If the glove doesn’t fit, you must acquit
The Study: A Norwegian trial recently demonstrated that using a smaller 7 mm PTV margin with daily cone-beam CT (CBCT) compared to using a 15 mm margin with weekly portal imaging for prostate radiation had no improvement in patient-reported GI or GU toxicity. Before that, an analysis of the TROG RADAR trial found that more complex treatment techniques may have been associated with worse disease control. A new complementary French trial compares daily versus weekly CBCT-image guidance. The target volume was a 1 cm expansion from the prostate in all directions except posteriorly where it was 5 mm. Let’s stop here for a second. If you think about it, this trial design actually makes no sense. If you image-align the prostate weekly versus daily, why would you assume similar margins of setup error? That’s where the Norwegians got it right. As one might expect, daily image guidance in this trial resulted in improved biochemical control and reduced rectal toxicity. But...it also resulted in worse overall survival. Side bar: The authors (and readers) are pretty quick to dismiss this outcome as a fluke, which is fine, but we should keep this same level of skepticism when survival outcomes are reversed.
Bottom Line: For prostate radiation, small PTV margins and daily image guidance go hand in hand. Put another way, smaller target volumes are not appropriate when using less than daily image guidance. | de Crevoisier, Int J Radiat Oncol Biol Phys 2018