How would you manage young female patients with a phyllodes tumor of the breast s/p excision with a positive margin at the posterior fascia?

The patient was in her 30s with a 2.8 cm benign phyllodes tumor with positive margin at the posterior fascia. Surgical resection is not considered ideal as doing so would require resection into pec muscle. Using the AMOS nomogram - pt has PFS rate at 1-, 3-, 5-, 10 years at 95%, 82%, 75%, and 68%, respectively. Transformation rate has been reported up to 20%+ on recurrence. Would you recommend surveillance or adjuvant RT (either WBI or APBI)?