ExacTrac X-Ray 6D Imaging During Stereotactic Body Radiation Therapy of Spinal and Nonspinal Metastases.

The objective was to investigate the possibility of using ExacTrac X-ray (ETX) for 6D image guidance in stereotactic body radiation therapy (SBRT) of bone metastasis and to propose a patient management protocol. The analyses were first obtained from measurements on a pelvic phantom and on 19 patients treated for bone metastasis. The phantom study consisted of applying known offsets and evaluating the ETX level of accuracy, where results were compared with kV-cone beam computed tomography (kV-CBCT). Two groups of patients, 10 spinal and 9 nonspinal SBRT cases, were analyzed to evaluate ETX imaging for different bone localisations. A comparison was made between kV-CBCT and ETX prior to the treatment fractions. During treatments, two other kV-CBCT/ETX image pairs were also acquired and a total of 224 shifts were compared. A second study, using the ETX monitoring module, analyzed the intrafraction motion of 8 other patients. In the phantom study, the root mean square (RMS) of the translational and rotational discrepancies between ETX and kV-CBCT were < 0.6 mm and < 0.4°, respectively. For both groups of patients, the RMS of the discrepancies observed between the two imaging systems were greater than the phantom experiment while still remaining < 1 mm and < 0.7°. In the nonspinal group, three patients (2 scapulas and 1 humerus) did not have consistent shift values with ETX due to a lack of anatomical information. When ETX monitoring was used during irradiation, the setup errors measured were on average less than 1 mm/1°. The results obtained validated the use of ETX for 6D image guidance during bone SBRT. Real-time tracking of the target position improves the accuracy of the irradiation. This strategy allowed for faster correction of out-of-tolerance positioning errors. The registration of bone lesions with poor anatomical information is a limitation of this 2D-kV imaging system.

Evaluation of a Dedicated Software "Elements™ Spine SRS, Brainlab®" for Target Volume Definition in the Treatment of Spinal Bone Metastases With Stereotactic Body Radiotherapy.

Introduction: Stereotactic body radiotherapy (SBRT) is a treatment option for spine metastases. The International Spine Radiosurgery Consortium (ISRC) has published consensus guidelines for target delineation in spine SBRT. A new software called Elements™ Spine SRS by Brainlab® that includes the module Elements SmartBrush Spine (v3.0, Munich, Germany) has been developed specifically for SBRT treatment of spine metastases, and the latter provides the ability to perform semiautomatic clinical target volume (CTV) generation based on gross tumor volume (GTV) localization and guidelines. The aims of our study were to evaluate this software by studying differences in volumes between semiautomatic CTV contours compared to manual contouring performed by an expert radiation oncologist and to determine the dosimetric impact of these differences on treatment plans. Methods: A total of 35 volumes ("Expert GTV" and "Expert CTV") from 30 patients were defined by a single expert. A semiautomatic definition of these 35 CTVs based on the location of "Expert GTV" and following ISRC guidelines was also performed in Elements SmartBrush Spine ("Brainlab CTV"). The spatial overlap between "Brainlab" and "Expert" CTVs was calculated using the Dice similarity coefficient (DSC). We considered a threshold of 0.80 or above to indicate that Elements SmartBrush Spine performed very well with adequate contours for clinical use. Two dosimetric treatment plans, each corresponding to a specific planning target volume (PTV; Expert PTV, Brainlab PTV), were created for 11 patients. Results: We showed that "Brainlab CTV" and "Expert CTV" mean volumes were 29.8 ± 16.1 and 28.7 ± 15.7 cm3, respectively (p = 0.23). We also showed that the mean DSC for semiautomatic contouring relative to expert manual contouring was 0.85 ± 0.08 and less than 0.80 in five cases. For metastases involving the vertebral body only (n = 13,37%), the mean DSC was 0.90 ± 0.03, and for ones involving other or several vertebral regions (n = 22.63%), the mean DSC was 0.81 ± 0.08 (p < 0.001). The comparison of dosimetric treatment plans was performed for equivalent PTV coverage. There were no differences between doses received by organs at risk (spinal cord and esophagus) for Expert and Brainlab PTVs, respectively. Conclusion: The results showed that the semiautomatic method had quite good accuracy and can be used in clinical routine even for complex lesions.

Evaluation of inter- and intra-fraction 6D motion for stereotactic body radiation therapy of spinal metastases: influence of treatment time.

