Postmastectomy Radiation Therapy: ASTRO-ASCO-SSO Guideline

Dr. Kathleen Horst, Dr. Rachel Jimenez, and Dr. Yara Abdou discuss the updated guideline from ASTRO, ASCO, and SSO on postmastectomy radiation therapy. They share new and updated recommendations on topics including PMRT after upfront surgery, PMRT after neoadjuvant systemic therapy, dose and fractionation schedules, and delivery techniques. They comment on the importance of a multidisciplinary approach and providing personalized care based on individual patient characteristics. Finally, they review ongoing research that may impact these evidence-based guidelines in the future. Read the full guideline, "Postmastectomy Radiation Therapy: An ASTRO-ASCO-SSO Clinical Practice Guideline" at www.asco.org/breast-cancer-guidelines"

TRANSCRIPT

This guideline, clinical tools, and resources are available at www.asco.org/breast-cancer-guidelines. Read the full text of the guideline and review authors' disclosures of potential conflicts of interest in the Journal of Clinical Oncology, https://ascopubs.org/doi/10.1200/JCO-25-01747

Brittany Harvey: Hello and welcome to the ASCO Guidelines podcast, one of ASCO's podcasts delivering timely information to keep you up to date on the latest changes, challenges, and advances in oncology. You can find all the shows, including this one, at asco.org/podcasts.

My name is Brittany Harvey, and today I am interviewing Dr. Kathleen Horst, expert panel chair from Stanford University; Dr. Rachel Jimenez, expert panel vice chair from Massachusetts General Hospital; and Dr. Yara Abdou, ASCO representative from the University of North Carolina, authors on "Postmastectomy Radiation Therapy: An American Society for Radiation Oncology, American Society of Clinical Oncology, and Society of Surgical Oncology Clinical Practice Guideline." Thank you for being here today, Dr. Horst, Dr. Jimenez, and Dr. Abdou.

Dr. Kathleen Horst: Thank you for having us.

Brittany Harvey: And then just before we discuss this guideline, I would like to note that ASCO takes great care in the development of its guidelines and ensuring that the ASCO conflict of interest policy is followed for each guideline. The disclosures of potential conflicts of interest for the guideline panel, including Dr. Horst, Dr. Jimenez, and Dr. Abdou who have joined us here today, are available online with the publication of the guideline in the Journal of Clinical Oncology, which is linked in the show notes.

Then to dive into the content that we are here today to talk about, Dr. Horst, could you start us off by describing what prompted the update for this joint guideline between ASTRO, ASCO, and SSO, and what is the scope of this 2025 guideline on postmastectomy radiation therapy?

Dr. Kathleen Horst: Thank you. This joint guideline was last updated in 2016. Over the past decade, the treatment of breast cancer has evolved substantially. Newer systemic therapy regimens have increasingly personalized treatment based on tumor biology, and local therapy management has explored both the de-escalation of axillary surgery and more abbreviated courses of radiation therapy. Given these advances, it was important to revisit the role of postmastectomy radiotherapy in this modern era of breast cancer therapy.

This updated guideline addresses four key questions, including postmastectomy radiation therapy after upfront surgery as well as after neoadjuvant systemic therapy. It also reviews the evolving role of various dose and fractionation schedules and optimal treatment techniques and dose constraints.

Brittany Harvey: Excellent. I appreciate that background, Dr. Horst. So then, next, Dr. Jimenez, I would like to review the recommendations of this guideline across those four key questions that Dr. Horst just mentioned. So first, what does the panel recommend for PMRT for patients who received initial treatment with mastectomy?

Dr. Rachel Jimenez: The panel provided pretty strong consensus that patients with positive lymph nodes or patients with large tumors involving the skin or the chest wall should receive postmastectomy radiation. However, the panel also recognized that the omission of postmastectomy radiation may be appropriate for select patients who have positive lymph nodes and have an axillary lymph node dissection if they have a low nodal burden and other favorable clinical or pathologic features. For patients without lymph node involvement at the time of surgery and no involvement of the skin or chest wall, postmastectomy radiation was not advised by the panel.

Brittany Harvey: Understood. It is helpful to understand those recommendations for that patient population. Following that, Dr. Abdou, what are the key recommendations for PMRT for patients who received neoadjuvant systemic therapy before mastectomy?

Dr. Yara Abdou: When we think about PMRT after neoadjuvant treatment, the key point is that the initial stage of presentation still matters a lot. So for example, if a patient comes in with more advanced disease, say a large primary tumor, like a clinical T4, or more extensive nodal disease, like an N2 or N3 disease, those patients should get PMRT, no matter how well they respond to neoadjuvant therapy, because we know it reduces the risk of recurrence and that has been shown pretty consistently.

