitted thermoplastic masks (IfTMs). To deal with this, an ad hoc adaptive radiotherapy (ART) may be required to ensure accurate and safe radiation delivery and to maintain treatment efficacy. Presently, the entire procedure for evaluating an eligible ART candidate is time-consuming, resource-demanding, and highly inefficient. In the artificial intelligence paradigm, the pre-treatment identification of NPC patients at risk for IfTMs has become greatly demanding for achieving efficient ART eligibility screening, while no relevant studies have been reported. Hence, we aimed to investigate the capability of computed tomography (CT)-based neck nodal radiomics for predicting IfTM-triggered ART events in NPC patients via a multi-center setting. Contrast-enhanced CT and the clinical data of 124 and 58 NPC patients from Queen Elizabeth Hospital (QEH) and Queen Mary Hospital (QMH), respectively, were retrospectively analyzed. Radiomic (R), clinical (C), and combined (RC) models were developed using the ridge algorithm in the QEH cohort and evaluated in the QMH cohort using the median area under the receiver operating characteristics curve (AUC). Delong's test was employed for model comparison. Model performance was further assessed on 1000 replicates in both cohorts separately via bootstrapping. The R model yielded the highest "corrected" AUC of 0.784 (BCa 95%CI: 0.673-0.859) and 0.723 (BCa 95%CI: 0.534-0.859) in the QEH and QMH cohort following bootstrapping, respectively. Delong's test indicated that the R model performed significantly better than the C model in the QMH cohort (p < 0.0001), while demonstrating no significant difference compared to the RC model (p = 0.5773). To conclude, CT-based neck nodal radiomics was capable of predicting IfTM-triggered ART events in NPC patients in this multi-center study, outperforming the traditional clinical model. The findings of this study provide valuable insights for future study into developing an effective screening strategy for ART eligibility in NPC patients in the long run, ultimately alleviating the workload of clinical practitioners, streamlining ART procedural efficiency in clinics, and achieving personalized RT for NPC patients in the future.

An innovative approach in the treatment of trigeminal trophic syndrome.

BACKGROUND: Trigeminal trophic syndrome (TTS) is a rare disorder that presents as ulceration of the nasal ala, and less often ipsilateral cheek, cutaneous lip, and forehead, following damage to the trigeminal nerve. TTS is often very difficult to treat due to anesthesia and paresthesia along the distribution of the trigeminal nerve, which often leads to a large behavioral component of this disease. METHODS: In this single case report, a thermoplastic mask was sutured in place overlying the chronic ulcerations of TTS in a 57-year-old man. RESULTS: Significant re-epithelization of the chronic ulcerations of TTS was seen after only 6 weeks of thermoplastic mask attachment. CONCLUSION: While TTS is a chronic and difficult to treat disease, the use of a thermoplastic mask in the treatment of this disease can significantly decrease the self-manipulation component and allow for re-epithelization and healing of the chronic ulcerations.

Evaluation of the inter- and intrafraction displacement for head patients treated at the particle therapy centre MedAustron based on the comparison of different commercial immobilisation devices.

In December 2016 the clinical operation has started at the particle therapy centre MedAustron, Wiener Neustadt, Austria. Different commercial immobilisation devices are used for head patients. These immobilisation devices are a combination of table tops (Qfix BoS™ Headframe, Elekta HeadStep™), pillows (BoS™ Standard pillow, Moldcare®, HeadStep™ pillow) and thermoplastic masks (Klarity Green™, Qfix Fibreplast™, HeadStep™ iCAST double). For each patient image-guided radiotherapy (IGRT) is performed by acquiring orthogonal X-ray imaging and 2D3D registration and the application of the resulting 6-degree of freedom (DOF) position correction on the robotic couch. The inter- and intrafraction displacement of 101 adult head patients and 27 paediatric sedated head patients were evaluated and compared among each other regarding reproducibility during the entire treatment and stability during each fraction. For the comparison, statistical methods (Shapiro-Wilk test, Mann-Whitney U-test) were applied on the position corrections as well as on the position verifications. The actual planning target volume margins of 3mm (adults) and 2mm (children) were evaluated by applying the van Herk formula on the intrafraction displacement results and performing treatment plan robustness simulations of twelve different translational offset scenarios including a HU uncertainty of 3.5%. Statistically significant differences between the immobilisation devices were found, but they turned out to be clinically irrelevant. The margin calculation for adult head patients resulted in 0.8mm (lateral), 1.2mm (cranio-caudal) and 0.6mm (anterior-posterior), and for paediatric head patients under anaesthesia in 0.8mm (lateral), 0.5mm (cranio-caudal) and 0.9mm (anterior-posterior). Based on these values, robustness evaluations of selected adult head patients and sedated children showed the validity of the currently used PTV margins.

