Is there any data on an effective RT regimen (dose/fractionation) for inducing an abscopal effect using a PD-1 inhibitor in refractory Hodgkin's lymphoma?  

I'm not aware of any published data for HL. There is a phase I-II study on low-grade B-cell lymphoma: "In situ vaccination with a TLR9 agonist induces systemic lymphoma regression: a phase I/II study" by Brody et al (J Clin Oncol. 2010 Oct 1;28(28):4324-32) that used a dose of 2 Gy x 2. There is a...