On the Feasibility of Stereotactic Radiosurgery With 5.0 and 10.0 mm MLC Leaves as a Function of Target Size and Shape.

Wassim Jalbout,Bassem Youssef,Bilal Shahine,Jana Abou Zahr

doi:10.3389/fonc.2019.00741

Wassim Jalbout, Bassem Youssef + Show 2 more

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https://doi.org/10.3389/fonc.2019.00741

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Abstract

Introduction: This study explores the feasibility of SRS/SRT treatment with MLC leaves wider than 2.5 mm at isocenter by inter-comparing treatment plans produced with 2.5, 5.0, and 10.0 mm leaves for various target sizes and shapes.Materials and methods: Forty previously treated patients were re-planned using 2.5, 5.0, and 10.0 mm wide MLC leaves. For each patient, all three plans were evaluated and contrasted between them in terms of five metrics: target dose homogeneity, conformity index, organs at risk dose, dose fall off outside the target, and dose to normal tissues. A regularity index RI was introduced that quantified the degree of target shape irregularity. The effect of target size and shape irregularity on feasibility of 5.0 and 10.0 mm leaves was analyzed.Results: Consistent plan degradation was observed for 10.0 mm (sometimes for 5.0 mm) compared to 2.5 mm MLC in terms of the above five plan metrics, but this degradation was small to clinically insignificant. As an exception, when target (PTV) size was smaller than about 1 cm diameter, clinically significant differences were found between 2.5, 5.0, and 10.0 mm MLC.Conclusion: 5.0 and 10.0 mm MLC can be used in SRS/SRT for targets (PTV) diameter larger than 1 cm. For smaller targets, 2.5 mm MLC is clinically superior, 5.0 is acceptable and 10.0 mm MLC is discouraged in terms of PTV dose conformity.

Highlights

This study explores the feasibility of SRS/SRT treatment with multileaf collimator (MLC) leaves wider than 2.5 mm at isocenter by inter-comparing treatment plans produced with 2.5, 5.0, and 10.0 mm leaves for various target sizes and shapes
According to Monk et al [1] in a study of 14 intracranial cases using BrainSCAN v5.1 for re-planning, 3 mm MLC leaves improved both target conformity and normal tissue sparing over 5.0 mm leaves but these improvements were not very significant clinically; no statistically significant differences were found in the maximum dose to critical structures
We propose a regularity index RI that quantifies target shape irregularity and we attempt to provide numerical limits for target dimension and irregularity within which 5.0 and 10.0 mm MLC could be acceptable for SRS/SRT treatments instead of 2.5 mm

Summary

Introduction

This study explores the feasibility of SRS/SRT treatment with MLC leaves wider than 2.5 mm at isocenter by inter-comparing treatment plans produced with 2.5, 5.0, and 10.0 mm leaves for various target sizes and shapes. Stereotactic radiosurgery/radiotherapy (SRS/SRT) treatment techniques are essential components of most modern radiotherapy departments’ treatment modalities. Their application is very common in the treatment of cranial metastases and is becoming progressively more so with recent indications favoring SRS/SRT over whole brain irradiation in certain cases of multiple brain metastasis. Some centers (mostly in developing countries) are still operating linacs with 10.0 mm MLC leaves, and the question arises as to whether it is acceptable to perform SRS/SRT treatments at such a leaf width. According to Monk et al [1] in a (small) study of 14 intracranial cases using BrainSCAN v5.1 for re-planning, 3 mm MLC leaves improved both target conformity and normal tissue sparing over 5.0 mm leaves but these improvements were not very significant clinically; no statistically significant differences were found in the maximum dose to critical structures

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Conclusion