Thank you for the additional information.
The most common cause of lumps in the upper chest and neck at or above the collar bones are enlarged lymph nodes, particularly lumps that are about a grape in size. There are other possible causes of lumps, but lymph nodes are the most common.
It is unusual that an ultrasound could not see lumps that are as large as a grape. An ultrasound may not always be able to determine the type of lump or the cause of the lump, but it will typically be able to see a lump that is this size. Since the ultrasound did not see the lumps, then it would be appropriate to consider an advanced imaging study, most often a CT scan of the neck.
If these are lymph nodes, then I could understand why your doctors have said that it is generally not something that would cause concern. The primary reason for enlarged lymph nodes is as a reaction to common infections, such as viral
infection of the head. These enlarged lymph nodes will typically resolve spontaneously. In someone that has swollen, tender lymph nodes of new onset, the usual approach is to provide symptomatic relief and see if the problem resolves. However, if the swollen lymph nodes persist for several months, then it would be appropriate to pursue further evaluation. Further evaluation may include the advanced imaging studies noted above, but it ultimately may require a biopsy of a lymph node. When there are several lymph nodes present, it is not necessary to biopsy all of the lymph nodes; they will usually only biopsy the most prominent lymph nodes.
Therefore, at this point, it would be appropriate to perform a CT scan next and then, based on the result of the CT scan, to consider a biopsy. The findings on exam and CT scan would guide which lumps would be biopsied.
If you have any further questions, please let me know.