Open AccessAn analytical expression for R50% dependent on PTV surface area and volume: A cranial SRS comparison
Author(s) -
Desai Dharmin D.,
Johnson E. L.,
Cordrey Ivan L.
Publication year - 2021
Publication title -
journal of applied clinical medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.83
H-Index - 48
ISSN - 1526-9914
DOI - 10.1002/acm2.13168
Subject(s) - nuclear medicine , volume (thermodynamics) , mathematics , imaging phantom , physics , medicine , quantum mechanics
The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (V IDC50% ) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SA PTV ), the volume of PTV (V PTV ), and the distance of dose drop‐off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50% Analytic ) and compare the values to clinical R50% values (R50% Clinical ) extracted from a previously published SRS data set that spans the V PTV range from 0.15 to 50.1 cm 3 . R50% Analytic is smaller than R50% Clinical in all cases by an average of 15% ± 7%, and the general trend of R50% Clinical vs V PTV is reflected in the same trend of R50% Analytic . This comparison suggests that R50% Analytic could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50% Analytic could provide useful guidance for what might be achievable in SRS planning.