Dosimetric impact of rotational errors in trigeminal neuralgia radiosurgery using CyberKnife.

PURPOSE: Trigeminal neuralgia (TN) can be treated on the CyberKnife system using two different treatment delivery paths: the general-purpose full path corrects small rotations, while the dedicated trigeminal path improves dose fall-off but does not allow rotational corrections. The study evaluates the impact of uncorrected rotations on brainstem dose and the length of CN5 (denoted as Leff) covered by the prescription dose. METHODS AND MATERIALS: A proposed model estimates the delivered dose considering translational and rotational delivery errors for TN treatments on the CyberKnife system. The model is validated using radiochromic film measurements with and without rotational setup error for both paths. Leff and the brainstem dose is retrospectively assessed for 24 cases planned using the trigeminal path. For 15 cases, plans generated using both paths are compared for the target coverage and toxicity to the brainstem. RESULTS: In experimental validations, measured and estimated doses agree at 1%/1 mm level. For 24 cases, the treated Leff is 5.3 ± 1.7 mm, reduced from 5.9 ± 1.8 mm in the planned dose. Constraints for the brainstem are met in 23 cases for the treated dose but require frequent treatment interruption to maintain rotational corrections <0.5° using the trigeminal path. The treated length of CN5, and plan quality metrics are similar for the two paths, favoring the full path where rotations are corrected. CONCLUSIONS: We validated an analytical model that can provide patient-specific tolerances on rotations to meet plan objectives. Treatment using the full path can reduce treatment time and allow for rotational corrections.

Ablative vs. Non-Ablative Radiotherapy in Palliating Locally Advanced Pancreatic Cancer: A Single Institution Experience and a Systematic Review of the Literature.

We studied the dose-local control (LC) relationship in ablative vs. non-ablative radiotherapy in a non-radical treatment setting of "locally advanced pancreatic cancer (LAPC)" by comparing our patients (n = 89) treated with SBRT on the CyberKnife unit vs. conventional radiation between January 2005 and January 2021, and by reviewing the literature. A systematic search was performed leveraging Medline for references on SBRT use in pancreatic cancer without date terms or language restrictions. A total of 3702 references were identified and the search was then repeated in Embase and the Cochrane database. Ultimately, 12 studies were eligible for inclusion, which either compared SBRT to conventional radiation, or SBRT use in dose escalation for primary LAPC in a non-neoadjuvant setting. Our cohort's median overall survival was 152 days (CI 95%, 118-185); including 371 days (CI 95%, 230-511) vs. 126 days (CI 95%, 90-161) favoring SBRT, p = 0.004. The median time to local progression was 170 days (48-923) for SBRT vs. 107 days (27-489) for the non-ablative group. In our SBRT patients, no local progressions were seen with BED10 > 60 Gy. Even when palliating LAPC, SBRT should be considered as an alternative to conventional radiation, especially in patients with a low disease burden. BED10 ≥ 60-70 Gy offers better local control without increasing toxicity rates. Less local progression may provide a better quality of life to those patients who already have a short life expectancy.

Stereotactic radiosurgery optimization with hippocampal-sparing in patients treated for brain metastases.

BACKGROUND AND PURPOSE: Cranial irradiation is associated with significant neurocognitive sequelae, secondary to radiation-induced damage to hippocampal cells. It has been shown that hippocampal-sparing (HS) leads to modest benefit in neurocognitive function in patients with brain metastases, but further improvement is possible. We hypothesized that improved benefits could be seen using HS in patients treated with stereotactic radiation (HS-SRS). Our study evaluated whether the hippocampal dose could be significantly reduced in the treatment of brain metastases using SRS, while maintaining target coverage. MATERIALS AND METHODS: Sixty SRS plans were re-planned to minimize dose to the hippocampus while maintaining target coverage. Patients with metastases within 5 mm of the hippocampus were excluded. Minimum, mean, maximum and dose to 40% (mean equivalent dose in 2 Gy per fraction, EQD2 to the hippocampus) were compared between SRS and HS-SRS plans. Median number of brain metastases was two. RESULTS: Compared to baseline SRS plans, hippocampal-sparing plans demonstrated Dmin was reduced by 35%, from 0.4 Gy to 0.3 Gy (p-value 0.02). Similarly, Dmax was reduced by 55%, from 8.2 Gy to 3.6 Gy, Dmean by 52%, from 1.6 Gy to 0.5 Gy, and D40 by 50%, from 1.8 Gy to 0.9 Gy (p-values <0.001). CONCLUSIONS: Our study demonstrated that further reduction of hippocampal doses of more than 50% is possible in the treatment of brain metastases with SRS using dose optimization. This could result in significantly improved neurocognitive outcomes for patients treated for brain metastases.

