As noted here by others, this is probably of no concern. However, the important word in the previous sentence in highlighted! There must have been a reason the rqadiologist mentioned this--was it out of thoroughness, possible real concern, or was it to just "cover himself or herself?" While I can indeed understand the latter, particularly in this age of lawsuit-happy people, perhaps we can give the radiologist the benefit of the doubt, and take it for what it is: it was seen, it may mean something, and therefore was officially reported. If you are a cynic, you might even think that the radiologist was unscrupulous and simply wanted to stimulate the "need" for additional follow-up X-rays! Let's hope not.
So if we return to the idea that this could (but probably doesn't) mean anything serious, there are only two possible responses: 1) decide this means nothing and ignore it. Probably OK, but there's that highlighted word again. What if you are wrong and it does mean something that could have been detected early? Was the radiologist willing to bet your life on that? Are you?
2) decide that this needs at least further discussion and maybe more follow-up. I'd start by asking your doctor to speak with the radiologist and determine what he or she meant by use of the term summation shadow. In itself, this is a radiologic normal occurrence. So why mention it? If it warrants further follow-up in your doctor's and radiologist's agreed opinion, likely one additional X-ray will conclusively answer the question.
Good luck, and in the meantime, don't worry!