Spatially fractionated radiotherapy for re-irradiation: feasibility, safety, treatment planning, and outcomes.

Spatially fractionated radiotherapy (SFRT) is emerging as an option to deliver high dose radiation to bulky disease sites with an improved therapeutic ratio compared to established radiotherapy techniques. SFRT has not been previously studied for re-irradiation. We describe three patients with previously irradiated bulky oligometastatic disease to different sites (lung, thigh, and adrenal gland) who were re-treated with SFRT, with intention to relieve or prevent worsening of symptoms. Dose prescription was 66.7 Gy in 5 fractions to uniformly spaced lattice vertices in the gross tumour volume (GTV), with the planning target volume (PTV) receiving 20 Gy at the periphery and a mean dose of approximately 27 Gy. Dose summation and image registration techniques, and selection of organs at risk and dose constraints, were defined on a case-by-case basis. GTV volumes ranged from 272 cm3 to 3183 cm3, and mean dose delivered to the PTV ranged from 26.2 to 27.4 Gy. Two treatment plans met all clinical goals, while one plan had marginal excess dose to two structures. All three patients tolerated SFRT at least as well as their initial radiotherapy courses, with no severe radiation-related adverse events. Two patients were symptomatic prior to SFRT and achieved clinically significant symptom improvement following treatment. SFRT offers a feasible and safe re-irradiation option for bulky metastatic disease in the palliative setting.