Abstract
We studied the dosimetry of single‐isocenter treatment plans generated to treat a solitary intracranial lesion using linac‐based stereotactic radiosurgery (SRS). A common metric for evaluating SRS plan quality is the volume of normal brain tissue irradiated by a dose of at least 12 Gy (V12), which is important because multiple studies have shown a strong correlation between V12 and incidence of radiation necrosis. Unrealistic expectations for values of V12 can lead to wasted planning time. We present a model that estimates V12 without having to construct a full treatment plan. This model was derived by retrospectively analyzing 50 SRS treatment plans, each clinically approved for delivery using circular collimator cone arc therapy (CAT). Each case was re‐planned for delivery via dynamic conformal arc therapy (DCAT), and then scaling arguments were used to extend dosimetric data to account for different prescription dose (PD) values (15, 18, 21, or 24 Gy). We determined a phenomenological expression for the total volume receiving at least 12 Gy (TV12) as a function of both planning target volume (PTV) and PD: TV12/1cc=n∗PD/1Gy+d∗PTV/1cca∗PD/1Gyc, where a,c,n,d are fit parameters, and a separate set of values is determined for each plan type. In addition, we generated a sequence of plots to clarify how the relationship between conformity index (CI) and TV12 depends on plan type (CAT vs DCAT), PTV, and PD. These results can be used to suggest realistic plan parameters and planning goals before the start of treatment planning. In the absence of access to more sophisticated pre‐planning tools, this model can be locally generated and implemented at relatively low cost with respect to time, money, and expertise.
Highlights
Stereotactic radiosurgery (SRS) is a course of radiotherapy delivered in a single high‐dose fraction with submillimeter spatial accuracy, and steep dose fall‐off outside the target volume.[1]
A radiation oncologist can use a plot such as this as a rough guide to determine if stereotactic radiosurgery (SRS) treatment is feasible, and if so, to choose the amount of margin added to the gross tumor volume (GTV), the prescription dose (PD), and the SRS delivery method to meet the desired volume of normal tissue receiving at least 12 Gy (V12 = total volume receiving at least 12 Gy (TV12) − GTV)
If we focus on the V12 goal, the task of the dosimetrist can be distilled to satisfying the other stated dosimetric goals while minimizing TV12 for the given planning target volume (PTV). (Though an exception can occur in the case where the 12 Gy isodose surface is not enclosed within the brain parenchyma, and an increase in TV12 does not necessarily imply the same increase in V12.) Using the TV12 model, the physician can quickly estimate V12 (=TV12 – GTV) and can modify pre‐planning details
Summary
Stereotactic radiosurgery (SRS) is a course of radiotherapy delivered in a single high‐dose fraction with submillimeter spatial accuracy, and steep dose fall‐off outside the target volume.[1] SRS is mostly used to treat intracranial lesions, which can be either malignant, such as brain metastases, recurrent glioma, or vestibular schwannoma; or benign, such as meningiomas or arteriovenous malformations. Before the start of computer‐based treatment design and dose calculation, the dosimetric task is specified by a radiation oncologist choosing plan features, goals, and constraints. If the corresponding pre‐planning process has specified a dosimetric task with unattainable goals, treatment planning time has been wasted, and disturbances in clinical workflow and treatment scheduling can follow
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