BACKGROUND: The objective of this study was to analyze the amplitude of translational and rotational movements occurring during stereotactic body radiotherapy (SBRT) of spinal metastases in two different positioning devices. The relevance of intra-fractional imaging and the influence of treatment time were evaluated. METHODS: Twenty patients were treated in the supine position either (1) on a body vacuum cushion with arms raised and resting on a clegecel or (2) on an integrated SBRT solution consisting of a SBRT table top, an Orfit™ AIO system, and a vacuum cushion. Alignments between the cone beam computed tomography (CBCT) and the planning computed tomography allowed corrections of inter- and intra-fraction positional shifts using a 6D table. The absolute values of the translational and rotational setup errors obtained for 329 CBCT were recorded. The translational 3D vector, the maximum angle, and the characteristic times of the treatment fractions were calculated. RESULTS: An improvement in the mean (SD) inter-fraction 3D vector (mm) from 7.8 (5.9) to 5.9 (3.8) was obtained by changing the fixation devices from (1) to (2) (p < 0.038). The maximum angles were less than 2° for a total of 87% for (1) and 96% for (2). The mean (SD) of the intra-fraction 3D vectors (mm) was lower for the new 1.1 (0.8) positioning fixation (2) compared to the old one (1) 1.7 (1.7) (p = 0.004). The angular corrections applied in the intra-fraction were on average very low (0.4°) and similar between the two systems. A strong correlation was found between the 3D displacement vector and the fraction time for (1) and (2) with regression coefficients of 0.408 (0.262-0.555, 95% CI) and 0.069 (0.010-0.128, 95% CI), respectively. An accuracy of 1 mm would require intra-fraction imaging every 5 min for both systems. If the expected accuracy was 2 mm, then only system (2) could avoid intra-fractional imaging. CONCLUSIONS: This study allowed us to evaluate setup errors of two immobilization devices for spine SBRT. The association of inter- and intra-fraction imaging with 6D repositioning of a patient is inevitable. The correlation between treatment time and corrections to be applied encourages us to move toward imaging modalities which allow a reduction in fraction time.

A new methodology to derive 3D kinetic parametric FDG PET images based on a mathematical approach integrating an error model of measurement.

BACKGROUND: In FDG-PET, SUV images are hampered by large potential biases. Our aim was to develop an alternative method (ParaPET) to generate 3D kinetic parametric FDG-PET images easy to perform in clinical oncology. METHODS: The key points of our method are the use of a new error model of PET measurement extracted from a late dynamic PET acquisition of 15 min, centered over the lesion and an image-derived input function (IDIF). The 15-min acquisition is reconstructed to obtain five images of FDG mean activity concentration and images of its variance to model errors of PET measurement. Our approach is carried out on each voxel to derive 3D kinetic parameter images. ParaPET was evaluated and compared to Patlak analysis as a reference. Hunter and Barbolosi methods (Barbolosi-Bl: with blood samples or Barbolosi-Im: with IDIF) were also investigated and compared to Patlak. Our evaluation was carried on Ki index, the net influx rate and its maximum value in the lesion (Ki,max). RESULTS: This parameter was obtained from 41 non-small cell lung cancer lesions associated with 4 to 5 blood samples per patient, required for the Patlak analysis. Compare to Patlak, the median relative difference and associated range (median; [min;max]) in Ki,max estimates were not statistically significant (Wilcoxon test) for ParaPET (- 3.0%; [- 31.9%; 47.3%]; p = 0.08) but statistically significant for Barbolosi-Bl (- 8.0%; [- 30.8%; 53.7%]; p = 0.001), Barbolosi-Im (- 7.9%; [- 38.4%; 30.6%]; p = 0.007) or Hunter (32.8%; [- 14.6%; 132.2%]; p < 10- 5). In the Bland-Altman plots, the ratios between the four methods and Patlak are not dependent of the Ki magnitude, except for Hunter. The 95% limits of agreement are comparable for ParaPET (34.7%), Barbolosi-Bl (30.1%) and Barbolosi-Im (30.8%), lower to Hunter (81.1%). In the 25 lesions imaged before and during the radio-chemotherapy, the decrease in the FDG uptake (ΔSUVmax or ΔKi,max) is statistically more important (p < 0.02, Wilcoxon one-tailed test) when estimated from the Ki images than from the SUV images (additional median variation of - 2.3% [- 52.6%; + 19.1%] for ΔKi,max compared to ΔSUVmax). CONCLUSION: None of the four methodologies is yet ready to replace the Patlak approach, and further improvements are still required. Nevertheless, ParaPET remains a promising approach, offering a non-invasive alternative to methods based on multiple blood samples and only requiring a late PET acquisition. It allows deriving Ki values, highly correlated and presenting the lowest relative bias with Patlak estimates, in comparison to the other methods we evaluated. Moreover, ParaPET gives access to quantitative information at the pixel level, which needs to be evaluated in the perspective of radiomic and tumour response. TRIAL REGISTRATION: NCT 02821936 ; May 2016.