Biological uncertainties in proton therapy for paediatric mediastinal Hodgkin Lymphoma
Aznar, Marianne Camille Rechner, L. A. Maraldo, M. V. Smith, E Lundgaard, A. Hjalgrim, L. L. Safwat, A. Aitkenhead, Adam H
Aznar, Marianne Camille
Rechner, L. A.
Maraldo, M. V.
Smith, E
Lundgaard, A.
Hjalgrim, L. L.
Safwat, A.
Aitkenhead, Adam H
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Abstract
Purpose or Objective
Proton Beam Therapy (PBT) is associated with different
types of uncertainties than photon therapy. For example,
it is unknown how much the increase in linear energy
transfer (LET) and relative biological effectiveness (RBE)
at the end of a proton’s range impact the doses to organs
at risk (OARs). For paediatric patients with mediastinal
Hodgkin lymphoma (HL), increased LET at the distal edge
of the beam could, in theory, result in “hot spots” in the
heart, oesophagus or lungs. Here, we investigate the
impact of different beam arrangements on the LET
distribution and dose to mediastinal OARs. In addition, we
explore the impact of using a variable RBE in the dose
calculation.
Material and Methods
Three paediatric patients with mediastinal HL previously
treated with photon-based radiotherapy were selected for
this study (ages 14-16 clinical stage 2A, one with bulky
disease). We retrospectively created proton plans for
each patient with 1 (anterior-posterior (AP)), 2 (APoblique),
and 3 (AP-oblique and posterior-anterior) beams
for a Varian Probeam delivery system, using a commercial treatment planning system (TPS, Eclipse v13.7). The
prescription dose was 19.8 Gy (RBE) in 11 fractions. Beam
arrangements were selected to reflect current clinical
practice for paediatric HL treatments. Robust optimisation
to the CTV, assuming an uncertainty of 3.5% in CT
calibration and 5 mm in positioning was used. Single field
uniform dose optimisation was used as a default and plans
were optimised for a fixed RBE of 1.1. All plans were
exported and recalculated with AUTOMC, an in-house
Monte Carlo (MC) software based on GATE v8.1 and
GEANT4 v10.3.3 (GATE RTion). Dose-averaged LET was
scored and variable RBE dose was calculated using the
McNamara model with an alpha/beta ratio of 2. Doses to
the heart, cardiac sub-structures, lungs, breasts,
oesophagus and spinal cord were recorded. A “hot spot”
was defined as an overlap of regions receiving ≥ 80%
prescription dose and regions with high LET (≥6 keV/μm). Results All treatment plans had acceptable target coverage and
robustness (evaluated with TPS dose). Dose differences
between all strategies (TPS with fixed RBE, MC with fixed
RBE and MC with variable RBE) were small (see figure 1).
LET generally decreased as the number of beams increased. No “hot spot” was observed in plans using > 1
field. However, 2 plans using a 1-field arrangements
revealed small “hot spots“ of 0.004 cc in muscle (Patient
1), or 0.11 cc located in the vertebra, carina, and
descending aorta (Patient 3). The use of MC had a larger
influence on the dose distribution than the use of a
variable RBE (see figure 2).
Conclusion
In this study, increased LET and RBE at the end of proton
range did not result in clinically meaningful hot-spots to
OARs for paediatric patients with mediastinal HL,
regardless of the beam arrangement. However,
uncertainties remain in models of variable RBE, and
further studies are needed to confirm these results.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Aznar M, Rechner LA, Maraldo MV, Smith E, Lundgaard A, Hjalgrim LL, et al. PD-0302: Biological uncertainties in proton therapy for paediatric mediastinal Hodgkin Lymphoma. Radiotherapy and Oncology . 2020 Nov;152:S155–6.