From a clinical trial report:
I think that Ps stands for parasternal (nodes) but what could Sp mean?
I haven't found a mention of "Sp" in the literature by googling for about 20 minutes.
P.S. Got some tentative googling results. Could it be "superior phrenic nodes"? Or "segmental pulmonary" area? (I conducted Google searches using "Breast cancer"+"Ps", "Chest wall"+"Sp" etc. in different combinations and looked into research articles related to breast cancer and its metastases).