ESTRO 36 Abstract Book

S940

ESTRO 36 2017

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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

comparison of planned to delivered plans for all TG-119

targets for IMRT and VMAT deliveries are shown in Figures

1 and 2 below, and for the two MLC systems. The

prototype MLC system produced higher passing rates for

both IMRT and VMAT than the TB MLC system for the

various test plans. In addition, the prototype MLC system

performs equally well for IMRT and VMAT, whereas the TB

MLC is less optimum for VMAT delivery compared to IMRT

(Fig. 1 and Fig. 2).

Conclusion

The TG-119 test plans were performed on a prototype MLC

system in comparison to the well-understood TB

Millennium MLC. Our investigation showed, in the context

of TG-119, this prototype MLC performs well for both IMRT

and VMAT plans.

Ezzel G., et al., 'IMRT commissioning: Multiple institution

planning and dosimetry comparisons, a report from AAPM

Task Group 119.” Med. Phys.

:5359-5373 (2009).

EP-1735 Total skin irradiation with helical

Tomotherapy: Planning and dosimetry feasibility

aspects

A. Haraldsson

, P. Engström

Skåne University Hospital, Radiation physics, Lund,

Sweden

Purpose or Objective

Mycosis fungoides (MF) is a lymhpatic disease that attacks

the skin. The primary treatment for treating MF is total

skin electron therapy (TSET). The procedure is technically

challenging both in terms of dosimetry and treatment

delivery. Helical TomoTherapy (HT) is due to its design

especially advantageous when irradiating very long and

complex targets. In this study we have explored the

possibility of employing HT in the treatment of total skin

irradiation (TSI).

Material and Methods

We used an anthropomorphic whole body phantom (PBU-

60 Kyoto Kaguka). The phantom was immobilized with

whole body vacuum bag, a five-point open head net mask

fixated to the couch and an individual neck rest. The

phantom was covered with a 7 mm thick wet suit made of

Neoprene (AquaLung) and CT scanned in two sets; from

vertex to thigh and from toes to hip. The CTV was defined

as skin with 5mm depth, with PTV as a 7 mm expansion.