Unresectable Foramen Magnum Meningioma Treated With CyberKnife Robotic Stereotactic Radiosurgery.

Meningiomas are tumors that originate from the meningeal or dural cover of the brain and are the most common primary benign brain tumors. Currently, the accepted management of these tumors is attempted total surgical excision when technically possible and associated with an acceptable risk. However, radiation therapy has been shown to provide excellent local control when used either as an adjunct to surgery or as a primary treatment. We present a case report of a 46-year-old female patient with an unresectable left foramen magnum meningioma resulting in headaches, neck pain, and swallowing difficulty. The patient underwent CyberKnife (Accuray Incorporated, Sunnyvale, CA) radiosurgery to a dose of 3,000 cGy in five fractions in March 2011. The patient tolerated treatment without complications and has remained clinically well with no ongoing cranial nerve deficits as of the last examination in late 2019. This demonstrates the excellent local control obtained when using radiosurgery as both a surgical adjunct and a primary treatment for meningiomas.

Reducing errors in prostate tracking with an improved fiducial implantation protocol for CyberKnife based stereotactic body radiotherapy (SBRT).

PURPOSE: Ultra-hypofractionated radiotherapy with SBRT is an established technique for treating localized prostate cancer. CyberKnife based SBRT requires implantation of fiducial markers for soft tissue target tracking by the orthogonal KV X-ray imaging system. The spatial distribution of fiducial markers must allow accurate calculation of a 3D transformation that describes the position of the prostate within the reference frame of the planning CT scan. Accuray provides a fiducial implantation guideline for tracking soft tissue lesions. Despite using the guideline we experienced an unacceptably high rate of rotational tracking failure due to problems with fiducial placement. We adapted the Accuray guideline to prostate SBRT for improved fiducial placement and more reliable target tracking.Methods and materials: 54 patients with prostate adenocarcinoma were treated with ultra-hypofractionated radiotherapy on CyberKnife. Patients had platinum fiducial markers implanted transrectally under ultrasound guidance by a Radiologist. For the first 26 patients, fiducial markers were positioned following the Accuray fiducial placement guidelines for soft tissue lesions (cohort 1). The initial rotational tracking error rate was unacceptably high (23%). On review, inappropriate fiducial placement was identified as the cause of error (especially insufficient spacing between seeds). In October 2016 we developed a seed placement protocol specifically for implanting fiducial markers within the prostate and a second cohort of patients was treated thereafter (cohort 2, 28 patients). The stipulations of the original guideline are maintained while the modified protocol requires that 4 fiducial markers be implanted in the postero-lateral peripheral zone in a single coronal plane. RESULTS: In cohort 1, patients had a median age of 64 years (50 - 74), PSA of 6.6mcg/L (1.1 - 14.7), and prostate volume of 56 cc (22 - 125), while in cohort 2 they had a mean age of 65 years (53 - 75), PSA of 6.2 mcg/L (1 - 12) and prostate volume of 47 cc (21 - 106). The fiducial markers were easily visualized and there were no cases of urosepsis related to fiducial implantation. In 6 of 26 patients (23%) from cohort 1, only translational mapping without accurate spatial rotations could be calculated. After adopting the prostate specific fiducial implantation protocol, rotational tracking error was eliminated. Accurate 6 degree tracking (accounting for translations and rotations) was achieved in all 28 patients from cohort 2. Using an in-house computer script we analyzed the dose distributions resulting from rotational misalignments of -10, -5, -3, 3, 5, and 10 degrees along all three rotational axes (pitch, roll and yaw). Rotational misalignments result in decreased minimum dose to the PTV and increased maximum dose to OARs. CONCLUSION: Implementing a prostate specific fiducial placement protocol for SBRT significantly improved our ability to track prostate motion in 6 degrees 77% to 100% reliability. Failure to track rotations can potentially lead to underdosing and overdosing of portions of the prostate and OARs respectively.