Prediction of GTV median dose differences eases Monte Carlo re-prescription in lung SBRT.

BACKGROUND AND PURPOSE: The use of Monte Carlo (MC) dose calculation algorithm for lung patients treated with stereotactic body radiotherapy (SBRT) can be challenging. Prescription in low density media and time-consuming optimization conducted CyberKnife centers to propose an equivalent path length (EPL)-to-MC re-prescription method based on GTV median dose. Unknown at the time of planning, GTV D50% practical application remains difficult. The current study aims at creating a re-prescription predictive model in order to limit conflicting dose value during EPL optimization. MATERIAL AND METHODS: 129 patients planned with EPL algorithm were recalculated with MC. Relative GTV_D50% discrepancies were assessed and influencing parameters identified using wrapper feature selection. Based on best descriptive parameters, predictive nomogram was built from multivariate linear regression. EPL-to-MC OARs near max-dose discrepancies were reported. RESULTS: The differences in GTV_D50% (median 10%, SD: 9%) between MC and EPL were significantly (p < .001) impacted by the lesion's surface-to-volume ratio and the average relative electronic density of the GTV and the GTV's 15 mm shell. Built upon those parameters, a nomogram (R2 = 0.79, SE = 4%) predicting the GTV_D50% discrepancies was created. Furthermore EPL-to-MC OAR dose tolerance limit showed a strong linear correlation with coefficient range [0.84-0.99]. CONCLUSION: Good prediction on the required re-prescription can be achieved prior planning using our nomogram. Based on strong linear correlation between EPL and MC for OARs near max-dose, further restriction on dose constraints during the EPL optimization can be warranted. This a priori knowledge eases the re-prescription process in limiting conflicting dose value.

[Multiple vertebral hemangiomas treated simultaneously with CyberKnife® Technology]. / Hémangiomes vertébraux multiples traites simultanément en condition stéréotaxique avec la technologie CyberKnife®.

Haemangioma are benign, vasoformative lesions of endothelial origin. A minority of patients with a vertebral location are symptomatic with neck or back pain. Neurological deficits due to spinal cord compression are rare and mostly observed at the thoracic and upper lumbar spine. Treatment is indicated in case of relevant symptomatology. Several invasive treatment modalities exist such as surgery and intralesional injections, but radiotherapy is the most common treatment for painful lesions. We report a case of a patient with symptomatic bifocal vertebral haemangioma (d9, l3) associated with dorsal and lumbar pain treated simultaneously using a cyberknife® vsi system, with a significant reduction of pain and a limited toxicity.

les hémangiomes sont des malformations vasculaires bénignes, d'origine endothéliale. Une minorité de patients avec des hémangiomes vertébraux sont symptomatiques et présentent des douleurs du cou ou du dos. Le déficit neurologique, occasionné par une compression de la moelle épinière, est rare et touche surtout la colonne thoracique et lombaire supérieure. Un traitement est indiqué si l'hémangiome vertébral est symptomatique. Les techniques invasives comme la chirurgie et les injections intra-lésionnelles peuvent être utilisées, mais la radiothérapie reste le traitement de choix pour les lésions douloureuses. Nous décrivons le cas d'un patient, présentant des douleurs dorso-lombaires, ayant deux hémangiomes localisés en d9 et l3 traités simultanément par le cyberknife® avec un certain succès, notamment du point de vue antalgique, et une toxicité très limitée.

Commissioning Monte Carlo algorithm for robotic radiosurgery using cylindrical 3D-array with variable density inserts.

INTRODUCTION: To commission the Monte Carlo (MC) algorithm based model of CyberKnife robotic stereotactic system (CK) and evaluate the feasibility of patient specific QA using the ArcCHECK cylindrical 3D-array (AC) with Multiplug inserts (MP). RESULTS: Four configurations were used for simple beam setup and two for patient QA, replacing water equivalent inserts by lung. For twelve collimators (5-60mm) in simple setup, mean (SD) differences between MC and RayTracing algorithm (RT) of the number of points failing the 3%/1mmgamma criteria were 1(1), 1(3), 1(2) and 1(2) for the four MP configurations. Tracking fiducials were placed within AC for patient QA. Single lung insert setup resulted in mean gamma-index 2%/2mm of 90.5% (range [74.3-95.9]) and 82.3% ([66.8-94.5]) for MC and RT respectively, while 93.5% ([86.8-98.2]) and 86.2% ([68.7-95.4]) in presence of largest inhomogeneities, showing significant differences (p<0.05). DISCUSSION: After evaluating the potential effects, 1.12g/cc PMMA and 0.09g/cc lung material assignment showed the best results. Overall, MC-based model showed superior results compared to RT for simple and patient specific testing, using a 2%/2mm criteria. Results are comparable with other reported commissionings for flattening filter free (FFF) delivery. Further improvement of MC calculation might be challenging as Multiplan has limited material library. CONCLUSIONS: The AC with Multiplug allowed for comprehensive commissioning of CyberKnife MC algorithm and is useful for patient specific QA for stereotactic body radiation therapy. MC calculation accuracy might be limited due to Multiplan's insufficient material library; still results are comparable with other reported commissioning measurements using FFF beams.