Dosimetric impact of mechanical movements of the Linac gantry during treatments with small fields

Abstract

ObjectiveCurrent accepted linac Quality Assurance (QA) guidelines used for Volumetric Modulated Arc Therapy (VMAT) suggest a mechanical isocentre tolerance level of 1 mm. However, this tolerance level has not been well-established for the specific case of small field stereotactic VMAT. This study aims to evaluate the clinical impact of mechanical uncertainty on this treatment modality by modelling systematic gantry sag derived isocentre variance in the Treatment Planning System (TPS).ApproachA previously reported dataset of gantry sag values in the literature served as a starting point for this study. Using an in-house developed VMAT arc splitting algorithm, isocentre shifts were applied at a Control Point (CP) level to DICOM-RT treatment plans. Dose distributions for varying isocentre shift magnitudes were calculated for a set of 29 stereotactic VMAT plans using the Eclipse Acuros XB dose algorithm. These plans had a range of Planning Target Volume (PTV) sizes. A quantitative comparison of each plan was conducted by evaluating five Dose Volume Histogram (DVH)-derived plan quality metrics.ResultsAll metrics exhibited a deterioration in plan quality with increasing magnitudes of isocentre shift. At small PTV sizes, these effects were amplified, exhibiting significant changes at 1 mm of average shift when typical targets and tolerances were considered. For plans with PTVs between 0 and 5 cm3, a 1 mm shift reduced PTV coverage by 6.6 ± 2.2% and caused a 12.1 ± 3.8% deterioration in the conformity index. Based on the results of this study, the prevalent tolerance of 1 mm may not be suitable for treatments of small PTVs with small fields.SignificanceIn contrast to commonly accepted values, an absolute mechanical isocentre of 0.5 mm with action level at 0.75 mm is recommended for stereotactic VMAT of PTV sizes below 10 cm3.

Effect of angular dependence for small‐field dosimetry using seven different detectors

“…and McCallum‐Hee et al. reported that the effect of gantry sag is conspicuous in rotational irradiation using small fields 28,29 . Gantry sag could be the reason for the variation in coplanar and non‐coplanar beams using the 0.5 × 0.5 cm 2 field in this study.…”

“…This effect is considered to be caused by reported that the effect of gantry sag is conspicuous in rotational irradiation using small fields. 28,29 Gantry sag could be the reason for the variation in coplanar and non-coplanar beams using the 0.5 × 0.5 cm 2 field in this study. In addition, owing to the variety of detectors in this study, there was a detector-specific uncertainty.…”

“…This effect of gantry sag is caused by the gravitational force acting on the gantry produces moment of force. During gantry rotation, this moment causes deviation from the ideal circular trajectory and induces a variation of the radiation field center around the radiation isocenter 27–29 …”

Assessing Focal Spot Alignment in Clinical Linear Accelerators: A Comprehensive Evaluation with Triplet Phantoms

“…Regular QA assessments of the FSP are uncommon, with the primary focus being on beam symmetry, flatness, field size, and radiation isocentre location. These aspects are under consideration in linac QA procedures as an indirect approach to FSP QA; however, a need to assess FSP directly and identify action levels related to FSP has been established 9,10 . The FSP will have an impact on the calibration of MLCs and jaws and the radiation isocentre position.…”

Đánh giá độ ổn định của điểm đồng tâm cơ khí, góc quay của collimator, gantry và bàn điều trị trong máy gia tốc tuyến tính

“…Các nhà vật lí y khoa lâm sàng có nhiệm vụ kiểm tra định kì cho thiết bị này luôn đạt chất lượng khi cung cấp bức xạ. Công việc này phức tạp và cần sự chính xác cao trong từng phép kiểm tra về cơ học cũng như về liều, và mỗi phép kiểm tra phải tuân thủ theo quy trình và sai số đã được quy định hoặc khuyến Hình 1 Ảnh hưởng của việc điểm đồng tâm của máy gia tốc tuyến tính bị lệch (từ lệch tâm quay và góc quay) lên khả năng bao phủ khối u khi xạ trị, khi điểm đồng tâm này lệch trên 1 mm, độ bao phủ này bị ảnh hưởng nhiều, có thể lên đến 20% [1].…”