Dose Gradient Index for Stereotactic Radiosurgery/Radiation Therapy

Abstract

Purpose: Steep dose falloff outside of tumors is a hallmark of stereotactic radiosurgery (SRS) and radiation therapy (SRT). Dose gradient index (DGI) quantifies the dose drop off. Tables of DGIs versus target volumes have been published for body sites, but none is available for brain. This study recommends guidelines for DGIs for brain SRS/SRT treatments based on clinical CyberKnife (CK) cases. Methods and Materials: Four hundred ninety-five plans for patients with central nervous system tumors treated with CK at our institution between March 2015 and May 2018 were analyzed. The CK treatment planning system MultiPlan was used for planning. SRS/SRT plans were stratified into 6 groups by tumor size (Group I [0-1 cm 3 ], II [1.0-3.0 cm 3 ], III [3.0-5.0 cm 3 ], IV [5.0-10.0 cm 3 ], V [10.0-15.0 cm 3 ], and VI [15.0-40.0 cm 3 ]). Ideal and minimally acceptable DGIs were determined for each size group. To evaluate the effect of target shape on DGI criteria, the plans were divided into 4 target shape groups: (1) homogeneous shape (circular), (2) adjacent to radiosensitive organs at risk (adjacent), (3) irregularly shaped (irregular), and (4) multiple target plans (multilesion). The mean for each target size group was defined as the ideal DGI. Minimally acceptable DGI criteria are specified to reject the lowest 10% of cases. Results: The minimal acceptable DGIs were 83 (Group I), 72 (II), 65 (III), 58 (IV), 52 (V), and 35 (VI). The ideal DGI is designated to evaluate SRS/SRT plans for homogeneous circular lesions, whereas minimal DGI is chosen to assess the plans for irregular, adjacent to organs at risk, and multilesions. SRS/SRT plans with higher DGI values are correlated with lower irradiated normal tissue volumes. Conclusions: This study provides a table of DGIs for brain SRS/SRT treatments as a tool for assessing the quality of intracranial SRS/SRT plans. DGI guidelines support SRS/SRT planning that results in lower risk of radionecrosis. Ó

What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans

“…A steep dose fall-off is important in order to decrease toxicity in tissues surrounding the target, especially in treatments involving high doses per fraction. Dose gradient indices quantify this property [23].…”

A novel dose fall‐off index and preliminary application in brain and lung stereotactic radiotherapy

“…In clinical SRT practice and dose fall-off researches, the 50% prescription isodose volume has received considerable attention. [25][26][27] Therefore, NGI50 V was recommended to evaluate SRT plans in clinical use and was used in the following comparisons to achieve consistency with previous SRT experiences. The curve of NGI50 V fitting to the PTV volume was satisfactory, as shown in Figure 2(b).…”

Genomic Analysis of Human Brain Metastases Treated with Stereotactic Radiosurgery Under the Phase-II Clinical Trial (NCT03398694) Reveals DNA Damage Repair at the Peripheral Tumor Edge

“…The main hypothesized benefit of pre-operative SRS is to ensure that metastatic tumors receive the maximal conformed dose before surgery disrupts and disperses the tumor cells into the surrounding cortical tissue. Although SRS delivers a concentrated dose of radiation to the lesion, there is differential dosing between the center and periphery of the treated lesion [45][46][47] . The optimal dosing for treating a lesion strives to strike the balance between maximizing radiation induced cancer cell apoptosis (disease-control) and minimizing radiation induced brain necrosis (radionecrosis).…”