Stereotactic and fractionated stereotactic radiosurgery (SRS/fSRS) utilization is growing in India, although planning studies are scarce. This study assessed clinical practices for SRS/fSRS treatment planning for brain metastases (BM) using ICRU-91 and explored the impact of planning tools.

Participants from 23 centers received two anonymized CT datasets with predrawn structures for single met (SM) and four BMs (MM) cases via email. Centers used local protocol to create plans. The plans were evaluated for target coverage, normal brain doses, and ICRU-91 dosimetric indices.

Monaco TPS overestimated mean GTV (PTV) by 3.7 (4.2)% and 2.1 (2.0)% for SM and MM respectively. Some institutions had good conformity and target coverage, whereas others had high OAR doses despite inadequate PTV dose coverage. Conformity index (CI) ranged from 1.07 to 1.45 (SM) and 1.06 to 1.25 (MM), and homogeneity index (HI) ranged from 0.07 to 0.28 (SM) and 0.13 to 0.32 (MM). Significant variation in GI and dose prescription isodose line selection was observed among centers.

There was a significant heterogeneity in the planning parameters noted among different centers. The study emphasized the importance of established planning protocols and comprehensive training for staff involved in SRS/fSRS. Notably, plans with finer MLC width outperformed, yet wider MLC plans achieved ICRU-91 indices comparable to published literature. The importance of our study is underscored by the absence of a national framework for SRS planning in India.

HyperArc (HA) is an automated planning technique enabling single-isocenter brain stereotactic radiotherapy (SRT); however, dosimetric outcomes may be influenced by the planner's expertise. This study aimed to assess the impact of institutional experience on the plan quality of HA-SRT for both single and multiple brain metastases.

Twenty patients who underwent HA-SRT for single metastasis between 2020 and 2021 comprised the earlier group, while those treated between 2022 and 2024 constituted the later group. For multiple metastases, 40 patients who received HA-SRT from 2020-2024 were divided into earlier and later treatment groups. Dosimetric parameters including gross tumor volume (GTV) doses (D98% and Dmean), volumes of the normal brain (Brain-GTV V25Gy and V30Gy), homogeneity index (HI), gradient index (GI), and total monitor unit (MU) were compared. A linear regression model was used to evaluate the effects of planning target volume (PTV) on volumes of normal brain via interaction between PTV volume and treatment era group (earlier vs. later).

The later group exhibited significantly higher D98% and Dmean values for both single and multiple metastases, while V25Gy and V30Gy and GI mean values were comparable. Consequently, mean HI and total MU values increased significantly. Both single and multiple metastases showed significant interaction between PTV volume and treatment era group.

Enhanced dosimetric outcomes in the later group suggested that accumulated experience contributed to improve GTV and brain dose in HA SRT. Institutional experience is important to improve the plan quality for SRT even with automatic planning such as HA.

The study aimed to investigate the optimal isodose line (IDL) in linear accelerator-based stereotactic radiotherapy for single brain metastasis, using HyperArc. We compared the dosimetric parameters for target and normal brain tissue among six plans with different IDLs.

This study included 30 patients with single brain metastasis. We retrospectively generated six plans for each tumor with different IDLs (80%, 70%, 60%, 50%, 40%, and 33%) using HyperArc. All treatment plans were normalized to the prescription dose of 35 Gy in five fractions which was covered by 95% of the planning target volume (PTV), defined by adding a 1.0 mm margin to the gross tumor volume (GTV). The dosimetric parameters were compared among the six plans.

For GTV > 0.1 cm3, the ratio of brain-GTV volumes receiving 25 Gy to PTV (V25Gy/PTV) was significantly lower at IDL 40%-70% than at IDL 80% and 33% (p < 0.01, retrospectively). For GTV < 0.1 cm3, V25Gy/PTV decreased continuously as IDL decreased. The values of D99% and D80% for GTV increased with decreasing IDL. An IDL of 50% or less was required to achieve D99% of greater than 43 Gy and D80% of greater than 50 Gy. The mean values of D99% and D80% for IDL 50% were 44.3 and 51.9 Gy.

The optimal IDL is 40%-50% for GTV > 0.1 cm3. These lower IDLs could increase D99% and D80% of GTV while lowering V25Gy of normal brain tissue, which may help reduce the risk of radiation necrosis and improve local control.

The purpose of this study was to systematically examine the normal tissue objective (NTO) function by comparing its variations for planning solitary brain metastasis with intensity-modulated and volumetric-modulated arc radiosurgery techniques.

