A clinical application of fair value estimate (FVE) for R50%: Separating overlapping 50% isodose volumes in single isocenter multiple targets SRS

IntroductionHyperArc (HA) auto‐planning offers simplicity for the end user and consistently high‐quality SRS plans. The “Ask For It” (AFI) optimization strategy offers a manual planning technique that, when coupled with R50%Analytic, can be guided to deliver a plan with an intermediate dose spill “as low as reasonably achievable” and high target dose conformity. A direct comparison of SRS plan quality obtained using the manual planning AFI strategy and HA has been performed.MethodsUsing a CT data set available from the Radiosurgery Society, 54 PTVs were created and used to generate 19 individual SRS/SRT cases. Case complexity ranged from single PTV plans to multiple PTV plans with a single isocenter. PTV locations ranged from relative isolation from critical structures to lesions within 1.5 mm of the optic apparatus and abutting the brainstem. All cases were planned using both the AFI and HA optimization strategies as implemented in the Varian Medical Systems Eclipse Treatment Planning System. A range of treatment plan quality metrics were obtained including Intermediate Dose Spill (R50%), Conformity Indices CIRTOG and CIPaddick, PTV Dose Coverage (Dn%), PTV Mean Dose, and Modulation Factor. The Wilcoxon Signed Rank Sum non‐parametric statistical method was utilized to compare the obtained plan quality metrics.ResultsStatistically significant improvements were found for the AFI strategy for metrics R50%, CIRTOG, CIPaddick, and PTV Mean Dose (p < 0.001). HA achieved superior coverage for Dn% (p = 0.018), while the Modulation Factors were not significantly different for AFI compared to HA optimization (p = 0.13).ConclusionThis study provides evidence that the AFI manual planning strategy can produce high‐quality planning metrics similar to the HA auto‐planning method.

AFI manual planning versus HyperArc auto‐planning: A head‐to‐head comparison of SRS plan quality

Desai,

Cordrey,

Johnson

et al. 2024

Automated hippocampal sparing whole brain radiotherapy with simultaneous integrated boost for multiple brain metastases: Halcyon, HyperArc on TrueBeam, and coplanar TrueBeam

McCarthy,

Clark,

Magliari

et al. 2024

PurposeTo demonstrate the ease and feasibility that hippocampal sparing whole brain (WB) simultaneous integrated boost (HSWB‐SIB) plans can be generated using knowledge‐based planning and Eclipse Scripting Application Programming Interface (ESAPI) for three different modalities, HyperArc on TrueBeam (TB‐HA), a coplanar beam arrangement on TrueBeam (TB‐Co), and the ring‐mounted Halcyon LINAC (Hal).MethodsTwelve patients with 2–14 brain metastases were retrospectively replanned for HSWB‐SIB using a published HSWB RapidPlan model with modifications for the automated addition of SIB to metastases. Prescribed dose was 30 Gy to the WB planning target volume (PTV) and 50 Gy to the metastases in 10 fractions. Eclipse treatment planning system (v16.1) was used with a 6 MV‐FFF beam and Acuros XB dose algorithm.ResultsThe methodology was successfully used for all modalities, generating plans in under 30 min. The plan doses were normalized to the WB PTV D95% receiving 30 Gy. Reporting values in the order of Hal, TB‐Co, and TB‐HA: The WB PTV received a V48 Gy of 4.58, 3.98, and 4.45 cc with statistically insignificant differences (p = 0.806). The boost PTVs received a D95% of 50.60, 50.43, and 51.13 Gy with statistically significant comparisons between TB‐HA and the other two modalities (p = 0.005). The hippocampus maximum dose was 11.81, 11.51, and 11.13 Gy with no statistically significant comparisons (p = 0.105). All other oragns‐at‐risk (OAR) doses were clinically acceptable. The modalities were evaluated using a dosimetric scorecard, achieving average scores of 84.85%, 86.45%, and 87.39%. End‐to‐end testing ensured the deliverability of the HSWB‐SIB plans for all modalities.ConclusionThe novel modification of the preexisting HSWB RapidPlan model with the automated inclusion of SIB objectives allows for easy, intuitive planning of complex HSWB‐SIB treatments. All modalities demonstrated can be used with clinically comparable results. Other institutions are recommended to pursue and validate this HSWB‐SIB technique to increase the accessibility of a single‐course of high‐quality treatment for patients with multiple brain lesions.

A predictive model for Gamma Knife intermediate dose spill: R50%_{Analytic‐GK}

Cordrey,

Kucuk,

Ramsey

et al. 2024

PurposeMinimizing intermediate dose spill in stereotactic radiosurgery (SRS) for brain treatment is crucial. Intermediate dose spill correlates with the exposure of normal brain tissue to high doses, which increases the risk of radionecrosis. R50%, defined as the volume of the 50% of prescription isodose cloud/planning target volume, is one metric for intermediate dose spill. A predictive model for R50% in linear accelerator VMAT‐delivered SRS has been developed Desai et al. (2020) and is called R50%Analytic. This study extends the R50%Analytic model to Gamma Knife (GK) delivered SRS, resulting in the R50%Analytic‐GK model.MethodsPhantom calculations were performed on 11 spherical target volumes ranging from 0.001 to 44 cm3 to develop the R50%Analytic‐GK model. R50%Analytic‐GK was tested against clinical data from 18 brain metastasis cases with one to 11 targets treated on GK Icon and planned in GammaPlan with lightning dose optimizer. Thirty‐five targets with volumes between 0.011 and 27.4 cm3 were analyzed by extracting the R50% achieved clinically (R50%Clinical) for comparison to the predicted intermediate dose spill from R50%Analytic‐GK.ResultsThe predicted R50%Analytic‐GK values generally represent a lower bound for the R50%Clinical values as the model would predict. The Difference, R50%Clinical − R50%Analytic‐GK, has a median value of 0.92, which quantifies the lower bound nature of R50%Analytic‐GK. The model reflected the character of intermediate dose spill for the clinical cases. A few outliers were likely due to specific planning complexities.ConclusionThe R50%Analytic‐GK model for intermediate dose spill successfully extends the theoretical framework of R50%Analytic to GK‐delivered SRS. It provides a method to predict the intermediate dose spill for GK Icon treatments. This model can aid in assessing SRS treatment plans by providing a benchmark for the intermediate dose spill for comparison.