On the other hand, if there are still positive lymph nodes after neoadjuvant treatment, basically residual nodal disease, PMRT is also strongly recommended because the risk of local-regional recurrence is much higher in that setting. The gray area is the group of patients who start with a lower burden of nodal disease, such as N1 disease, but then become node negative at surgery. For those patients, we tend to individualize the decision. So if the patient is young or has triple-negative disease, or if there is a lot of residual disease in the breast even though the nodes are cleared, then radiation is probably helpful. But if everything has melted away with pCR in both the breast and the nodes, then it may be safe to omit PMRT in those patients.

For patients with smaller tumors and no nodal involvement to begin with, like a clinical T1-T2 N0, if they are still node negative after neoadjuvant treatment, then PMRT is generally not recommended because their baseline recurrence risk is low.

And finally, if the margins are positive and cannot be re-excised, then PMRT is recommended after neoadjuvant therapy.

Brittany Harvey: Yes, those distinctions are important for appropriate patient selection.

So then, Dr. Horst, we have just reviewed the indications for PMRT, but for those patients who receive PMRT, what are the appropriate treatment volumes and dose fractionation regimens?

Dr. Kathleen Horst: The guideline addresses coverage of the chest wall and regional nodes with a specific discussion of the data regarding internal mammary nodal irradiation, which has been an area of controversy over many years. The guideline also reviews the data exploring moderate hypofractionation, or shorter courses of radiation therapy. The task force recommends utilizing moderate hypofractionation for the majority of women requiring postmastectomy radiation, which is likely to have a large impact on clinical practice. This recommendation is based on the evolving data demonstrating that a 3-week course of radiotherapy after mastectomy provides similar oncologic outcomes and minimal toxicity for most patients compared to the standard 5-week treatment course.

Brittany Harvey: Thank you for reviewing that set of recommendations as well.

So then, Dr. Jimenez, to wrap us up on the key questions here, what delivery techniques are recommended for treating patients who receive PMRT?

Dr. Rachel Jimenez: So this portion of the guideline is likely to be most helpful for radiation oncologists because it represents the most technical part of the guideline, but we do believe that it offers some important guidance that has, to this point, been lacking in the postmastectomy radiation setting.

So first, the panel recommends that all patients should undergo 3-dimensional radiation planning using CAT scan based imaging, and this includes contouring. So contouring refers to the explicit identification, using a drawing interface on the CAT scan imaging, by the radiation oncologist to identify the areas that are targeted to receive radiation, as well as all of the nearby normal tissues that could receive unintended radiation exposure. And we also provide radiation oncologists in the guideline with suggestions about how much dose each target tissue should receive and what the dose limits should be for normal tissues.

Additionally, we make some recommendations regarding the manner in which radiation is delivered. So for example, we advise that when conventional radiation methods are not sufficient for covering the areas of the body that are still at risk for cancer, or where too high of a dose of radiation would be anticipated to a normal part of the body, that providers employ a technique called intensity modulated radiation therapy, or IMRT. And if IMRT is going to be used, we also advise regular 3-dimensional imaging assessments of the patient's body relative to the treatment machine to ensure treatment fidelity.

When the treatments are delivered, we further advise using a deep inspiration breath-hold technique, which lowers the exposure to the heart and to the lungs when there is concern for cardiopulmonary radiation exposure, and again, that image guidance be used along with real-time monitoring of the patient's anatomy when those techniques are employed.

And then finally, we advise that patients receiving postmastectomy radiation utilize a bolus, or a synthetic substance placed on the patient's skin to enhance radiation dose to the superficial tissue, only when there is involvement of the skin with cancer or other high-risk features of the cancer, but not for every patient who receives postmastectomy radiation.

Brittany Harvey: Understood. And then, yes, you just mentioned that section of the guideline is probably most helpful for radiation oncologists, but I think you can all comment on this next question. What should all clinicians, including radiation oncologists, surgical oncologists, medical oncologists, and other oncologic professionals, know as they implement all of these updated recommendations?

Dr. Rachel Jimenez: So I think one of the things that is most important when we consider postmastectomy radiation and making recommendations is that this is a multidisciplinary panel and that we would expect and encourage our colleagues, as they interpret the guidelines, to employ a multidisciplinary approach when they are discussing each individual patient with their surgical and medical oncology colleagues, that there is no one size fits all. So these guidelines are intended to provide some general guidance around the most appropriate techniques and approaches and recommendations for the utilization of postmastectomy radiation, but that we recognize that all of these recommendations should be individualized for patients and also represent somewhat of a moving target as additional studies, both in the surgical and radiation oncology realm as well as in the systemic therapy realm, enter our milieu, we have to adjust those recommendations accordingly.

