Spinal cord constraints in the era of high-precision radiotherapy : Retrospective analysis of 62 spinal/paraspinal lesions with possible infringements of spinal cord constraints within a minimal volume.

OBJECTIVE: Current constraints aim to minimize the risk of radiation myelitis by the use of restrictive maximal spinal cord doses, commonly 50 Gy. However, several studies suggested that a dose-volume effect could exist. Based on these observations, we evaluated patients receiving potentially excessive doses to the spinal cord within minimal volumes. PATIENTS AND METHODS: Patients receiving radiotherapy between June 2010 and May 2015 using the NovalisTM (Varian, Palo Alto, CA, USA; Brainlab, Heimstetten, Germany) radiosurgery system were retrospectively analyzed. A total of 56 patients with 62 treated lesions that had been prescribed radiation doses close to the spinal cord potentially higher than the common 50 Gy 2­Gy equivalent-dose (EQD2) constraint were selected for further analysis. Of these patients, 26 with 31 lesions had no history of previous irradiation, while 30 patients with 31 lesions had been previously irradiated within the treatment field. RESULTS: According to different dose evaluation approaches (spinal canal, spinal cord contour), 16 and 10 out of 31 primary irradiated lesions infringed constraints. For the 16 lesions violating spinal canal doses, the maximum doses ranged from 50.5 to 61.9 Gy EQD2. Reirradiated lesions had an average and median cumulative dose of 70.5 and 69 Gy, respectively. Dose drop-off was steep in both groups. Median overall survival was 17 months. No radiation myelitis or radiomorphological alterations were observed during follow-up. CONCLUSION: This study adds to the increasing body of evidence indicating that excessive spinal cord doses within a minimal volume, especially in a reirradiation setting with topographically distinct high-point doses, may be given to patients after careful evaluation of treatment- and tumor-associated risks.

Novalis frameless image-guided noninvasive radiosurgery: initial experience.

OBJECTIVE: To evaluate our initial experience with Novalis (BrainLAB, Heimstetten, Germany) frameless image-guided noninvasive radiosurgery. METHODS: The system combines the dedicated Novalis linear accelerator with ExacTrac X-Ray 6D, an infrared camera and a kilovolt stereoscopic x-ray imaging system, a noninvasive mask system, and ExacTrac robotics for patient positioning in six degrees of freedom. Reference cranial skeletal structures are radiographically imaged and automatically fused to digital reconstructed radiographs calculated from the treatment planning computed tomographic scan to find the target position and accomplish automatic real-time tracking before and during radiosurgery. We present the acceptance testing and initial experience in 15 patients with 19 intracranial lesions treated between December 2005 and June 2006 at the Charité by frameless image-guided radiosurgery with doses between 12 and 20 Gy prescribed to the target-encompassing isodose. RESULTS: Phantom tests showed an overall system accuracy of 1.04 +/- 0.47 mm, with an average in-plane deviation of 0.02 +/- 0.96 mm for the x-axis and 0.02 +/- 0.70 mm for the y-axis. After infrared-guided patient setup of all patients, the overall average translational deviation determined by stereoscopic x-ray verification was 1.5 +/- 1.3 mm, and the overall average rotational deviation was 1.0 +/- 0.8 degree. The data used for radiosurgery, after stereoscopic x-ray verification and correction, demonstrated an overall average setup error of 0.31 +/- 0.26 mm for translation and 0.26 +/- 0.23 degree for rotation. CONCLUSION: This initial evaluation demonstrates the system accuracy and feasibility of Novalis image-guided noninvasive radiosurgery for intracranial benign and malignant lesions.