Twenty-two cases were retrospectively planned with two NTO parameter sets named A and B using intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques. The Type A set used slope, k = 0.4 mm-1 plus end dose, De = 20%, whereas the Type B set used k = 1.0 mm-1 plus De = 10%. The resulting four plan types were assessed using mean dose to 5 mm exterior ring, normal brain receiving 12 Gy (V12), 5 Gy total brain dose volume (V5), gradient index (R50%), focal index (FI), Paddick conformity index (PCI), prescription isodose surface (PIDS), and MU/Gy.

Brain doses were significantly lower for VMAT than for IMRT. R50% was more favorable for VMAT than for IMRT for each planning target volume (PTV). The mean FI was comparable between the corresponding IMRT and VMAT plan types. PCI was better for the IMRT_A plan type. PIDS was significantly lower for Type B plans than Type A for both techniques. For PTVs <3 cm3, IMRT plans showed poor dosimetry and required NTO settings stricter than Type B.

The application of NTO variations demonstrated varied dosimetry for IMRT and VMAT techniques. The NTO parameter variations produced field size and/or beamlet size/shape variations. The strict NTO parameter set generated more conformal beam apertures to reduce the brain dose. VMAT plan types showed significantly lower brain doses and better dosimetry for all target sizes.

Stereotactic radiotherapy (SRT) for brain metastases (BM) allows very good local control (LC). However, approximately 20%-30% of these lesions will recur. The objective of this retrospective study was to evaluate the impact of dosimetric parameters on LC in cerebral SRT.

Patients treated with SRT for 1-3 BM between January 2015 and December 2018 were retrospectively included. A total of 349 patients with 538 lesions were included. The median gross tumour volume (GTV) was 2 cm3 (IQR, 0-7). The median biological effective dose with α/β = 10 (BED10) was 60 Gy (IQR, 32-82). The median prescription isodose was 71% (IQR, 70-80). Correlations with LC were examined using the Cox regression model.

The median follow-up period was 55 months (min-max, 7-85). Median overall survival was 17.8 months (IQR, 15.2-21.9). There were 95 recurrences and LC at 1 and 2 years was 87.1% (95% CI, 84-90) and 78.1% (95% CI, 73.9-82.4), respectively. Univariate analysis showed that systemic treatment, dose to 2% and 50% of the planning target volume (PTV), BED10 > 50 Gy, and low PTV and GTV volume were significantly correlated with better LC. In the multivariate analysis, GTV volume, isodose, and BED10 were significantly associated with LC.

These results show the importance of a BED10 > 50 Gy associated with a prescription isodose <80% to optimize LC during SRT for BM.

Isodose, BED, and GTV volume were significantly associated with LC. A low isodose improves LC without increasing the risk of radionecrosis.

There is wide variation in treatment planning strategy for central nervous system (CNS) stereotactic radiosurgery. We sought to understand what relationships exist between intratumor maximum dose and local control (LC) or CNS toxicity, and dosimetric effects of constraining hotspots on plan quality of multiple metastases volumetric modulated arc therapy radiosurgery plans.

We captured brain metastases from 2015 to 2017 treated with single-isocenter volumetric modulated arc therapy radiosurgery. Included tumors received single-fraction stereotactic radiosurgery, had no previous surgery or radiation, and available follow-up imaging. Our criterion for local failure was 25% increase in tumor diameter on follow-up MRI or pathologic confirmation of tumor recurrence. We defined significant CNS toxicity as Radiation Therapy Oncology Group irreversible Grade 3 or higher. We performed univariate and multivariate analyses evaluating factors affecting LC. We examined 10 stereotactic radiosurgery plans with prescriptions of 18 Gy to all targets originally planned without constraints on the maximum dose within the tumor. We replanned each with a constraint of Dmax 120%. We compared V50%, mean brain dose, and Dmax between plans.

Five hundred and thirty tumors in 116 patients were available for analysis. Median prescription dose was 18 Gy, and median prescription isodose line (IDL) was 73%. Kaplan-Meier estimate of 12-month LC only tumor volume (HR 1.43 [1.22-1.68] P < .001) was predictive of local failure on univariate analysis; prescription IDL and histology were not. In multivariate analysis, tumor volume impacted local failure (HR 1.43 [1.22-1.69] P < .001) but prescription IDL did not (HR 0.95 [0.86-1.05] P = .288). Only a single grade 3 and 2 grade 4 toxicities were observed; tumor volume was predictive of CNS toxicity (HR 1.58 [1.25-2.00]; P < .001), whereas prescription IDL was not (HR 1.01 [0.87-1.17] P = .940).

