Top Line: Spatially-fractionated radiation is a fascinating theoretical way to minimize toxicity when treating a large target with ablative radiation.
The Study: Spatially-fractionated RT can be delivered in a grid-like pattern with 3D techniques (aka GRID) or as a more complex lattice-like simultaneous integrated boost (SIB) to gross tumor within a lower-dose distributed to a larger planning target volume with intensity modulated radiation therapy (IMRT) techniques (aka Lattice). With ablative radiation coming on the scene for even soft tissue sarcomas (STS), a disease site with notoriously large target volumes, the time is ripe for exploring such out-of-the box planning techniques. The phase 1 LITE SABR M1 trial prospectively evaluated Lattice radiation for tumors 4.5 cm or larger. The prescription was 20 Gy to the planning target volume (PTV), a 5-10 mm expansion on the gross tumor volume (GTV), with a SIB of 66.7 Gy to the GTV all in 5 fractions delivered every other day. Gating was used for tumor motion > 5 mm, and patients could not receive any systemic therapy 2 weeks before or after radiation.. There were 22 tumors treated across 20 patients, the majority had STS (n=9) or non-small cell lung cancer (n=7). Median tumor volume was a whopping 579 cc. The primary endpoint of grade 3+ toxicity occurred in one patient who experienced urosepsis after 2 of 5 planned treatments to a retroperitoneal STS. This occurred in a patient with preexisting nephrostomy tubes in place so our money is on true, true, and unrelated. Median tumor shrinkage at 2 months was 25% and at 5 months was nearly 50%.
TBL: Larger ongoing studies at Wash U will further confirm whether Lattice radiation is a safe and effective way to tackle large tumors with ablative radiation doses. | Duriseti, Radiother Oncol 2021