Dr. Kathleen Horst: Yeah, I would agree, and I wanted to comment as a radiation oncologist, we recognize that local-regional considerations are intertwined with systemic therapy considerations. So as the data evolve, it is critical to have these ongoing updates in a cross-disciplinary manner to ensure optimal care for our patients. And as Dr. Jimenez mentioned, these multidisciplinary discussions are critical for all of us to continue to learn and understand the evolving recommendations across disciplines but also to individualize them according to individual patients.

Dr. Yara Abdou: I could not agree more. I think from a medical oncology perspective, systemic therapy has gotten much better with adjuvant CDK4/6 inhibitors, T-DM1, capecitabine, and immune therapy. So these are all newer adjuvant therapies, so the baseline recurrence risks are lower than what they were in the trials that established PMRT. So the absolute benefit of radiation varies more now, so smaller for favorable biology but still relevant in aggressive subtypes or with residual disease. So it is definitely not a one-size-fits-all.

Brittany Harvey: Yes, I think it is important that you have all highlighted that multidisciplinary approach and having individualized, patient-centric care.

So then, expanding on that just a little bit, Dr. Abdou, how will these guideline recommendations affect patients with breast cancer?

Dr. Yara Abdou: So basically, reiterating what we just talked about, these guidelines really move us towards personalized care. So for patients at higher risk, so those with larger tumors, multiple positive nodes, or residual nodal disease after neoadjuvant therapy, PMRT remains essential, consistently lowering local-regional recurrence and improving survival.

But for patients at intermediate or lower risk, the recommendations support a more selective approach. So instead of a blanket rule, we now integrate tumor biology, response to systemic therapy, and individual patient factors to decide when PMRT adds meaningful benefit. So the impact for patients is really important because those at high risk continue to get the survival advantage of radiation while others can be spared the unnecessary treatment and side effects. So in short, we are aligning PMRT with modern systemic therapy and biology, making sure each patient receives the right treatment for their situation.

Brittany Harvey: Absolutely. Individualizing treatment to every patient will make sure that everyone can achieve the best outcomes as possible.

So then, Dr. Jimenez, to wrap us up, I believe Dr. Horst mentioned earlier that data continues to evolve in this field. So in your opinion, what are the outstanding questions regarding the use of PMRT and what are you looking to for the future of research in this space?

Dr. Rachel Jimenez: So there are a number of randomized phase III clinical trials that are either in active accrual or that have reported but not yet published that are exploring further de-escalation of postmastectomy radiation and of axillary surgery. And so we do not yet have sufficient data to understand how those two pieces of information integrate with each other. So for example, if you have a patient who has a positive lymph node at the time of diagnosis and forgoes axillary surgery aside from a sentinel lymph node biopsy, we do not yet know that we can also safely forgo radiation entirely in that setting. So we expect that future studies are going to address these questions and understand when it is appropriate to simultaneously de-escalate surgery and radiation.

Additionally, there is a number of trials that are looking at ways in which radiation could be omitted or shortened. So there is the RT CHARM trial, which has reported but not yet published, looking at a shorter course of radiation. And so we do make recommendations around that shorter course of radiation in this guideline, but we anticipate that the additional data from the RT CHARM study will provide further evidence in support of that.

Additionally, there is a study called the TAILOR RT trial, which looks at forgoing postmastectomy radiation in patients who, to Dr. Abdou's point, have a favorable tumor biology and a low 21-gene recurrence score. And so we are going to anticipate the results from that study to help guide who can selectively forgo postmastectomy radiation when they fall into that favorable risk category.

So there are a number of questions that I think will help flesh out this guideline. And as they publish, we will likely publish a focused update on that information to help provide context for our colleagues in the field and clarify some of these recommendations to suit the latest data.

Brittany Harvey: Absolutely. We will look forward to those de-escalation trials and ongoing research in the field to build on the evidence and look for future updates to this guideline.

So I want to thank you for your work to update these guidelines, and thank you for your time today, Dr. Horst, Dr. Jimenez, and Dr. Abdou.

Dr. Rachel Jimenez: Thank you.

Dr. Yara Abdou: Thank you.

Dr. Kathleen Horst: Thank you.

Brittany Harvey: And then finally, thank you to all of our listeners for tuning in to the ASCO Guidelines podcast. To read the full guideline, go to www.asco.org/breast-cancer-guidelines. You can also find many of our guidelines and interactive resources in the free ASCO Guidelines app, which is available in the Apple App Store or the Google Play Store. If you have enjoyed what you have heard today, please rate and review the podcast and be sure to subscribe so you never miss an episode.

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