A standard dose of brentuximab vedotin will be administered to all patients by intravenous infusion 3 weeks prior to initiation of low-dose TSEB and then every 3 weeks for a total of 3 cycles. Cohort A patients will complete brentuximab vedotin after 3 cycles; patients in Cohort B will continue brentuximab vedotin until disease progression or unacceptable toxicity, whichever occurs first. In the absence of progression or unacceptable toxicity, the patient may receive brentuximab vedotin for up to 2 years as a study participant. A total of 12 Gy TSEB (ie, low-dose TSEB) will be administered to both cohorts per standard protocol in 6 fractions (2 fractions per week) beginning 3 weeks after the first dose of brentuximab vedotin.