Investigation of Dose Falloff for Intact Brain Metastases and Surgical Cavities Using Hypofractionated Volumetric Modulated Arc Radiotherapy

Abstract

Introduction: Intact brain metastases tend to be small and spherical compared to postsurgery brain cavities, which tend to be large and irregular shaped and, as a result, a challenge with respect to treatment planning. The purpose of the present study is to develop guidelines for normal brain tissue dose and to investigate whether there is a dependence on target type for patients treated with hypofractionated volumetric modulated arc radiotherapy (HF-VMAT). Methods: Treatment plans from a total of 100 patients and 136 targets (55 cavity and 81 intact) were retrospectively reviewed. All targets were treated with HF-VMAT with total doses ranging between 20 and 30 gray (Gy) in 5 fractions. All plans met institutional objectives for organ-at-risk constraints and were clinically delivered. Dose falloff was quantified using gradient index (GI) and distance between the 100% and 50% isodose lines (R50). Additionally, the dose to normal brain tissue (brain contour excluding all gross tumor or clinical target volumes) was assessed using volume receiving specific doses (Vx) where x ranged from 5 to 30 Gy. Best-fit curves using power law relationships of the form y ¼ ax b were generated for GI, R50, and Vx (normal brain tissue) versus target volume. Results: There was a statistically significant difference in planning target volume (PTV) for cavities versus intact metastases with mean volumes of 37.8 cm 3 and 9.5 cm 3 , respectively (P < .0001). The GI and R50 were statistically different: 3.4 and 9.8 mm for cavities versus 4.6 and 8.3 mm for intact metastases (P < .0001). The R50 increased with PTV with power law coefficients (a, b) ¼ (6.3, 0.12) and (5.9, 0.15) for cavities and intact, respectively. GI decreased with PTV with coefficients (a, b) ¼ (5.9, À0.18) and (5.7, À0.14) for cavities and intact, respectively. The normal brain tissue Vx also exhibited power law relationships with PTV for x ¼ 20 to 28.8 Gy.In conclusion, target volume is the main predictor of dose falloff. The results of the present study can be used for determining target volume-based thresholds for dose falloff and normal brain tissue dose-volume constraints.Keywords hypofractionated VMAT, brain, stereotactic radiosurgery, treatment planning Abbreviations CBCT, cone beam computed tomography; CI, conformity index; CT, computed tomography; CTV, clinical target volume; GI, gradient index; GTV, gross tumor volume; Gy, gray; HF-VMAT, hypofractionated volumetric modulated arc radiotherapy; HI, heterogeneity index; IMRT, intensity modulated radiation therapy; MD, maximum dose; MLC, multileaf collimator; MU, monitor unit; OAR, organ at risk; PD, prescribed dose; PIV, prescription isodose volume; PIV half , volume of 50% of the prescription isodose; PTV, planning target volume; QA, quality assurance; R50, radial difference between equivalent spherical volumes corresponding to the 100% and 50% isodose lines; R Eq (V), radius of an equivalent sphere with volume V; RI, regularity index; RTOG, radiation therapy oncology group; SRS, stereotactic radiosurg...

Stereotactic radiosurgery alone for multiple brain metastases? A review of clinical and technical issues

“…After accounting for target volume, further differences in dose fall-off can be realized through differences in clinical practice and technique. [103][104][105] For example, allowing multiple non-coplanar arcs with heavy beam modulation and being willing to allow hotspots in the target in excess of 150% of the prescription may produce more rapid dose fall-off than a single arc with limited modulation and a target hotspot of <120%. Furthermore, GK is planned and prescribed fundamentally differently than VMAT, and it is not surprising that there is variability in the literature regarding dose fall-off due to planning practice differences.…”

Investigation of two linear accelerator head designs for treating brain metastases with hypofractionated volumetric-modulated arc radiotherapy

“…All treatment planning (original and replanned) was performed using the Pinnacle 3 treatment planning system (TPS) v. 9.2 (Philips Healthcare, Andover, MA) with the SmartArc option. The specifics of our dose calculation and QA process follows the approach taken by Ruschin et al 11 Treatment planning for multiple targets consisted of placing isocentres such that the maximum off-axis distance of any leaf would be ,5 cm, thereby limiting the susceptibility of plan deliverability to rotational setup errors, leaf speed accuracy and off-axis small-field beam modelling. 1,12 The arc configurations used for a total of 28 arcs are shown in Table 2.…”

“…According to Ruschin et al, 11 a power-law fit was made to the data. The 95% prediction band was generated around each fit to establish a range of expected outputs for any given PTV volume made from the given set of cases.…”