@laurademaine,
Hello Laura,
I was diagnosed spring of 2018 with mild MAC, Bronchiectasis and small nodes on the lung. My pulmo said to hold off on any treatment but to just "watch". I went to Mayo for a second opinion and they were in agreement. I was to have yearly cat scans to make sure the nodes did not change. I did that for several years, they did not change and now are no longer followed. When you say mass on your lungs, are you referring to nodes? The Mayo Dr who I really liked also thought my acid reflux had something to do with my condition. Aspirating fluids into my lung at night. He suggested sleeping on a wedge and no food or water 3 hrs before bedtime. That worked for me!!
It has been 4 years now with no need for antibiotics. I have a little shortness of breath and fatigue sometimes. Like you, I exercise (walking bout 2.5 miles a day) eat well...plant based only and meditate. I am so opposed to taking the 3 antibiotics and hope I never have to. The only other thing I do is nebulize the 6% saline solution and take NAC which is a wonderful supplement that I feel is helpful for the lung. The nebulizing makes good sense in clearing out the mucus in the lung. I suggest a second opinion from a Dr that specializes in MAC. Pulmonologists (including my local Dr) sometimes do not seem to keep up with MAC or just do not treat many patients with the disease. It sounds like you are fortunate with your lack of symptoms. I would not hurry into antibiotic treatment and suggest that you review posts on this site as often the antibiotics have a negative side effect and sometimes do not kill off the MAC.
Oh...including a good snippet on NAC.
"N-acetyl-cysteine (NAC) is included in the World Health Organization’s list of essential medicines; a list that details the most relevant medications needed for a basic health system [1]. Acetyl-cysteine is a derivative of cysteine in which an acetyl group is attached to nitrogen. Due to its disulfide reducing activity, NAC is used as a mucolytic agent to promote expectoration [2]. NAC is commonly prescribed as an adjunct therapy in patients with a wide range of respiratory diseases characterized by formation of thick mucus, such as cystic fibrosis [2–4]. At high doses, NAC results in significantly improved small airway function and decreased exacerbation frequency in patients with stable chronic obstructive pulmonary disease (COPD) [3, 4]. NAC’s mucolytic activity is also the basis of its use in liquefying sputum samples for the microscopic detection of acid-fast bacilli (AFB) in suspected pulmonary tuberculosis (TB) patients [5]. Furthermore, in both experimental animal models and clinical studies, NAC displays a protective effect on acute liver injury induced by anti-TB drugs in acetaminophen-dependent or independent conditions [6–11]. In patients with type 2 diabetes, NAC holds promise in primary prevention of cardiovascular complications and systemic inflammation [12–14]."

This excerpt is from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5084440/ It is entitled N-acetyl-cysteine exhibits potent anti-mycobacterial activity in addition to its know anti-oxidative functions. I personally take NAC 600 mg 2x a day. It is over the counter.
Take care.
Kate alleycatkate