Altered breathing pattern can jeopardize PBS proton therapy of locally advanced NSCLC
Camilla Grindeland Boer,
Norway
MO-0792
Abstract
Altered breathing pattern can jeopardize PBS proton therapy of locally advanced NSCLC
Authors: Liv Bolstad Hysing1,2, Camilla Grindeland Boer1, Kristine Fjellanger1,2, Inger Marie Sandvik1, Maren Ugland1, Grete May Engeseth1,3
1Haukeland University Hospital, Oncology and Medical physics, Bergen, Norway; 2University of Bergen, Physics and technology, Bergen, Norway; 3University of Bergen, Clinical science, Bergen, Norway
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Purpose or Objective
Patients
with locally advanced non-small cell lung cancer (LA-NSCLC) have poor
prognosis, with treatment intensifications prevented by high toxicity rates from
state-of-the-art multimodal therapy. Pencil beam scanning proton therapy
(PBS-PT) has potential to spare the lungs and heart compared to IMRT. However,
little knowledge exists on how uncertainties occurring between planning (Plan) and
start of treatment (Start) influence organ at risk sparing and target coverage.
The purpose of this prospective simulation study was to evaluate if the
clinical potential of PBS-PT persists from Plan to Start, in order to guide its
use in LA-NSCLC.
Material and Methods
4DCT
imaging at Plan and Start (fraction 2 or 3) was carried out for 15 patients
that received state-of-the-art IMRT with prescribed doses of 60-66 Gy in 2 Gy
fractions. Three PBS-PT plans were created per patient: 3D-robust single field
uniform dose (SFUD), 3D-robust intensity-modulated proton therapy (IMPT) and 4D-robust
IMPT (4DIMPT). Target coverage and dose-volume parameters relevant for toxicity
were compared across PBS-PT and IMRT. Robustness towards setup and range, breathing
motion and interplay were investigated
at Plan, and robustness towards changes
in breathing motion and anatomy was investigated at Start (Table).
Uncertainty
| Robustness evaluations at Plan
| Robustness evaluations at Start
| Criterion for target
|
Setup
and range
| Simulations
up to 5 mm setup and 3.5% calibration uncertainty
| - | D98%
> 95% of prescribed dose
|
Interplay
| Spot
scanning simulations on all 10 breathing phases
| - | D98%
> 90%
|
Breathing
motion
| Recalculation
on maximum and minimum inspiration
| Recalculation
on maximum and minimum inspiration
| D98%
> 95%
|
Changes
in breathing motion and anatomy
| - | Recalculation
on 4DCT average intensity projection
| D98%
> 95%
|
Results
Sparing
at Plan with PBS-PT compared to IMRT was significant and largest with IMPT,
followed by 4DIMPT, SFUD and IMRT, and persisted at Start (Figure 1). All plans met the preset criteria for target
robustness at Plan (Figure 2). At
Start, D98% was >95% for 12/15 patients with PBS-PT. Three patients had lack
of CTV coverage (worst for IMPT with D98% of 82-93%), mainly in the mediastinal
lymph nodes, caused by changes in breathing-motion pattern between Plan and
Start (Figure 2).
Conclusion
The
potential of PBS-PT to reduce heart and lung toxicity compared to IMRT was
significant and persistent from Plan to Start. All PBS-PT techniques responded
similarly to uncertainties and were sufficiently robust at Plan, and for the
majority of patients at Start. Altered breathing patterns between Plan and
Start jeopardized target coverage for 3/15 patients with all PBS-PT techniques.
Adaptive protocols should therefore include imaging at onset of or early in
treatment.