Compare of Interfractional Setup Reproducibility Between Vacuum-Lock Bag and Thermoplastic Mask in Radiotherapy for Breast Cancer.

Background: This study aimed to analyze the difference of setup reproducibility between Vacuum-lock bag and Thermoplastic mask in the radiotherapy for breast cancer. Methods: A total of 100 invasive breast carcinoma patients were collected, among whom 50 patients were immobilized with Vacuum-lock bag (VB group), and the other 50 patients were immobilized with Thermoplastic mask (TM group). Set up reproducibility in different axes and comfort levels between two groups at three treatment progress points during the radiation therapy were collected and analyzed. Results: The linear regression model showed that fixed device was an independent factor of radiotherapy setup error (SE). Further subgroup analysis based on different axes showed that the SE caused by the fixed device was obvious in all directions. The comfort level in the VB group was significantly larger than that in the TM group at the beginning of treatment, reduced as the treatment progress going on, and finally disappeared within three weeks. Conclusions: Thermoplastic mask could significantly reduce positioning errors in the radiotherapy of breast cancer. Although more discomfort was found in the TM group, it could be eliminated as the treatment progresses.

A retrospective analysis of setup and intrafraction positional variation in stereotactic radiotherapy treatments.

PURPOSE: The aim of this study was to provide a comprehensive assessment of patient intrafraction motion in linac-based frameless stereotactic radiosurgery (SRS) and radiotherapy (SRT). METHODS: A retrospective review was performed on 101 intracranial SRS/SRT patients immobilized with the Klarity stereotactic thermoplastic mask (compatible with the Brainlab frameless stereotactic system) and aligned on a 6 Degree of Freedom (DoF) couch with the Brainlab ExacTrac image guidance system. Both pretreatment and intrafraction correction data are provided as observed by the ExacTrac system. The effects of couch angle and treatment duration on positioning outcomes are also explored. RESULTS: Initial setup data for patients is shown to vary by up to ±4.18 mm, ±2.97°, but when corrected with a single x-ray image set with ExacTrac, patient positions are corrected to within ±2.11 mm, ±2.27°. Intrafraction patient motion is shown to be uniformly random and independent of both time and couch angle. Patient motion was also limited to within approximately 3 mm, 3° by the thermoplastic mask. CONCLUSIONS: Our results indicate that since patient intrafraction motion is unrelated to couch rotation and treatment duration, intrafraction patient monitoring in 6 DoF is required to minimize intracranial SRS/SRT margins.

Intrafractional 6D head movement increases with time of mask fixation during stereotactic intracranial RT-sessions.

BACKGROUND: The present study investigates the intrafractional accuracy of a frameless thermoplastic mask used for head immobilization during stereotactic radiotherapy. Non-invasive masks cannot completely prohibit head movements. Previous studies attempted to estimate the magnitude of intrafractional inaccuracy by means of pre- and postfractional measurements only. However, this might not be sufficient to accurately map also intrafractional head movements. MATERIALS AND METHODS: Intrafractional deviation of mask-fixed head positions was measured in five patients during a total of 94 fractions by means of close-meshed repeated ExacTrac measurements (every 1.4 min) conducted during the entire treatment session. A median of six (range: 4 to 11) measurements were recorded per fraction, delivering a dataset of 453 measurements. RESULTS: Random errors (SD) for the x, y and z axes were 0.27 mm, 0.29 mm and 0.29 mm, respectively. Median 3D deviation was 0.29 mm. Of all 3D intrafractional motions, 5.5 and 0.4% exceeded 1 mm and 2 mm, respectively. A moderate correlation between treatment duration and mean 3D displacement was determined (rs = 0.45). Mean 3D deviation increased from 0.21 mm (SD = 0.26 mm) in the first 2 min to a maximum of 0.53 mm (SD = 0.31 mm) after 10 min of treatment time. CONCLUSION: Pre- and post-treatment measurement is not sufficient to adequately determine the range of intrafractional head motion. Thermoplastic masks provide both reliable interfractional and intrafractional immobilization for image-guided stereotactic hypofractionated radiotherapy. Greater positioning accuracy may be obtained by reducing treatment duration (< 6 min) and applying intrafractional correction. TRIAL REGISTRATION: Clinicaltrials.gov, NCT03896555, Registered 01 April 2019 - retrospectively registered.

A prospective examination of mask anxiety during radiotherapy for head and neck cancer and patient perceptions of management strategies.

