The favorable oncologic and aesthetic and oncologic outcomes associated with OPM have led to an increase in uptake of these procedures [7, 13]. There have been well described concerns in the literature about radiation boost delivery and accurate localization for these patients [14–16]. This study aimed to identify if there was a propensity to omit boost radiotherapy as a result of surgical technique, presumably for concern regarding localization. Not surprisingly, given the typical patient selection for OPM, we did find there were differences between the groups in terms of patient age, stage, margin status and chemotherapy administration, which likely resulted in significantly more patients within the OPM group receiving BRT. However, when conditioned on BRT indication criteria, there were no significant differences found in the proportion of patients receiving BRT. This suggests that the recommendation of BRT was predicated on tumor characteristics rather than surgical technique. We also found no significant difference in delays to adjuvant radiotherapy. Any potential differences in incidence of IBTR could not be estimated due to the relatively short follow-up times and very few IBTR events, which were limitations also noted in previous studies [8, 11, 14, 15, 17, 18]. Finally, we evaluated the relationship between the ratio of post-operative seroma volume to tumor size and reception of BRT. The rationale was to assess whether a radiation oncologist might alter the BRT volumes (either knowingly or subconsciously) in the context of OPM and the known associated architectural distortion. Interestingly, there were no significant differences seen in this measure, which could suggest radiation oncologist’s utilized supplementary means (in addition to CT simulation) in volume definition. These could reasonably include any pre-operative imaging (mammography, CT staging or MRI), operative notes and involvement of the surgeon’s input directly at the time of contour delineation.
To our knowledge, this is the first study reporting on whether surgical technique influences the decision to proceed with radiation boost delivery in adjuvant breast radiotherapy. A systematic review by Schaverien et al. demonstrated that the majority of studies reporting on oncologic outcomes of OPM did not provide sufficient radiotherapy details regarding the application of boost radiotherapy to assess whether OPM impacted delivery of BRT [12]. This was despite the propensity of OPM to be performed in younger patients with more advanced tumors, a characteristic shared in our study as well. Similarly, a recent retrospective study of 965 patients by Borm et al. showed that while there was a trend to reduced boost utilization in patients with OPM in comparison to non-OPM patients, however overall rates were still high (94.2% vs. 91% p = 0.06) [8]. Furthermore, there were no significant differences in IBTR between OPM and non-OPM patients, supporting similar oncologic outcomes with adoption of OPM.
The biggest challenge faced in investigating the oncologic impact of OPM with BRT are the low incidence rates of IBTR. In the Cochrane meta-analysis of BRT, an absolute risk reduction (ARR) of 2.5% was found in IBTR incidence rates amongst all-comers [4]. If one assumes ‘perfectly’ localized radiotherapy confers an ARR of 2.5%, the assumption would be that any degradation in localization based on OPM could reduce this number but it is unclear as to what extent. Would there be no benefit, or could a lesser benefit still exist? The combination of generally high control rates and potentially small differences in ARR conferred by BRT, would mean that definitive results produced by clinical trials or patient data analyses would require very high patient numbers. As such it seems unlikely that the field can expect a definitive answer in this regard. The results of our study demonstrate that the OPM procedure itself did not impact our radiation oncologists’ decision to administer BRT or the BRT volume relative to the tumor volume. Unfortunately, our data analysis cannot answer the question as to how effective the boost delivery was in target localization or ultimately reduction in IBTR incidence rates.
Without definitive quantification of treatment effect, oncologists have instead relied on first principals, and the primary tenant of radiotherapy – that if one intends to treat, one must accurately target the region at risk. The concern with OPM and boost radiotherapy relates to the architectural distortion in local breast tissue and the implications this has on tumor (and tumor cavity) localization. Several studies have investigated fiducial placement within the tumor cavity at the time of OPM in order to aid in boost delineation at the time of radiotherapy, and results have shown that clip location can be outside the original tumor quadrant in up to 50% of cases [16, 17]. Several other localization devices, such as radio-opaque gels and films, have also been investigated as intra-operative markers to aid in future radiation boost planning [19, 20]. Consequently there have been calls for increased collaboration and a multi-disciplinary approach to patient selection for treatment planning in OPM cases [10, 21].
Limitations of this study include the observational nature, particularly in regards to BRT indication as there were no firm institutional care pathways at the time of analysis. However, this limitation was mitigated by the strong concordance shown between our retroactively applied “BRT indicated” criteria and patients who actually received BRT. The lack of long-term follow-up in this study is also a limitation, particularly in regards to secondary outcomes of IBTR incidence rates, but as mentioned above, with the low anticipated rates of recurrence and expected differences with BRT treatment, it is unlikely that we can expect significant differences with longer follow-up given the number of patients analyzed. Finally, we were unable to comment on the accuracy of tumor cavity delineation for BRT treatment as no fiducials or targeting materials were used, and we agree that this represents an interesting component of further research and an opportunity for future projects.