Journal of Applied Clinical Medical Physics最新文献

Purpose: Treatment planning for CyberKnife (CK) (Accuray, USA) can be performed with Precision (Accuray, USA) or RayStation (RS) (RaySearch Laboratories, Sweden) treatment planning systems (TPS). RaySearch recently released a new version of the CK module in RS 12A. The objective of the study was to compare plan quality between RS 12A and Precision.

Methods: Fifty nine plans were optimized with both TPS and compared; 39 were for brain metastases and 20 were for vertebral metastases. To avoid bias in plan comparison, Precision plans were recomputed in RS with the dose algorithm and beam model of RS, and then compared to RS plans. The comparison was divided into 3 parts in order to reflect the potential of RS and the differences with Precision, in terms of technical aspects of delivery efficiency and dose distribution. We compared the dose to the target and to the organs at risk (OAR), the conformity index (CI), the gradient, as well as the number of monitor units (MU), and the number of beams and nodes. Finally, a global plan quality index (PQI) was calculated.

Results: RS plans showed an equivalent target coverage for brain metastases but worse for vertebrae. OAR sparing was improved in RS but with a lower CI compared to Precision. Using an appropriate planning methodology in RS, plans with comparable quality to Precision could be obtained, but at the cost of a longer optimization time. The PQI obtained with RS was better than Precision, except for some brain cases.

Conclusion: RS is an adequate alternative for CK planning as it is possible to obtain plan quality comparable to Precision. However, the optimization time is longer compared to Precision and more attention must be paid to the choice of the initial conditions in terms of the number of beams and nodes.

Background: Immobilization devices are essential for maintaining accurate and repeatable patient positioning in radiotherapy. This study aimed to evaluate the setup errors and dosimetric deviation induced by the deformation of immobilization devices in thoracic cancer radiotherapy using CT-linac.

Materials and methods: A retrospective analysis was conducted on 40 thoracic cancer patients who underwent radiotherapy, using vacuum cushion (VC) and thermoplastic mask (TM) for immobilization. A total of 206 weekly Fan-beam CT (FBCT) images (4-7 per patient) were analyzed to manually delineate immobilization devices and assess their geometric deformations against setup errors. Dosimetric deviations between the clinical plan (CT-plan) and the delivered plan (FBCT-plan) were compared for planning target volume (PTV) and organs at risk (OARs). Correlations between dose variations and setup errors were analyzed in lateral (LAT), longitudinal (LNG), and vertical (VRT) axes.

Results: The conformity of the VC (N_up) and TM (N_down) with the patient in simulation CT exhibited moderate to strong correlations with VRT setup errors (N_down: r = -0.484, p < 0.01;N_up: r = -0.697, p < 0.01). However, intra-fraction deformation of immobilization devices (in FBCT) showed no significant correlation with setup errors. In the dosimetric analysis of OARs, lung dose parameters (D_mean, V₅, V₂₀) and heart D_mean exhibited a consistent absolute difference with increasing setup errors. Dose variation decreased significantly when errors exceeded 5 mm, particularly in the VRT direction for most PTV indices, with the exception of CI and HI. Spinal cord D_max variation correlated significantly with setup accuracy along the LNG axis, but not along other axes.

Conclusion: The conformity of immobilization devices in simulation CT exhibits a stronger correlation with setup accuracy than the deformation of these devices in intra-fraction FBCT. FBCT is recommended for improving treatment precision through dosimetric assessment and planning adjustments.

This case report describes a 45-min active learning lesson plan that engages 4th–5th and 6th–8th grade school students in spatial reasoning through a review of medical imaging. The lesson plan reviews different planar orientations and cross-sections of computed tomography (CT) images of familiar objects. The lesson is designed to introduce students to the idea that scientists are key contributors to healthcare, including in medical imaging technologies that facilitate the visualization of internal structures of patients without invasive procedures. The lesson demonstrates the three standard anatomical planes, axial, sagittal and coronal, by guiding students through CT image datasets of various objects. Students then are led in an interactive “dissection” of fruit to compare internal structures with medical images. The lesson plan aligns with key aspects of Next Generation Science Standards and aims to spark interest in the field of medical physics among a young student population through an introduction to imaging technologies. Worksheets and imaging datasets are included as supplementary materials to facilitate interested physicists adapting this work for educational purposes in their own communities, with minimal repeated effort.

Purpose: The purpose of this work was to experimentally quantify MR-compatible ionization chamber response for 1.5T Elekta Unity and 0.35T ViewRay MRIdian MR-linac systems through the determination of the magnetic field quality conversion factor, k_B,Q.

Methods: Seven MR-compatible ionization chamber models from Standard Imaging and PTW were evaluated. Both the quality conversion factor k_Q and the magnetic field quality conversion factor k_B,Q were experimentally determined through a cross-calibration method. Specifically, the ratio of absorbed dose measured with a reference A1SL chamber under reference conditions to corrected output measured with each test chamber at the same point of measurement allowed for the determination of k_B,Q. The angular dependence of the magnetic field quality conversion factor for MR-compatible chamber models was assessed for the 1.5T Elekta Unity system by measuring k_B,Q with the chamber axis and magnetic field direction aligned at cardinal angles (0°, 90°, 180°, 270°).

Results: Beam quality conversion (k_Q) factors for MR-compatible ionization chambers measured in a standard linac beam showed an average percent difference of -0.09 ± 0.18% compared to computed k_Q values for their conventional chamber versions. Similarly, magnetic field quality conversion (k_B,Q) factors for corresponding MR and non-MR ionization chamber models measured using the same cross-calibration technique demonstrated average percent differences of -0.1 ± 0.3% and 0.0 ± 0.2% for the Elekta Unity and ViewRay MRIdian, respectively. Investigation of the angular dependence of this correction factor demonstrated identical chamber response for equivalent MR-compatible and conventional chamber models.

Conclusions: This work provides critical experimental validation of MR-compatible ionization chamber performance, with a direct comparison of measured k_B,Q values to corresponding conventional chamber models demonstrating nearly equivalent chamber response. k_B,Q values determined using our experimental method will serve as an important reference for upcoming MR-linac reference dosimetry protocols and ultimately represent an important step towards accurate output calibration of MR-linac systems.

Purpose: This report describes routine machine quality assurance (QA) (daily, monthly, and annual QA) tests for the Zap-X^® Gyroscopic Radiosurgery^® platform.

Methods: Following the recommendations of the American Association of Physicists in Medicine Task Group (AAPM TG)-142 and Medical Physics Practice guideline (MPPG) 8.b, routine machine QA tests for the Zap-X system were implemented. The implementation included (1) daily, monthly, and annual QA tests encompassing dosimetry, mechanical, safety and imaging tests, (2) QA methods of each test specific to the Zap-X, (3) a tolerance value for each test, and (4) necessary QA equipment.

Results: Baseline values and key results of daily, monthly, and annual QA tests are presented in this report. This report also discusses QA tests not adopted from TG 142 or MPPG 8.b (e.g., distance indicator) due to unique features of the Zap-X system as well as additional QA tests added from the vendor's recommendations (e.g., self-check) and from TG-135 recommendations (e.g., monthly end-to-end testing) because of similarities between Zap-X and CyberKnife systems.

Conclusions: The comprehensive information on routine machine QA tests presented in this report will assist Zap-X teams in other Neurosurgery centers or Radiation Oncology clinics in establishing and maintaining their QA programs until AAPM endorsed guidelines become available.