Patterns of Dose Prescription and Recording in Stereotactic Body Radiation Therapy: A Multi-institutional Study

Abstract

Hypofractionated stereotactic body radiation therapy (SBRT) has become standard of care for small inoperable lesions in many disease sites. Dose prescription and delivery play an important role; however there is no consistent way of reporting dose and treatment outcome (1). This study evaluates the variability in dose prescription and reporting among academic institutions. The SBRT data of 428 patients from 8 academic institutions were collected. Prescription and delivery parameters were examined such as treatment site, technique (3DCRT, IMRT, VMAT, etc.), treatment planning system, inhomogeneity correction algorithm, dose prescription, size of target volume, normalization and monitor unit (MU). The dose-volume histogram (DVH) for each patient was anonymously analyzed at a central location following international guidelines, ICRU-50 and ICRU-83 for 3DCRT and IMRT, respectively. Dose parameters; D100, D98, D95, D50, D2 were extracted for analysis of SBRT plan and variability among institutions. Use of 3DCRT, IMRT and VMAT was 12%, 31%, and 57%, respectively. Surprisingly every institution used same treatment planning and algorithm for dose calculation. Disease sites were mainly lung (71%) followed by liver (15.5%), prostate (7.5%), spine (6.8%), brain (4.1%), and pancreas (2.5%). Target volume in lung cases varied and largely fell into two groups (22.0-28.7) and (48.0-67.1) cm3 indicating differences in margin or selection of larger tumors. The MU was inversely proportion to the treatment volume. A hot spot ranging from 120-150% was noted in nearly half of the patients and was more specific to some institutions. A D50 ≥110% was found in nearly half of the institutions. Three of the four institutions that delivered D50 values <110% still had a difference between D50 and D98 of ≥10%, suggesting the relative dose distributions among these 3 institutions was similar in general to that of the other four with high D50 values. These same seven institutions had relatively high inhomogeneity index (HI) as summarized in Table 1. As shown by the large variation in dosimetry among academic institutions, a single dose parameter to characterize SBRT prescriptions in the literature or in guidelines can be very misleading. Large variation in volume indicates larger margin characteristics of institutional practice. An attempt to unify dose prescription and delivery is needed for outcome comparison as we move towards more hypofractionated treatments.