Impact of prescription isodose level and collimator selection on dose homogeneity and plan quality in robotic radiosurgery

Purpose In stereotactic radiosurgery (SRS), prescription isodoses and resulting dose homogeneities vary widely across different platforms and clinical entities. Our goal was to investigate the physical limitations of generating dose distributions with an intended level of homogeneity in robotic SRS. Methods Treatment plans for non-isocentric irradiation of 4 spherical phantom targets (volume 0.27–7.70 ml) and 4 clinical targets (volume 0.50–5.70 ml) were calculated using Sequential (phantom) or VOLOTM (clinical) optimizers (Accuray, Sunnyvale, CA, USA). Dose conformity, volume of 12 Gy isodose (V12Gy) as a measure for dose gradient, and treatment time were recorded for different prescribed isodose levels (PILs) and collimator settings. In addition, isocentric irradiation of phantom targets was examined, with dose homogeneity modified by using different collimator sizes. Results Dose conformity was generally high (nCI ≤ 1.25) and varied little with PIL. For all targets and collimator sets, V12Gy was highest for PIL ≥ 80% and lowest for PIL ≤ 65%. The impact of PIL on V12Gy was highest for isocentric irradiation and lowest for clinical targets (VOLOTM optimization). The variability of V12Gy as a function of collimator selection was significantly higher than that of PIL. V12Gy and treatment time were negatively correlated. Plans utilizing a single collimator with a diameter in the range of 70–80% of the target diameter were fastest, but showed the strongest dependence on PIL. Conclusion Inhomogeneous dose distributions with PIL ≤ 70% can be used to minimize dose to normal tissue. PIL ≥ 90% is associated with a marked and significant increase in off-target dose exposure. Careful selection of collimators during planning is even more important.

Salvage Radiosurgery for Recurrent Supratentorial Primitive Neuroectodermal Tumors: A Single Institutional Series and Review of the Literature

<b><i>Introduction:</i></b> Supratentorial primitive neuroectodermal tumor is a rare, aggressive intrinsic brain tumor with limited treatment options for recurrent disease. SRS as a treatment modality in the recurrent setting was investigated. <b><i>Methods:</i></b> A retrospective review of 8 patients treated with SRS for local or distant recurrence of supratentorial PNET from 1999 to 2014 was conducted. <b><i>Results:</i></b> Thirty-six tumors were treated in 15 sessions in 8 patients. The median patient age was 22.5 (interquartile range [IQR], 14.75–43.5 years) with a median 21-month period from diagnosis until SRS (IQR, 16–23.75 months). The median prescription isodose volume was 1.85 cm³ (IQR, 1.85–7.02 cm³); median tumor margin dose was 18 Gy (IQR 14–20 Gy); and median isocenters was 2 (range 1–13). No patients experienced adverse radiation effects. All but 1 patient died, and the median overall survival was 32 months (IQR, 26.75–53.5 months) with median overall survival following SRS of 9.5 months (IQR, 5.25–30 months). Univariate analysis failed to demonstrate a statistically significant association between age, number of gamma knife treatments, interval to gamma knife, and margin radiation dose with overall survival. <b><i>Discussion/Conclusion:</i></b> This series supports the use of SRS in patients with recurrent supratentorial PNET following multimodal therapy.

Focus on the expected quality of reporting in SBRT/radiosurgery prospective studies: how far have we come in 30 years?

Objectives: We aimed at describing and assessing the quality of reporting in all published prospective trials about radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Methods: The Medline database was searched for. The reporting of study design, patients’ and radiotherapy characteristics, previous and concurrent cancer treatments, acute and late toxicities and assessment of quality of life were collected. Results: 114 articles – published between 1989 and 2019 - were analysed. 21 trials were randomised (18.4%). Randomisation information was unavailable in 59.6% of the publications. Data about randomisation, ITT analysis and whether the study was multicentre or not, had been significantly less reported during the 2010–2019 publication period than before (respectively 29.4% vs 57.4% (p < 0.001), 20.6% vs 57.4% (p < 0.001), 48.5% vs 68.1% (p < 0.001). 89.5% of the articles reported the number of included patients. Information about radiation total dose was available in 86% of cases and dose per fraction in 78.1%. Regarding the method of dose prescription, the prescription isodose was the most reported information (58.8%). The reporting of radiotherapy characteristics did not improve during the 2010 s-2019s. Acute and late high-grade toxicity was reported in 37.7 and 30.7%, respectively. Their reporting decreased in recent period, especially for all-grade late toxicities (p = 0.044). Conclusion: It seems necessary to meet stricter specifications to improve the quality of reporting. Advances in knowledge: Our work results in one of the rare analyses of radiosurgery and SBRT publications. Literature must include necessary information to first, ensure treatments can be compared and reproduced and secondly, to permit to decide on new standards of care.

