Consideration of Margin in SRS/SRT Plans for Brain Metastases Treatment With Volumetric Modulated Radiotherapy

Abstract

It is well known that when treating brain metastases tumors local control depends not only on the amount of dose delivered but also on the margin to the clinical target volume in SRS/SRT treatment. The margin is commonly thought to be contributed from uncertainties of planning/treatment delivery, such as setup, imaging and target contouring, etc. It is defined as the margin expanding clinical target volume (CTV) to plan target volume (PTV). This margin, however, is the intended planning margin (PM) of clinicians. When considering contribution to the local control/toxicities from margin, it should be the delivery margin (DM), which is defined as the volume between CTV and the isodose surface at the minimum dose to PTV. The DM is contributed both from the high dose region outside of PTV and from the PM. The former has been ignored in current clinical practice though it is also a crucial part to affect the local control and treatment toxicities since this part is the high dose region in normal tissue but low dose region for the target.To access the contribution from the high dose region outside of PTV, two indexes were proposed: the modified conformity index (mCI) and modified Paddick confirmity index (mPCI). mCI = Vmin/PTV = CI + Vextra/PTV, where Vmin is the volume of brain tissue received the minimum dose to PTV or more and Vextra is the volume of brain tissue received the dose between the minimum dose to PTV and the prescribed dose. mPCI = TV2min/(PTV*Vmin), where TVmin is the target volume that is covered by the minimum dose to PTV. This retrospective study evaluated the DM with mCI, mPCI, V12 Gy, etc. for volumetric modulated radiotherapy (VMAT) planning techniques in 50 clinically treated brain metastasis cases.After implementing a set of planning techniques conformity has largely improved in the retrospective plans. Comparing the clinically treated plans with the retrospective study plans, the average CI and mCI were decreased by (6.5 ± 4.1)% and (28.1 ± 5.7)%, and the PCI and mPCI have been increased by (8.9 ± 2.9)% and (17.2 ± 2.8)% respectively (all data is expressed as ± std). The V12 (normal tissue volume irradiated within 12 Gy) was reduced by more than 35%. Comparison of historic data from 3D conformal, Dynamic conform arc and VMAT was made, and it showed that the typical CI decreased about 36% from the pioneer modality of SRS/SRT (1.67 ± 0.07) to the latest modality, VMAT (1.07 ± 0.05). This indicates that the more conformal is treatment plan the smaller the DM margin.Proposed mCI and mPCI to evaluate the DM in SRS/SRT plans for BM treatment with VMAT. Implementation of a set of planning techniques in VMAT largely improved the conformity of the VMAT plans. Comparison of CI from the progress of treatment modalities showed that the DM has been reduced 36% and its clinical impact on local control/toxicity is unknown.D. Wang: None. A.S. DeNittis: None.