is radiation necessary post bilateral mastectomy

"Because it's in the guidelines" is a way of saying that breast cancer experts decided that there was solid evidence that it leads to better outcomes, but it's a shortcut. Your doctors should be able to explain to you what was the evidence, what was the nature of the better outcomes, how much of the evidence is based directly on women in a situation comparable to yours (receptor/age/grade, etc) and help you do your own risk benefit analysis. Since it's to do with the benefit of radiation, I think your radiation oncologist or someone in her office should be best equipped to answer this question.

I think it's definitely worth asking your doctor about this. The guidelines are usually pretty conservative, which can be both a good and a bad thing. I found some sources suggesting that this guideline be changed, but I believe that the process for change isn't super fast.

To summarize what I found (and I'm a layperson, no medical credentials, qualifications, etc), it seems like this may be an area of controversy in the current guidelines. There is older evidence that T3N0 women have fewer recurrences after post mastectomy radiation, but newer studies hint that it might be more associated with clinical high risk factors rather than inherent to all T3N0 situation. (In your situation, the fact of multiple tumors in both breasts may be considered a higher risk factor)

Your doctor should be able to explain why they have chosen this *for you*, especially given the controversy in the guidelines. The guidelines are there to help doctors and patients, not to replace decision making and patient counseling. Then they should support you in taking into consideration all the factors in your particular situation and coming to your own decision.

I thought these two articles did a good job clearly explaining the controversy and might be a good place to start a conversation with your doctor.

From: Remick J, Amin NP. Postmastectomy Breast Cancer Radiation Therapy. [Updated 2019 Sep 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519034/

T3N0

A similar controversy exists among node-negative patients with a large primary tumor size (greater than 5 cm) [9]. Isolated locoregional failure was as low as 7% among patients with T3N0 disease treated with mastectomy alone or with adjuvant systemic therapy on five NSABP chemotherapy trials [9]. Furthermore, a recent SEER analysis of 568 patients who received postmastectomy radiation therapy for T3N0 breast cancer showed no improvement in overall survival compared to a case-matched control analysis [10]. However, the risk of locoregional recurrence is higher in the presence of high-risk pathologic features including LVSI, high tumor grade, close/positive margins, and premenopausal status which again implies a subset of patients who may benefit from postmastectomy radiation therapy [11].

Post mastectomy radiation therapy in T3 node-negative breast cancer (2016) (https://www.sciencedirect.com/science/article/pii/...)

Despite a large body of research investigating the use of radiation therapy in women with breast cancer, no consensus regarding its use in locally advanced node-negative disease exists among the literature. Although the evidence is unclear, a survey of radiation oncologists in North America and Europe published in 2005 revealed that the vast majority would offer radiation to women with T3N0 disease. In North America, 88% of radiation oncologists and in Europe 84% of practitioners responded that they would favor radiation in this subset of breast cancer patients. Much of this practice is driven by two landmark trials conducted by the Danish Breast Cancer Cooperative Group (DBCCG). The first trial published in 1997 was a randomized trial designed to evaluate PMRT in 1708 premenopausal women [5]... A subsequent study by the DBCCG published in 1999 conducted a randomized trial of radiation therapy compared with systemic therapy alone in 1375 postmenopausal women [8]... Although the findings from these two trials were critical to our knowledge on the implications or radiation, the results are not directly applicable to the T3N0M0 population...

...Since the publication of the DBCCG trials, several studies have been published that provide conflicting data on radiation therapy within the T3 node-negative population.

...Because of the conflicting reports provided by small case series and subgroup analyses from larger randomized trials, the present study sought to use a population-based approach to evaluate the role of radiation therapy through the SEER database... Our analysis found no difference in breast cancer-specific mortality at any time point throughout the study...

Our data did demonstrate a trend toward improved overall survival with radiation therapy use... After adjusting for confounders through propensity score analysis, no difference in overall mortality was seen between the two groups. The disparity in overall survival before propensity score analysis may reflect the fact the women receiving radiation were on average younger than those who did not.

...Despite the limitations of the present study, our data seem to indicate that PMRT in women with locally advanced node-negative breast cancer may not be beneficial. Given the risks of radiation therapy, careful consideration before delivery is critical. Prospective data in the form of a randomized clinical trial are needed to identify whether certain patient and tumor characteristics in the T3 node-negative population predict locoregional failure to allow for improved patient selection for this treatment modality.