Proton vs Photon PMRT Capsular Contracture
ForPost-mastectomy breast cancer, implant-based reconstruction (TE/I or DTI), receiving PMRT
TL;DR2yr CC 50% with proton+DTI vs 12% photon+TE/I; proton HR 2.3 univariate, 1.76 (0.93-3.32) multivariable, ns.
The actionable variable is reconstruction type, not beam: DTI carried HR 3.0 (1.7-5.5) for CC independent of modality, and proton+DTI was the worst cell at 50% at 2 years vs 12% for photon+TE/I. That reframes the pre-RT conversation with plastic surgery toward staged TE/I when protons are planned, rather than toward declining protons outright.
In a woman heading to PMRT after mastectomy with implant reconstruction, this informs the timing and type of reconstruction discussed with plastic surgery when proton is on the table; it says nothing about autologous reconstruction or prepectoral placement, neither of which was studied.
The proton CC signal did not survive adjustment (HR 1.76, 0.93-3.32, P=.083), so this is not on its own grounds to decline protons where cardiac sparing is the indication. It is grounds to know the reconstruction plan before simulation: proton with DTI hit 50% CC at 2 years, and every TE/I patient here was irradiated with the expander in place.
Reconstruction type outweighed beam modality: DTI carried HR 3.0 (1.7-5.5) for CC versus staged TE/I on multivariable analysis. When PMRT is planned, particularly proton PMRT, that argues for staging the reconstruction rather than direct-to-implant, in subpectoral placement at least, which is all this cohort covers.
9 details
IRB-approved retrospective cohort at 2 centers within one institution, PMRT delivered 2017-2023. N=175 (89 PBS proton, 86 IMRT photon). CC estimated by Kaplan-Meier, with Cox proportional hazards for covariates and a binary logistic model as verification.
Breast cancer pts who underwent subpectoral 2-stage TE/I or DTI reconstruction and then PMRT. Median age 49 (range 24-78), 63% Hispanic. Groups were balanced except on tumor laterality (P < .001) and reconstruction type (P < .001), the two axes the analysis turns on.
Pencil beam scanning proton PMRT vs IMRT photon PMRT. All TE/I pts had the tissue expander in place and irradiated, so this cohort speaks to expander-in-situ RT, not to post-exchange irradiation of a permanent implant. Dose and fractionation are not reported in source.
Proton was associated with CC on univariate analysis (HR 2.3, 1.26-4.30, P=.007) but the association did not hold after adjustment (HR 1.76, 0.93-3.32, P=.083). DTI vs TE/I carried HR 3.0 (1.7-5.5), P < .001 in the multivariable model. No other factor was significantly associated with CC.
| Group | n | 2yr CC rate |
|---|---|---|
| Proton + DTI | 36 | 50% |
| Photon + DTI | 15 | 35% |
| Proton + TE/I | 53 | 23% |
| Photon + TE/I | 71 | 12% |
Modality was assigned by practice pattern, not randomized, so the residual proton association could be confounding the model did not capture. CC is clinician-graded on unblinded chart review, and the DTI cells are thin (36 proton, 15 photon), which is where the widest rate gap sits.
The paper set out to test a prespecified suspicion that protons increase CC and returned a trend that did not clear significance once reconstruction type entered the model. What it does establish is the interaction cell worth counseling on: proton + DTI at 50% CC at 2 years.
Retrospective, non-randomized modality assignment with baseline imbalance in reconstruction type and laterality; the proton signal loses significance once adjusted.
- Does prepectoral placement change the proton CC signal?
- Expander-in-situ vs post-exchange RT sequencing for implant reconstruction
- Proton PMRT reconstruction toxicity in a prospective randomized comparison
📚 Sources · 📄 2 papers
Abstract
The longer read
Proton PMRT has been argued into practice largely on dosimetry: better heart and lung sparing than photons in a population with decades of life ahead of them, where late cardiac events are the toxicity of record. The unresolved question has always been what the flat, skin-sparing-free entrance dose of pencil beam scanning does to the reconstruction sitting directly under it. This cohort is an attempt to answer it, and it is worth reading for what it does and does not establish.
What it establishes cleanly is that reconstruction type dominates. DTI carried HR 3.0 (1.7-5.5) for capsular contracture against TE/I on multivariable analysis, and that estimate is both large and precise enough to act on. The modality signal is the weaker of the two: it started at HR 2.3 (1.26-4.30) univariate and fell to HR 1.76 (0.93-3.32, P=.083) once the model accounted for the covariates. A reader is entitled to read that attenuation two ways. The generous reading is that the study is underpowered at N=175 for a hazard of that size and the point estimate is stable, so the effect is likely real and the confidence interval simply has not closed. The skeptical reading is that reconstruction type was imbalanced between the arms at P < .001, which is precisely the variable that turned out to matter, and the univariate proton signal was partly reading that imbalance back to us. The paper's own framing, a trend, is the honest one.
The four-cell breakdown is the most useful output and also the most fragile. Proton with DTI at 50% at two years against photon with TE/I at 12% is a fourfold spread, but the DTI cells hold 36 and 15 patients. A difference of a handful of events moves the proton-versus-photon DTI comparison substantially, and no interaction test is reported in the source. Treat the 50% as a counseling figure for the highest-risk configuration rather than as an estimate of proton's specific contribution within DTI.
Two design features limit how far this transfers. All TE/I patients were irradiated with the expander in place, so this speaks to expander-in-situ PMRT and not to the alternative sequence where the exchange precedes radiation, a choice that plausibly changes capsule biology. And capsular contracture is a clinician-graded endpoint assessed on unblinded retrospective review by teams who knew the modality; the investigators had a prior hypothesis that protons would look worse, which is the setting in which grading drift is hardest to rule out.
Where this should land in practice is not as a reason to decline protons in a patient whose cardiac anatomy warrants them. It is a reason to move the reconstruction decision earlier in the conversation. If proton PMRT is planned and the patient is choosing between DTI and staged TE/I, this cohort argues the staged approach is the safer pairing, and the magnitude of the reconstruction effect is larger and better estimated than the modality effect it is being weighed against.