Background and purpose: We evaluated the effect of block margin on small fields when point dose prescription (ICRU) or isodose line prescription (RTOG) formats are used.
Material and methods: A total of 11 clinical SBRT cases, one 4-field prostate case and 2 phantom cases using 0, 0.5 or 1 cm block margins were analysed. Integral dose and target coverage were compared using DVHs and isodose volumes for either isodose line prescription (100% Rx dose to 95% PTV volume) or isocenter point prescription (100% Rx dose to the isocenter) were calculated.
Results: Tight planning target margins using isodose line prescription leads to good target coverage but high dose heterogeneity with hot spots possibly exceeding 140% of the prescription dose for small target volumes. As block margin is increased, target coverage converges for the two methods but point dose prescriptions result in better dose homogeneity. For a given block margin, integral doses are consistently larger for isodose line prescription over point prescription, but are similar when block margins are adjusted to produce equal target coverage. As target size increases dose heterogeneity and integral dose differences disappear.
Conclusions: For small targets, the ICRU point prescription method can produce comparable PTV coverage to the isodose line prescription method with less dose heterogeneity and comparable integral dose. Reduction of hot spots in potentially normal tissue and reporting clarity makes this internationally recommended prescription standard preferable.