Abstract
PurposeTo compare linac-based mono-isocentric radiosurgery with Brainlab Elements Multiple Brain Mets (MBM) SRS and the Gamma Knife using a specific statistical method and to analyze the dosimetric impact of the target volume geometric characteristics. A dose fall-off analysis allowed to evaluate the Gradient Index relevancy for the dose spillage characterization.Material and methodsTreatments were planned on twenty patients with three to nine brain metastases with MBM 2.0 and GammaPlan 11.0. Ninety-five metastases ranging from 0.02 to 9.61 cc were included. Paddick Index (PI), Gradient Index (GI), dose fall-off, volume of healthy brain receiving more than 12 Gy (V12Gy) and DVH were used for the plan comparison according to target volume, major axis diameter and Sphericity Index (SI). The multivariate regression approach allowed to analyze the impact of each geometric characteristic keeping all the others unchanged. A parallel study was led to evaluate the impact of the isodose line (IDL) prescription on the MBM plan quality.ResultsFor mono-isocentric linac-based radiosurgery, the IDL around 70–75% was the best compromise found. For both techniques, the GI and the dose fall-off decreased with the target volume. In comparison, PI was slightly improved with MBM for targets < 1 cc or SI > 0.78. GI was improved with GP for targets < 2.5 cc. The V12Gy was higher with MBM for lesions > 0.4 cc or SI < 0.84 and exceeded 10 cc for targets > 5 cc against 6.5 cc with GP. The presence of OAR close to the PTV had no impact on the dose fall off values. The dose fall-off was higher for volumes < 3.8 cc with GP which had the sharpest dose fall-off in the infero-superior direction up to 30%/mm. The mean beam-on time was 94 min with GP against 13 min with MBM.ConclusionsThe dose fall-off and the V12Gy were more relevant indicators than the GI for the low dose spillage assessment. Both evaluated techniques have comparable plan qualities with a slightly improved selectivity with MBM for smaller lesions but with a healthy tissues sparing slightly favorable to GP at the expense of a considerably longer irradiation time. However, a higher healthy tissue exposure must be considered for large volumes in MBM plans.
Highlights
For the multiple brain metastases treatment, whole brain radiation therapy (WBRT) tends to disappear in favor of stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT)
A higher healthy tissue exposure must be considered for large volumes in Multiple Brain Mets (MBM) plans
The gradient index increased with the isodose line (IDL) with a minimum value found for IDL 55% and a maximum for IDL 90% going from 3.91 to 5.74 (Fig. 2b) whereas the volume for the normal brain receiving at least 12 Gy decreased from IDL 50% with 10.92 cc to reach a minimum value at 8.25 cc for IDL 70% and after increased until 11.72 cc for the IDL 90% (Fig. 2c)
Summary
For the multiple brain metastases treatment, whole brain radiation therapy (WBRT) tends to disappear in favor of stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT). One prospective study [1] indicated that there was no significant difference between two to four and five to ten metastases for the overall survival with SRS alone. No prospective studies have evaluated the use of SRS relative to whole brain radiotherapy (WBRT) for patients with more than four brain metastases. The current tendency is to avoid WBRT due to the attendant toxicity and neurological deterioration after such a treatment. Since 1968, Gammaknife radiosurgery is the gold standard for stereotactic treatments. The Gamma Knife (Elekta, Stockholm, Sweden) contains 192 60Co sources and 4, 8 and 16 mm collimator options providing 192 narrow beams precisely focused on a target
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