Organ Deformation and Dose Coverage in Robotic Respiratory-Tracking Radiotherapy

The objectives of this study were to estimate global uncertainty for patients with thoracic tumors treated in our center using the CyberKnife VSI after placement of fiducial markers and to compare our findings with the standard CTV to PTV margins used to date. Datasets for 16 patients (54 fractions) treated with the CyberKnife and the Synchrony Respiratory Tracking System were analyzed retrospectively based on CT planning, tracking information, and movement data generated and saved in the logs files by the system. For each patient, we analyzed all the main uncertainty sources and assigned a value. We also calculated an expanded global uncertainty to ensure a robust estimation of global uncertainty and to enable us to determine the position of 95% of the CTV points with a 95% confidence level during treatment. Based on our estimation of global uncertainty and compared with our general margin criterion (5 mm in all three directions: superior/inferior [SI], anterior/posterior [AP], and lateral [LAT]), 100% were adequately covered in the LAT direction, as were 94% and 94% in the SI and AP directions. We retrospectively analyzed the main sources of uncertainty in the CyberKnife process patient by patient. This individualized approach enabled us to estimate margins for patients with thoracic tumors treated in our unit and compare the results with our standard 5 mm margin.PACS number: 87.55‐x

“…Tumor deformations, correlations with surrounding tissue, and fiducial variation in CoM compared to

{CoM}_{Tumor}

have been studied by other authors 19 , 27 , 28 , 29 . In our split methodology, these data would be useful for determination of the uncertainties associated with Steps 1 and 2.…”

Section: Discussionmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 99%

{CoM}_{CT}

position is very difficult to modify, as it is not possible to change a configuration once the seed has been implanted.…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Retrospective evaluation of CTV to PTV margins using CyberKnife in patients with thoracic tumors

Floriano

García²,

Moreno³

et al. 2014

“…It should be noted that our results were based on rigid geometry and rigid target motion. However, errors due to differential target motion and varying lung density should be negligible if adequate safety margin is used 5 , 24 . Errors due to variation of beam paths as treatment beams traversing through different materials were also expected to averaged out for a large number of beams

(> 100 beams)

used in Synchrony treatment.…”

Section: Discussionmentioning

confidence: 99%

“…Because internal organ motion is nonrigid, the positions of the target and hence the radiation beam relative to other critical organs, such as esophagus and spinal cord, can change during respiration. A few studies have demonstrated that failure to account for the organ deformation and the dynamic tracking beam movement in the treatment planning process may cause unexpected loss of target dose coverage and increase of critical organ doses 4 , 5 . To resolve this problem, a 4D dose optimization and calculation algorithm has been added to the CyberKnife treatment planning system (TPS).…”

Section: Introductionmentioning

confidence: 99%

Accuracy and sensitivity of four‐dimensional dose calculation to systematic motion variability in stereotatic body radiotherapy (SBRT) for lung cancer

Chan

Kwong

et al. 2012

The dynamic movement of radiation beam in real‐time tumor tracking may cause overdosing to critical organs surrounding the target. The primary objective of this study was to verify the accuracy of the 4D planning module incorporated in CyberKnife treatment planning system. The secondary objective was to evaluate the error that may occur in the case of a systematic change of motion pattern. Measurements were made using a rigid thorax phantom. Target motion was simulated with two waveforms (sin and cos4) of different amplitude and frequency. Inversely optimized dose distributions were calculated in the CyberKnife treatment planning system using the 4D Monte Carlo dose calculation algorithm. Each plan was delivered to the phantom assuming (1) reproducible target motion, and (2) systematic change of target motion pattern. The accuracy of 4D dose calculation algorithm was assessed using GAFCHROMIC EBT2 films based on 5%/3 mm γ criteria. Treatment plans were considered acceptable if the percentage of pixels passing the 5%/3 mm γ criteria was greater than 90%. The mean percentages of pixels passing were 95% for the target and 91% for the static off‐target structure, respectively, with reproducible target motion. When systematic changes of the motion pattern were introduced during treatment delivery, the mean percentages of pixels passing decreased significantly in the off‐target films (48%; p < 0.05), but did not change significantly in the target films (92%; p=0.324) compared to results of reproducible target motion. These results suggest that the accuracy of 4D dose calculation, particularly in off‐target stationary structure, is strongly tied to the reproducibility of target motion and that the solutions of 4D planning do not reflect the clinical nature of nonreproducible target motion generally.PACS numbers: 87.53.Ly, 87.55.km

Detectability of fiducials’ positions for real‐time target tracking system equipping with a standard linac for multiple fiducial markers

Ueda

Ohira

et al. 2020

Purpose To investigate the detectability of fiducial markers’ positions for real‐time target tracking system equipping with a standard linac. The hypothesis is that the detectability depends on the type of fiducial marker and the gantry angle of acquired triggered images. Methods Three types of ball fiducials and four slim fiducials with lengths of 3 and 5 mm were prepared for this study. Triggered images with three similar fiducials were acquired at every 10° during the conformal arc irradiation to detect the target position. Although only one type of arrangement was prepared for the ball fiducials, a three‐type arrangement was prepared for the slim fiducials, such as parallel, orthogonal, and oblique with 45° to the gantry‐couch direction. To measure the detectability of the real‐time target tracking system for each fiducial and arrangement, detected marker positions were compared with expected marker positions at every angle of acquired triggered images. Results For the ball‐type fiducial, the maximum difference between the detected marker positions and expected marker positions was 0.3 mm in all directions. For the slim fiducial arranged parallel and oblique with 45°, the maximum difference was 0.4 mm in all directions. When each slim fiducial was arranged orthogonal to the gantry‐couch direction, the maximum difference was 1.5 mm for the length of 3 mm, and 3.2 mm for the length of 5 mm. Conclusions The detectability of fiducial markers’ positions for the real‐time target tracking system equipping with a standard linac depends on the form and insertion angles of the fiducials.