The prescription isodose line had no impact on local tumor control or CNS toxicity. Penalizing radiosurgery hotspots resulted in worse radiosurgery plans with poorer gradient. Limiting maximum dose in gross tumor causes increased collateral exposure to surrounding tissue and should be avoided.

To compare the dosimetric parameters of automated noncoplanar volumetric modulated arc therapy plans using single-isocentric (SIC) and multi-isocentric (MIC) techniques for patients with two brain metastases (BMs) in stereotactic irradiation and to evaluate the robustness of rotational errors. The SIC and MIC plans were retrospectively generated (35 Gy/five fractions) for 58 patients. Subsequently, a receiver operating characteristic curve analysis between the tumor surface distance (TSD) and V_25Gy was performed to determine the thresholds for the brain tissue. The SIC and MIC plans were recalculated based on the rotational images to evaluate the dosimetric impact of rotational error. The MIC plans showed better brain tissue sparing for TSD > 6.6 cm. The SIC plans provided a significantly better conformity index for TSD ≤ 6.6 cm, while significantly lower gradient index was obtained (3.22 ± 0.56vs. 3.30 ± 0.57, p < 0.05) in the MIC plans with TSD > 6.6 cm. For organs at risk (OARs) (brainstem, chiasm, lens, optic nerves, and retinas), D_0.1 cc was significantly lower (p < 0.05) in the MIC plans than in the SIC plans. The prescription dose could be delivered (D_99%) to the gross tumor volume (GTV) for patients with TSD ≤ 6.6 cm when the rotational error was < 1°, whereas 31% of the D_99% of GTV fell below the prescription dose with TSD > 6.6 cm. MIC plans can be an optimal approach for reducing doses to OARs and providing robustness against rotational errors in BMs with TSD > 6.6 cm.

The following Indexes: Homogeneity, Gradient, Conformity, Paddick Conformity and New Conformity of the dose distribution were compared. The parameters to assess a high dose to the organs at risk: V₁₀/TV, V_90%/TV and the Integral Dose were discussed. The higher the prescription isodose, the more uniform the dose distribution in the target, which is highly beneficial in the case of larger tumor sizes due to the lower risk of complications. For smaller tumors, higher dose heterogeneity is desirable. This can be obtained with a 40% prescription isodose.

The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (V_IDC50% ) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SA_PTV ), the volume of PTV (V_PTV ), and the distance of dose drop-off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50%_Analytic ) and compare the values to clinical R50% values (R50%_Clinical ) extracted from a previously published SRS data set that spans the V_PTV range from 0.15 to 50.1 cm³ . R50%_Analytic is smaller than R50%_Clinical in all cases by an average of 15% ± 7%, and the general trend of R50%_Clinical vs V_PTV is reflected in the same trend of R50%_Analytic . This comparison suggests that R50%_Analytic could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50%_Analytic could provide useful guidance for what might be achievable in SRS planning.

The possibility of intentionally triggering non targeted effects (NTEs) using spatially fractionated radiotherapy (SFRT) alone or combined with immunotherapy is an intriguing and fascinating area of research. Among different techniques for SFRT, stereotactic body radiotherapy targeting exclusively the central hypoxic segment of bulky tumors, (SBRT-PATHY) might trigger immunogenic cell death more efficiently. This in silico study aims to identify the best possible dosimetric trade-off for prescribing SFRT with volumetric modulated arc (VMAT) based stereotactic radiotherapy (SRT).

Eight spherical volumes defined "Gross Tumor Volumes" (GTVs) were generated with diameters of 3-10 cm (with incremental steps of 1 cm), simulating tumor lesions. The inner third part of each GTV (GTV_central) was selected to simulate the central hypoxic area and a ring structure was derived around it to simulate the tumor periphery (GTV_peripheral). Volumetric modulated arc radiation treatment (VMAT) plans were calculated to deliver a single fraction of 10 Gy to each GTV_central with different dose prescription methods: target mean and isodose driven (40, 50, 60, 70, 80 and 90%).The volume of GTV_peripheral receiving less than 2 Gy was recorded as dosimetric performance indicator.

56 possible dosimetric scenarios were analyzed. The largest percentage of GTV_peripheral spared from the dose of 2 Gy was achieved with dose prescription methods to the 70% isodose line for lesions smaller than 6 cm (range 42.9-48.4%) and to the target mean for larger ones (range 52.9-64.5%).

Optimizing the dose prescription method may reduce the dose to tumor periphery in VMAT-based SFRT, thus potentially sparing tumor infiltrating immune cells. The optimal method may vary according to the size of the lesion. This should be taken into account when designing prospective trials using SFRT.