The Effect of Prescription Isodose Variation on Tumor Control and Toxicities in Stereotactic Radiosurgery for Sporadic Vestibular Schwannoma: Propensity Score-Matched Case–Control Study

Objective Vestibular schwannoma (VS) treated with Gamma Knife stereotactic radiosurgery (SRS) was typically performed at 50% isodose line (IDL50); however, the impact of IDL variation on outcomes is poorly understood. This study aimed to compare tumor control (TC) and toxicities between treatment at 40% (IDL40) and 50% (IDL50). Methods and Materials Sporadic/unilateral VS patients treated with SRS dose 12 to 14 Gy and prescription isodose volume ≤10cm3 were included. Propensity score matching was applied to IDL40 cohort to generate an IDL50 companion cohort, adjusting for age and prescription isodose volume. After exclusion of patients with follow-up <24 months, there were 30 and 28 patients in IDL40 and IDL50 cohorts, respectively. Results Median follow-up time was 96 months (24–225 months). Actuarial and radiographic TC rates were 91.8% and clinical TC was 96.2% both at 5 and 10 years. TC was higher in IDL40 cohort but not significant (96.4 vs. 86.7%; p = 0.243). Hearing preservation (HP) rates were 71.9 and 39.2% at 5- and 10-year intervals, with significantly higher rates of HP noted in IDL40 cohort (83.3 vs. 57.1% at 5-year interval; 62.5 vs. 11.4% at 10-year interval; p = 0.017). Permanent facial neuropathy occurred in two patients, both from the IDL50 cohort (3.5%). Rates of post-SRS steroid treatment or shunt placement for hydrocephalus were slightly higher in IDL50 patients (6.9 vs. 17.9%; p = 0.208 and 3.3 vs. 7.1%; p = 0.532). Conclusion For treatment of VS with SRS, dose prescription at IDL40 or IDL50 provides excellent long-term TC and toxicity profiles. IDL40 may be associated with improved long-term HP.

Does the Diffuseness of the Nidus Affect the Outcome of Stereotactic Radiosurgery in Patients with Unruptured Cerebral Arteriovenous Malformations?

<b><i>Background:</i></b> We proposed an algorithm to automate the components within the identification of components within the nidus of cerebral arteriovenous malformations (AVMs) which may be used to analyze the relationship between its diffuseness and treatment outcomes following stereotactic radiosurgery (SRS). <b><i>Objectives:</i></b> to determine the impact of the diffuseness of the AVM nidus on SRS outcomes. <b><i>Methods:</i></b> This study conducted regular follow-ups of 209 patients with unruptured AVMs who underwent SRS. The diffuseness of the AVM nidus was estimated by quantifying the proportions of vascular nidal component, brain parenchyma, and cerebrospinal fluid in T2-weighted MRIs. We used Cox regression analysis to characterize the association between nidal diffuseness and treatment outcomes in terms of obliteration rate and radiation-induced change (RICs) rate following SRS. <b><i>Results:</i></b> The median AVM volume was 20.7 cm³. The median duration of imaging follow-up was 51 months after SRS. The overall AVM obliteration rate was 68.4%. RICs were identified in 156 of the 209 patients (74.6%). The median proportions of the nidus of AVM and brain parenchyma components within the prescription isodose range were 30.2 and 52.2%, respectively. Cox regression multivariate analysis revealed that the only factor associated with AVM obliteration rate after SRS was AVM volume. However, a larger AVM volume (&#x3e;20 mL) and a larger proportion of brain parenchyma (&#x3e;50%) within the prescription isodose range were both correlated with a higher RIC rate following SRS. <b><i>Conclusions:</i></b> The diffuseness of the nidus indeed appears to affect the RIC rate following SRS in patients with unruptured AVMs.