Dosimetric Discounting Of Extra-parenchymal Dose Spill In The Treatment Of Metastatic Lesions By Gamma Knife Stereotactic Radiosurgery

Paul Young1, ROJYMON JACOB2, John Fiveash1, Ruby Meredith1, Sharon Spencer1, Michael Dobelbower1

1Univ. AL, Birmingham, Birmingham, AL, USA 2UNIVERSITY OF ALABAMA

Keywords: gamma knife, dosimetry, brain metastasis, dose planning, physics

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       Dose planning for brain metastases can be based on a number of factors.
     The Purpose of this project was to investigate the dosimetric effect of neglecting, or “discounting”, the extra-parenchymal portion of the prescription isodose volume (PIV) applied to peripheral, metastatic lesions treated with Gamma Knife Stereotactic Radiosurgery.
     A series of 60, clinically treated, peripheral, metastatic lesions was investigated.
     For each lesion, the Leksell Gamma Plan Software LGP 5.34 was used to determine the total PIV and the extra-parenchymal portion, or “spill”, of the PIV. This dose spill was subtracted from the total PIV to determine a discounted PIV. Using a dose-volume, iso-effect curve, historically employed in our clinic, the total and discounted PIV were separately employed to determine two prescription dose values for each lesion. The difference in these two dose values was recorded as a potential prescription dose increase. Similarly, the total and discounted PIV values were separately employed in calculations of dose conformality index (CI = PIV/GTV where GTV = Gross Tumor Volume).
     The maximum, potential increase in dose prescription found was 1 Gy, with 10% 6/60 of lesions being assigned a potential increase of 0.5 to 1.0 Gy. Of these, 83% 5/6 were surgical resection beds, having a significant portion of their PIV spilling into the resection cavity. 12% 7/60 of the lesions, initially having CI values > 2, had their CI reduced to < 2. Only 2% 1/60 had a final CI > 2. 17% 1/6 was an unresected lesion at the extreme edge of treatable frame space. The position of this lesion necessitated the use of larger than usual shots to cover the anterior edge of the lesion.
     This was a retrospective study.
     In the treatment of peripheral lesions, especially those with resection cavities, discounting extra-parenchymal dose spill results in effectively improved dose conformality.
     The potential selection of higher prescription dose values may be important.


Project Roles:

P. Young (), R. JACOB (), J. Fiveash (), R. Meredith (), S. Spencer (), M. Dobelbower ()