ESTRO 36 Abstract Book

S953

ESTRO 36

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Table 1: Optimal and mandatory dose constraints

Conclusion

Two sequential planning excercises have demonstrated

dose escalation in anal cancer patients is achievable

without sacrifice of OAR sparing. This shows OAR sparing

is achievable across multiple centres using a variety of

planning techniques, giving expectation of consistent

quality plans for trial patients.

Over 30 sites will join the trial in the next phase and will

complete the same RTQA process.

References

[1]

A Computational Environment for Radiotherapy

Research, CERR; Online:

http://www.cerr.info/about.php

EP-1733 Proton grid therapy (PGT): a parameter study

T. Henry

, A. Valdman

, A. Siegbahn

Stockholm University, Department of Medical Physics,

Stockholm, Sweden

Karolinska Institutet, Department of Oncology and

Pathology, Stockholm, Sweden

Purpose or Objective

Proton grid therapy (PGT) with the use of crossfired and

interlaced proton pencil beams has recently been

proposed by our research group. A clear potential for

clinical applications has been demonstrated. The beam

sizes used in our proof-of-concept study were in the range

7-12 mm, full-width at half maximum (FWHM),

representing the typical range of available proton pencil-

beam widths at a modern proton therapy facility.

However, to further take advantage of the dose-volume

effect, on which the grid therapy approach is based, and

thereby improve the overall outcome of such treatment,

smaller beams are desirable. In this present study, Monte-

Carlo (MC) simulations of a simple PGT treatment were

performed with varying beam sizes and center-to-center

(c-t-c) distances between the beams. The aim was to

determine which combinations of those two parameters

would produce the most therapeutically desirable dose

distributions (high target dose and low valley dose outside

of the target).

Material and Methods

MC calculations were performed using TOPAS version 2.0

in a 20x20x20 cm

water tank. The beam grids were aimed

towards a 2x2x2 cm

cubic target at the tank center. Two

opposing (or 2x2 opposing) grids were used. The target was

cross-fired in an interlaced manner. Grids containing

planar beams (1-D grids) or circular beams (2-D grids) were

considered. Three beam widths (1, 2 and 3 mm FWHM) and

a wide range of c-t-c distances (3-12 mm) were studied.

Peak and valley doses outside the target and the

minimum, maximum and mean doses inside the target

were scored. The objective of the planning was to obtain

a nearly homogeneous target dose in combination with low

peak doses in normal tissue as well as high peak-to-valley

dose ratios (PVDRs) close to the target.

Results

The most appropriate c-t-c distances, according to our

planning objectives, for 1, 2 and 3 mm beam-element

widths, were 7, 8 and 10 mm, respectively. With these c-

t-c distances, a very high entrance PVDR was obtained for

the 3 beam sizes (>10000). At 1 cm distance from the

target, the PVDR was 9, 10 and 14, for the three beam

widths studied. Inside the target, a high dose homogeneity

could be obtained for these cases (σ= ±4%). When

decreasing the c-t-c distance further, the PVDR decreased

dramatically outside of the target. With increasing c-t-c

distances, the PVDRs also increased as expected, but the

overall target dose homogeneity decreased due to the

appearances of cold spots.

Conclusion

In this work we studied the possibility to use beam-

element widths in the mm range for PGT combined with

crossfiring. For each proton beam-element size studied,

an optimal c-t-c distance was determined according to the

selected planning objectives. With the optimal parameter

setting, a high target dose homogeneity could be obtained

together with high PVDRs outside of the target.

EP-1734 AAPM TG-119 benchmarking of a novel

jawless dual level MLC collimation system

D. Mihailidis

, R. Schuermann

, C. Kennedy

, J. Metz

University of Pennsylvania, Radiation Oncology,

Philadelphia, USA

Purpose or Objective

To study delivery accuracy for fixed beam and volumetric

intensity modulated RT (IMRT & VMAT) of a new jawless

MLC collimation system mounted on a straight through

linac. The AAPM TG-119

recommended IMRT

commissioning process was used to benchmark the new

MLC system and compare it with the TrueBeam Millennium

(120-MLC). This new MLC has faster moving leaves that

may be more optimum for faster intensity modulated

deliveries.

Material and Methods

A prototype jawless MLC system with 28 pairs of 1cm

leaves provides a 28x28cm

field size at 100 cm. The

leaves have maximum over-travel, i.e. over 28 cm, and

100% inter-digitization. After acquiring beam data and

deducing the dosimetric leaf gaps (DLG) for modeling the

MLC in the planning system, we applied the test plans in

TG-119 IMRT for fixed IMRT and VMAT delivery. The same

test plans, using 6X-FFF (filter-free), were planned and

delivered, in an identical way, on a solid water phantom

with a cc-13 ion chamber (IC), a MapCheck2 (for IMRT),

and an ArcCHECK (for VMAT). Results obtained with the

millennium and the new MLC system were compared based

on γ-criteria of 3%/3mm-G (global normalization), and a

more stringent 2%/2mm-L (local normalization).

Results

The TB DLG values (1.3mm) were adjusted to balance the

confidence intervals for the IC measurements between

IMRT and VMAT. For the new MLC system, the DLG values

(0.1mm) were not adjusted. The TG-119 required IC

measurements resulted for prototype MLC: 1.19% (mean),

1.28% (SD), 3.71% (CL) and 0.19% (mean), 0.47% (SD),

1.11% (CL) for high dose and low dose regions,

respectively. For the TB MLC: 1.93% (mean), 0.5% (SD),

2.91% (CL) and 1.32% (mean), 1.17% (SD), 3.62% (CL) for

high dose and low dose regions, respectively. The