INTRODUCTION: Distress related to wearing an immobilisation mask for radiotherapy treatment (RT) is a common experience for the person undergoing RT for head and neck cancer (HNC). Described as 'mask anxiety', there is little known about the patterns of this distress through the course of the treatment or what strategies are being used by people to help alleviate mask anxiety. METHODS: The study used a prospective cohort design to examine the patterns of patient-reported mask anxiety during the course of RT, using a modified Distress Thermometer (DT) and a survey to explore strategies patients used to assist their mask anxiety. RESULTS: Thirty-five participants, who identified as experiencing mask anxiety, were followed throughout RT treatment. At baseline, females were more likely to experience higher mask anxiety (P = 0.03). Across the course of treatment, mask anxiety significantly (P < 0.001) reduced within the total cohort. In 72% of participants, the level of initial distress was found to reduce over time. Only 22% experienced mask anxiety that remained constant. Few (6%) experienced an increase in mask anxiety across the course of RT. Participants reported relying on intervention from health professionals, self-taught strategies, music, visualisation and medication to manage their mask anxiety. CONCLUSIONS: Due to its high prevalence and variable patterns over time, it is recommended that routine screening for mask anxiety be implemented as standard care throughout the course of RT for HNC. Multiple, diverse strategies are being used by patients and studies are needed to develop effective interventions for managing mask anxiety.

Factors associated with set-up errors in intensity-modulated radiotherapy after breast conserving surgery / 中华放射医学与防护杂志

Objective To evaluate the difference of set-up errors between thermoplastic mask and breast bracket in patients receiving intensity-modulated radiotherapy after breast conserving surgery,and the impact of clinical factors associated with set-up errors.Methods A total of 34 patients treated with intensity-modulated radiotherapy after breast conserving surgery from January 2016 to June 2018 were reviewed.Eighteen patients were fixed with thermoplastic mask,and sixteen were with breast bracket.Weekly CBCT scan records were used to analyze set-up errors,and group systematic and random errors were computed.The influence of clinical factors on set-up errors was also analyzed.Results The immobilization technique with thermoplastic mask showed great superior in comparison with breast bracket;however,only in the Ty(translation) and Ry(rotation),the differences had significance.Based on group systematic and random errors,PTV margins in Tx,Ty and Tzwere 2.65,4.36 and 2.87 mm in thermoplastic mask group,as well as 5.71,6.07 and 4.20 mm in breast bracket group,respectively.Multi-factor regression analysis showed that BMI was independent factors affecting set-up errors.Conclusions Compared with breast bracket,the immobilization technique with thermoplastic mask has the potential of reducing set-up errors and PTV margins in patients receiving intensity-modulated radiotherapy after breast conserving surgery,especially in patients with high BMI.

Exploring the prevalence and experience of mask anxiety for the person with head and neck cancer undergoing radiotherapy.

INTRODUCTION: While use of a thermoplastic mask during radiotherapy (RT) treatment for head and neck cancer (HNC) is an essential component of safe patient care, there is little understanding of the extent to which this evokes anxiety (i.e. "mask anxiety") for the person undergoing treatment. METHODS: A mixed method, convergent design was used to examine the prevalence and experience of mask anxiety using two clinical cohorts. In phase one, a cohort of 100 patients undergoing RT for HNC were assessed for self-perceived mask anxiety using a modified distress thermometer screening tool. In phase two, a separate cohort of 20 patients who identified as having mask anxiety participated in individual interpretative descriptive interviews to explore the nature of their experience. RESULTS: In phase one, 26% of participants self-identified as being anxious about the use of a thermoplastic mask. In phase two thematic analysis of the interviews revealed two over-arching themes relating to the person's experience of mask anxiety: contributors to the mask anxiety (vulnerability, response to experience and expectations); and how the person was going to manage the mask anxiety during treatment (strategies and mindset). CONCLUSIONS: Mask anxiety impacted a quarter of participants undergoing radiotherapy for HNC. In line with the themes elicited from the participants, implementation of routine screening to ensure early identification, and patient education to assist preparation for wearing the mask during RT are strategies that could improve current management of mask anxiety.

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.

Effects of cleaning and disinfection of thermoplastic masks on hospital infection in patients receiving precise radiotherapy for nasopharyngeal carcinoma / 中华放射肿瘤学杂志

