ESTRO 36 Abstract Book

S838

ESTRO 36

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specialising in H&N RT and their assessments were

compared with quantitative analysis. Optimisation

objectives and priorities and the use of dummy organs

were analysed in conjunction with the dose

distributions.

Results

Each plan met the pre-defined PTV

and PORV doses. There was no significant variation in HI

(1.05-1.09) regardless of the priorities applied in the

optimisation. A larger variation in CI (1.07-1.18) was

attributed to use of the Normal Tissue Objective function.

There were variations in dose to normal tissues as planners

applied varying dose constraints to keep doses as low as

reasonably practicable without compromising PTV

coverage. Clinician reviews picked up more subtle but

potentially clinically relevant variations between plans –

high dose spill, dose spread across mid-line and over-

zealous sparing of normal tissues. The plans scored most

highly by the clinicians were created by the most

experienced planners who took less time to reach an

“optimal” solution.

Conclusion

There is a relatively

consistent approach to H&N VMAT planning within our

institution. This study has highlighted where differences

between optimisation objectives and priorities and the use

of dummy structures can lead to subtle variations in dose

distributions which may not be detected in quantitative

analysis but may affect acceptability. More experienced

planners

familiar

with

clinician

expectations

demonstrated improved judgement when determining an

optimal plan. Shared learning has enabled a more

consistent approach to H&N plan optimisation and an

improved understanding of what is achievable and

clinically acceptable. This has benefitted both planners

and clinicians. In addition, the creation of plan

optimisation templates, based on the findings of this

study, are aimed at a consistent optimisation resulting in

improvements in the patient pathway.

EP-1556 Dosimetric commissioning of a TPS for a

synchrotron-based proton PBS delivery system

G. Kragl

, T. Böhlen

, A. Carlino

1,2

, L. Grevillot

, H.

Palmans

, A. Elia

, B. Knäusl

, J. Osorio

, R. Dreindl

, J.

Hopfgartner

, S. Vatnitsky

, M. Stock

EBG MedAustron GmbH, Medical Department, Wiener

Neustadt, Austria

University of Palermo, Department of Physics and

Chemistry, Palermo, Italy

National Physics Laboratory, Radiation Dosimetry,

Teddington, United Kingdom

Purpose or Objective

To provide an overview regarding dosimetric

commissioning of the TPS RayStation for proton PBS

delivery installed at a synchrotron-based dual particle

facility. 1D/2D commissioning consisted of benchmarking

the dose calculation algorithm against measured IDDs, on-

axis lateral spot profiles in air and field size factors as well

as comparisons of absolute dosimetry. 3D commissioning

consisted of absolute dose comparisons in the SOBP of

cubic targets in water as well as the characterization of

3D dose distributions with increasing complexity. A robotic

patient positioning system was used rather than

extendable snouts to reduce the air gap between patient

and nozzle. Therefore, special attention was paid to non-

isocentric setups.

Material and Methods

Commissioning was performed for the PB algorithm

(version 3.5) integrated in RayStation (version 5.0.2). IDDs

were acquired with a Bragg peak chamber (PTW) and

corrected for insufficient detector size by means of MC

simulations (GATE/GEANT4). Spot profiles in air were

acquired with a scintillating screen (Lynx, IBA) at 7 air

gaps. Absolute dosimetry was performed with a Roos

chamber (PTW) in 12 x 12 cm

fields (2 mm lateral spot

spacing) for 20 energies. Field size factors were acquired

with a semiflex ionization chamber (PTW) at 3 depths in

water for field sizes ranging from 2 x 2 to 20 x 20 cm

. 3D

dose distributions were characterized using 24-PinPoint

chamber arrays (PTW).

Results

Calculated ranges agreed within 0.2 mm with measured

ranges. The integrals of measured and calculated IDDs

agreed within 0.5% for clinically relevant ranges. At

isocenter, calculated and measured spot sizes (FWHM)

differed on average less than 0.4 mm in x- and y-

directions. For non-isocentric setups differences were

within 0.5 mm. Field size factors always agreed within 4%;

deviations were generally low (<1%) and increased only at

small field sizes and the highest energies (range >30 cm).

For isocentric arrangements, absolute dose agreed within

2.2% in the center of SOBPs of cubic targets with different

sizes and at different depths. As expected, the deviations

increased for plans with range shifter for non-isocentric

arrangements. Variations of up to 3.5% were obtained for

modulation widths of 6 cm. Results for more complex

geometries are currently under investigation.

Conclusion

Clinically acceptable results were obtained for open

beams. For plans with range shifter, a scaling of dose

distributions might be considered until the upcoming MC

dose calculation algorithm is available. Minimizing the air

gap to reduce modelling inaccuracies with respect to

scattered protons in air is beneficial for these cases and

realized by non-isocentric treatments.

EP-1557 Minimum prescription concept for dose

painting increases robustness towards geometrical

uncertainty

S. Korreman

Aarhus University Hospital, Department of Oncology,

Aarhus C, Denmark

Purpose or Objective

Dose painting radiotherapy with heterogeneous dose

escalation is vulnerable to geometrical errors, which

potentially deteriorate the benefits of dose escalation

substantially. This study investigates use of a minimum

prescription concept to increase plan robustness towards

geometrical uncertainties.

Material and Methods

Dose escalation was prescribed based on PET Cu-ATSM

tracer uptake for a head and neck cancer patient, with a

high degree of heterogeneity in the uptake. The minimum

dose was 60Gy, and dose escalation was prescribed based

on a linear correspondence model to the tracer uptake,

with a maximum escalation up to ~90Gy. Dose painting

plans were optimized using the Eclipse treatment planning

system, using modulated arc therapy technique in a

contour-based dose escalation scheme (5 levels). Two

planning strategies were tested: (1) Minimum and

maximum dose constraints imposed on all subvolumes

(exact-map), and (2) minimum constraints on all

subvolumes with only one overall maximum constraint

(minimum-map). Geometrical error was simulated by

displacing the isocenter with up to 2 mm. Quality index

metrics were compared for the two planning strategies.

Results

For both strategies, optimizations could be performed

with good adherence to dose constraints. For the exact-

map technique, the fraction of voxels with quality index

within plus/minus 5% of prescription dose was ~79%, and

the fraction of voxels above 95% of prescription dose was

~93%. For the minimum-map technique, the fraction of

voxels above 95% of prescription dose was ~97%. With

displacement of 2 mm, the >95% fraction changed to ~85%

for exact-map, and ~95% for minimum-map technique.