GEC-ESTRO HNS critical review: skin superficial brachytherapy with flaps and customized moulds
ForLarge NMSC infiltrating >5 mm, interstitial BT or EBRT not viable
TL;DRPrescription >5 mm deep judged acceptable for selected large NMSC using tailored bolus; at 5 mm depth, 5→10 mm bolus cut skin Dmax 187%→148%.
The planning lever is bolus thickness, not the depth ceiling: at 5 mm depth on a 6×6 cm² CTV, going from 5 to 10 mm bolus cut skin Dmax 187% to 148% while R100-R50 widened 16.0 to 18.7 mm. With EQD2 at a 150% Dmax spanning 121.5 to 189 Gy across ACROP schedules, a flat percentage cap is a weak surface constraint.
In large NMSC infiltrating beyond 5 mm where interstitial BT or EBRT is not viable, this supports mould or flap BT with thicker or variable bolus to hold surface Dmax down; it does not extend to tumours suited to interstitial BT or EBRT.
The planning lever is bolus thickness, not the depth ceiling: at 5 mm depth on a 6×6 cm² CTV, going from 5 to 10 mm bolus cut skin Dmax 187% to 148% while R100-R50 widened 16.0 to 18.7 mm. EQD2 at a 150% Dmax spanning 121.5 to 189 Gy across ACROP schedules makes a flat percentage cap a weak surface constraint.
10 details
PRISMA systematic review by the GEC-ESTRO H&N and Skin Working Group, PubMed search through 31 May 2019, plus a theoretical TG-43 planning exercise (Oncentra Brachy, Ir-192 HDR) on 6×6 and 10×10 cm² CTVs.
Clinical studies of NMSC superficial HDR Ir-192 BT with flaps or custom moulds; conference papers, surveys, case reports and reviews included, English only.
Planning model used parallel catheters at 1 cm spacing, 2 mm source step, prescription depths of 3, 5, 10 and 20 mm crossed with bolus of 3, 5, 10 and 20 mm. ACROP-listed mould/flap schedules range from 17-18 × 3 Gy to 8 × 5 Gy twice daily.
Thicker bolus lowers surface Dmax at the cost of a shallower gradient; the bolus × Dmax × R100-R50 trade is tabulated above for the 5 mm depth case.
| Parameter | ACROP / ABS / GEB guidance | HNS WG position |
|---|---|---|
| Max prescription depth | 5 mm | >5 mm acceptable in selected large tumours if interstitial BT or EBRT not viable |
| Bolus thickness | Catheters ideally 5 mm from skin (ACROP, ABS) | Boli of different thicknesses acceptable to optimize plan |
| Skin surface Dmax | 125% flaps, 140% custom moulds (ABS, GEB) | No PROM-based value can be recommended |
| Bolus | Skin Dmax | R100-R50 |
|---|---|---|
| 3 mm | 228% | 14.3 mm |
| 5 mm | 187% | 16.0 mm |
| 10 mm | 148% | 18.7 mm |
GEC-ESTRO ACROP, ABS, AAPM and GEB guidance converge on a 5 mm prescription ceiling and ABS/GEB on 125% (flaps) / 140% (moulds) surface Dmax. The ABS Dmax limit was modelled on its APBI consensus, where skin is an OAR rather than the target. Zaorsky's meta-analysis (EBRT 9965 vs BT 553 pts) favoured BT cosmesis but carried no bolus or Dmax data.
Dosimetry is TG-43 without scatter correction, with MC deviations up to 5% for flaps. Skin toxicity shows a volume and layer effect that a single surface Dmax point cannot capture, and telangiectasia needs 2-3 yr follow-up to assess.
The 5 mm and 125-140% limits are not tied to PROMs in any published series, and deviating centres report similar results. The group calls for standardized dosimetry reporting and homogeneous multicentre data via the COBRA project before revising thresholds.
Working-group critical review of retrospective series and surveys; no trial data, and the group declines to propose any new PROM-based Dmax or depth threshold.
- Does skin surface Dmax predict toxicity or cosmesis in superficial BT?
- Separate outcomes for prescription depth beyond 5 mm
- Fractionation-aware surface dose constraint for moulds and flaps
📚 Sources · 📄 1 paper
The longer read
The working group is not overturning the 5 mm prescription-depth ceiling so much as exposing what holds it up. The ACROP, ABS and GEB documents agree on that ceiling and on surface Dmax caps of 125% for flaps and 140% for moulds, yet the review finds no published series tying either figure to local control, toxicity or cosmesis. The caps read as inherited convention. The ABS skin Dmax limit followed the template of its APBI consensus, where skin is an organ at risk sitting over a deeper target. In NMSC the skin is the target, so a tight surface cap risks underdosing tumour rather than protecting normal tissue.
The planning exercise is the part a physicist and radiation oncologist can use tomorrow. On a theoretical 6×6 cm² CTV prescribed at 5 mm, replacing the standard 5 mm bolus with 10 mm drops surface Dmax from 187% to 148%, while the 100% to 50% isodose separation widens from 16.0 to 18.7 mm. That is the bargain IMRT strikes against conformal RT: a cooler surface bought with more low-dose spread. The model also shows that a textbook 5 mm bolus at 5 mm depth does not meet the 125%-140% caps, which raises the question of how often current limits are actually being met. These are TG-43 numbers without scatter correction, and the review cites deviations up to 5% against Monte Carlo for flaps, so the absolute percentages carry calculation error. The direction of the bolus effect is geometric, though, and should survive a model-based algorithm.
The clinical support for prescribing deeper is thin in a specific way. Series reporting depths of 9, 10, 15 or 20 mm describe good control and cosmesis, but none separates the deep-prescription pts or states what share of the cohort they were. A reader cannot tell whether the favourable outcomes belong to the deep cases or are carried by the shallow majority. Surveys showing 2 of 16 ABS centres working past 5 mm, and Canadian centres treating depth as only a relative contraindication, establish that the practice exists, not that it is safe. The fractionation argument compounds the problem: at a 150% hotspot, the ACROP mould and flap schedules map to EQD2 anywhere from 121.5 to 189 Gy, so one percentage cap does not describe one biological exposure.
What the position moves is narrow. For a large NMSC infiltrating beyond 5 mm where interstitial BT and EBRT are not options, surface BT with tailored or 3D-printed bolus becomes a defensible plan rather than an off-guideline one. The thin lesion within current limits is untouched, and no new Dmax number is licensed; the group explicitly declines to propose one. The real weight of this paper rests on COBRA producing homogeneous dosimetry linked to outcomes, because until then Dmax itself is unstable: none of the reviewed papers says how it was computed, and DVH bin and sample settings shift it. The position would be wrong if thick-bolus deep plans turn out to trade surface reactions for higher dose in the hypodermal layers, a shift the review itself flags and that a skin-surface point metric would never detect.