Briere, Tina M.

Background/Objectives: Stereotactic body radiation therapy (SBRT) for skull base reirradiation is particularly challenging, as patients have already received substantial radiation doses to the region, and nearby normal organs may have approached their tolerance limit from prior treatments. In this study, we reviewed the characteristics and capabilities of four advanced external beam radiation delivery systems and four modern treatment planning systems and evaluated the treatment plan quality of each technique using skull base reirradiation patient cases. Methods: SBRT plans were generated for sixteen skull base reirradiation patients using four modalities: the GK plan for the Elekta Leksell Gamma Knife Perfexion/ICON, the CyberKnife (CK) plan for the Accuray CyberKnife, the intensity-modulated proton therapy (IMPT) plan for the Hitachi ProBeat-FR proton therapy machine, and the volumetric-modulated arc therapy (VMAT) plan for the Varian TrueBeam STx. These plans were evaluated and compared using two novel gradient indices in addition to traditional dosimetry metrics for targets and organs at risk (OARs). The steepest border gradient quantified the percent prescription dose fall-off per millimeter at the boundary between the target and adjacent critical structures. This gradient index highlighted the system’s ability to spare nearby critical OARs. The volume gradient assessed the extent of dose spread outside the target toward the patient’s body. Results: All plans achieved comparable target coverage and conformity, while IMPT and VMAT demonstrated significantly better uniformity. The GK plans exhibited the highest border gradient, up to 20.9%/mm, followed by small-spot-size IMPT plans and CK plans. Additionally, IMPT plans showed the benefit of reduced dose spread in low-dose regions and the lowest maximum and mean doses to the brainstem and carotid artery. Conclusions: The advanced external beam radiotherapy modalities evaluated in this study are well-suited for SBRT in skull base reirradiation, which demands precise targeting of tumors with highly conformal doses and steep dose gradients to protect nearby normal structures.

Abstract PURPOSE/OBJECTIVE We seek to establish the feasibility of using spine stereotactic radiosurgery (SSRS) allowing for spinal cord dose constraint relaxation as the primary management of metastatic epidural spinal cord compression (MESCC) in inoperable patients monitoring for radiation myelopathy (RM) and radiographic local control (LC). MATERIAL/METHODS Patients with radiation naïve MESCC were enrolled on this prospective Phase 1 trial. Single fraction SSRS was delivered to a dose of 18 or 24 Gy. Spinal cord constraint relaxation was performed in sequential cohorts from an initial allowable Dmax cohort of 10 Gy up to 16 Gy only if tumor progression occurred without RM. RESULTS Thirty two patients enrolled on the trial of which 4, 12, 8 and 8 were in the 10 Gy, 12 Gy, 14 Gy and 16 Gy cord Dmax cohorts, respectively. At baseline, there were 10 sites with MESCC Grade 1B, 10 sites with Grade 1C, 9 sites with Grade 2, 2 sites with Grade 1A, and 1 site with Grade 3 disease. Of the 28 evaluable patients, the median overall survival was 32.3 months (range 5.8-122.6 mo). The 1-year and 5-year LC was 84.5% and 79.5%. The 2-year LC did not statistically differ between the 14-16 Gy cord Dmax combined cohorts and the 10-12 Gy cord Dmax combined cohorts (92.9% vs. 62.9%, p = 0.15). With a median clinical follow-up of 22.7 months (range 3-122.3 mo), there were no cases of RM including in the 13 patients who survived at least 3 years. In the cohort receiving a cord Dmax of 16 Gy, there were no cases of RM with a median follow-up of 18.6 mo (range 9.0-97.1 mo). CONCLUSION With long-term follow-up, these data demonstrate that SSRS is a safe and effective tool in patients with MESCC. Cord constraint relaxation may be considered in inoperable patients with MESCC.