A plea for the GTV median dose reporting in SBRT/can the ICRU 83 reporting way be applied to SBRT plans?

Abstract

Purpose Plan comparison is difficult with various prescriptions (80% of maximum dose, on the 70% or 50%). In 2010 the AAPM TG 101 suggests to report SBRT with prescription ICRU reference point or isodose covering PTV to a particular percentage. At the same time ICRU report 83 for IMRT was published, the ICRU point is abandoned and prescription is based on median target dose. Can we conciliate these 2 reports? Methods Theoretical plans with Cyberknife for spherical GTV of from 2 to 60 mm were created with prescription of 10 Gy with based on % of maximum dose perfectly adjusted to cover 95% of PTV. GTV median doses were collected. Plans with a shift equal to the PTV margin, in the direction of the minimum observed in the dose distribution, were created and the GTV D50% were again collected. Three different clinical situations: brain metastases, prostate and lung lesion were assessed with different percentage of maximum dose used for prescription and again applying a shift. Results The GTV median dose is little sensitive to the minimum in the PTV, and thus remains almost constant in all cases i.e. when we imagine a systematic error equal to the PTV margin. With the 6 mm PTV and a prescription isodose of 54%, i.e. with a fall-off of 20%/mm at the edge of the PTV, the GTV median dose is 14.77 Gy and 14.75 Gy with the shift. For the particular case of lung where the PTV includes a low density region, using Monte-Carlo calculation, the GTV median dose is also stable. Conclusions The GTV median dose appears to be a convenient way to describe the dose distribution delivered, whatever the % of maximum dose used for prescription. For a better understanding of the dose distributions, every team should report PTV D98%, PTV D95%, PTV D2% and GTV median dose like ICRU report 83 recommends for IMRT, in order to compare clinical studies.