Objective To investigate the effects of cleaning and disinfection of thermoplastic masks on the hospital infection in patients receiving precise radiotherapy for nasopharyngeal carcinoma ( NPC). Methods A prospective study was performed among 102 patients receiving precise radiotherapy for NPC from 2013 and 2016, consisting of 18 patients with early?stage ( I, Ⅱ) disease and 84 patients with advanced (Ⅲ, IV) disease. All patients were randomly divided into group A and group B using a random number table. For group A, the marker lines of thermoplastic masks were sandwiched by double plastic tapes;cleaning and disinfection plus ultraviolet ( UV ) disinfection were applied to the masks 1 h prior to radiotherapy and immediately after radiotherapy. For group B, only conventional UV disinfection was applied to the masks. The surface of the masks was examined and hospital infection during radiotherapy was evaluated. Results At the 18th radiotherapy, group A had a significantly lower mask surface colony count than group B (7.90±650 vs. 139.05±12929 CFU/cm2, P=0000). Group A also had a significantly lower incidence of hospital infection than group B (725% vs. 882%, P=0046). For the patients with early stage NPC, particularly, there was no significant difference in the incidence of infection between the two groups (556% vs. 667%, P=0629). For patients with advanced NPC, group A had a significantly lower incidence of infection than group B ( 762% vs. 929%, P=0035) . There were no significant differences in incidence rates of oral mucosal, skin, and respiratory system infections between the two groups ( 471% vs. 510%, P=0692;176% vs. 235%, P=0463;78% vs. 137%, P=0338) . In both groups A and B, the incidence of oral mucosal infection was significantly higher than the incidence rates of skin infection ( P=0001, 0000) and respiratory system infection ( P=0004, 0000) . Conclusions Thermoplastic mask is one of the risk factors for hospital infection in patients receiving precise radiotherapy for NPC. Timely cleaning and disinfection plus UV disinfection can significantly reduce the surface colony count and the incidence of hospital infection in patients with NPC, particularly in those with advanced NPC receiving precise radiotherapy. The incidence of hospital infection is the highest in the oral mucosa, followed by the skin and the respiratory system.

The Continuous Assessment of Cranial Motion in Thermoplastic Masks During CyberKnife Radiosurgery for Trigeminal Neuralgia.

Stereotactic radiosurgery (SRS) treatment is characterized by high doses per fraction and extremely steep dose gradients. This requires a great degree of accurate localization to the appropriate treatment position, and continuous immobilization during the treatment session. In the case of Trigeminal Neuralgia (TGN) treatment this is especially true as the very small target volume makes positional accuracy critical. In this study we carried out a quantitative analysis of patient motion during the full treatment fraction within a radiosurgery immobilization mask system. Patient cranial movement was assessed by using the image guidance stereo x-ray cameras on a CyberKnife (CK) M6 robotic radiosurgery system (Accuray, Sunnyvale, CA). A total of five patients received treatments for either right or left TGN. The duration of treatment varied from 24-64 minutes. Orthogonal images were taken every 15 seconds during the treatment to assess patient movement. Approximately 60 stereo images were taken per patient and a total of 560 images were analyzed in this study. The mean absolute movement in each of longitudinal, lateral or vertical directions was approximately 0.3 mm for the duration of the treatment; however, on occasion much greater movement was observed during a fraction. The maximum displacement was in the longitudinal direction and reached 2.4 mm compared to the initial setup. Images taken at the end of the treatment session showed that the patients typically return to a position closer to the original setup position than the maximum excursion that occurred. This data suggests that although this mask system appears stable during much of the treatment session; for some patients there may be momentary patient movements that take place. Frequent imaging and correction can help mitigate the effect of this movement. It is important to understand the limitations of non-invasive mask systems when used for very high precision treatment.

Helical Tomotherapy of the breast: can thermoplastic immobilization improve the reproducibility of the treatment setup and the accuracy of the delivered dose?

PURPOSE: To evaluate the impact of thermoplastic mask immobilization in the setup reproducibility and delivered dose for Helical Tomotherapy (HT) of the breast/chest wall. METHODS: 16 patients treated with Accuray Hi-Art HT for breast-cancer were considered. Patients were positioned supine with arms extended above the head using Civco Wing Board (WB) system. In 50% of patients an Orfit thermoplastic mask was added in order to improve immobilization. Before each treatment fraction a megavoltage CT (MVCT) scan was taken and registered to the planning CT by experienced medical staff. The impact of thermoplastic mask was investigated analysing MVCT shift-roll data and MVCT dose distribution using Planned Adaptive software. RESULTS: In the analysed cases, the addition of thermoplastic mask had minor impact on the lateral, longitudinal and roll data distribution. Variance of vertical shifts was significantly reduced in the WB + Orfit group. Van Herk's margins were not affected by addition of thermoplastic immobilization. In both groups, target coverage (V95) and maximum dose (D1) were almost identical to planned values. D1 of organs at risk were not significantly different in the two groups. CONCLUSIONS: Analysis of shift-roll data shows no improvement in the group of patients immobilized with the addition of thermoplastic mask. Van Herk's margin is quite large (7-10 mm) in both groups evidencing the need to perform daily setup correction. The adapted dose distribution complies well with the planned one, showing that if MVCT is used before each treatment fraction, a 3 mm margin (setup component) for CTVs expansion could be adequate.