SABR re-irradiation of pelvic cancer recurrences: photon vs proton beam therapy
Glassborow, E. Richardson, J. Clarke, Mathew F Murray, L. Speight, R. Aspin, L. Gregory, S. Handley, J Chuter, Robert
Glassborow, E.
Richardson, J.
Clarke, Mathew F
Murray, L.
Speight, R.
Aspin, L.
Gregory, S.
Handley, J
Chuter, Robert
Citations
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Abstract
Purpose or Objective
Patients who experience a pelvic cancer recurrence in or
near the region that received initial radiotherapy for the
primary disease, typically have few treatment options.
OARs have often reached their dose constraint limits
leaving standard re-irradiation (reRT) unavailable.
Alternatives include surgery, which may be extensive, and
chemotherapy, which is typically reserved for wide-spread
or symptomatic disease.
Despite OAR dose concerns, photon SABR reRT has been
utilised with promising initial results but can still struggle
to meet constraints. Proton beam therapy (PBT) could
offer an improvement through characteristic distal dose
fall off.
This project aims to establish whether PBT can achieve
reduced OAR doses for equal or improved target coverage,
compared to photon SABR.
Material and Methods
PBT plans have been retrospectively created in Eclipse
v13.7 on CT data sets for 15 patients treated with photon
SABR reRT for pelvic recurrence reRT under NHS England’s
Commissioning through Evaluation (CtE) programme
between 2016-2017. The photon SABR plans were created
in Monaco v5.1, using a VMAT technique and had been
prescribed to a 5mm CTV-PTV expansion with an ALARP
OAR approach. For the PBT plans a spot scanning technique
was used, and plans were created to match the SABR
prescription of 30Gy in 5 fractions and the CTV coverage
aim of V100% ≥ 95%. An ALARP approach was taken for OAR
constraints, aiming to reach an ‘ideal’ constraint obtained
by cumulative EQD2 calculations subtracting the primary radiation dose from the CtE constraints. PBT plans were
prescribed to the CTV and assessed under uncertainty
conditions of ±5mm shifts in all axes, and a nominal ±3.5%
stopping power uncertainty.
The CTV and OAR dose statistics from the nominal PBT
plans were then compared to those from the original
clinically delivered photon SABR plans.
Results
Dosimetric comparison of relevant OAR statistics seen in
Figure 1 shows a marked decrease in OAR dose using PBT.
Only the small bowel saw any sizeable increase in dose in
the PBT plan, seen in only 3 patients. A smaller increase
by PBT was seen in the cauda equina, vessels, and colon,
for a 4th patient. The largest dose reductions were seen for
the sacral plexus, up to 18Gy.
Conclusion
PBT has the potential to make significant dose reductions
for OARS in the pelvic reRT setting. These savings are most
pronounced for OARs that abut the target on one
side. Less impact is seen when OARs surround the target,
as with small bowel. This offers the possibility of increased
eligibility within this patient group, to include those who
due to OAR doses may have been ineligible for photon
SABR, and may even provide room for dose escalation.
Whilst this study has made initial steps in the development
of the PBT planning technique to align with practical
deliverability, further work is required to investigate
robustness under inter- and intra-fraction patient variation
to determine effective set up and imaging strategies for a
PBT technique for this patient group.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Glassborow E, Richardson J, Clarke M, Murray L, Speight R, Aspin L, et al. PD-0305: SABR re-irradiation of pelvic cancer recurrences: photon vs proton beam therapy. Radiotherapy and Oncology . 2020 Nov;152:S157–8.