"p" in this case should mean pathological.

I found more information at this web site:

http://training.seer.cancer.gov/module_staging_cancer/unit03_sec03_part04_ajcc_guidelines.html


The excerpts below explain the subject.

16. Each component may be individually defined as clinical or pathological, such as pT3 cN0 cM0. When components with mixed staging bases are stage grouped, the general clinical and pathologic rules for classification apply. For example, clinical M categories may be used for pathologic stage grouping, but pathologic T categories should not be stage grouped with clinically stage grouped cases.

17. The category X is used in each element to indicate that there has been no assessment of that characteristic of the tumor. It is important to differentiate the fact that no attempt was made to assess the element from the fact that nothing was found. For example, NX means that no diagnostic tools were used to evaluate the status of lymph nodes, and N0 means that no lymph node involvement was found by radiography or other methods.

18. If a stage grouping contains the term "any," such as Any T or Any N, the stage grouping is based on other elements and it does not matter what that T or N is. Consequently, even missing information including the X (not assessed) category is acceptable for that element in that stage group. For example, the chest CT scan for a lung cancer patient documents that there is a large mass on the right side and contralateral mediastinal lymph nodes involved. This case would be staged as N3. Assuming that distant metastases are ruled out (M0), it would not matter the size or location of the primary tumor, because the N3 M0 combination is a stage group IIIB. If distant metastases were present (M1), neither the size of the primary nor the lymph node involvement would matter, because the case is automatically stage group IV.