ESTRO 36 Abstract Book

S443

ESTRO 36 2017

_______________________________________________________________________________________________

Oncology, New Delhi, India

Humanitas Research Hospital and Cancer Center,

Radiation Oncology, Milan-Rozzano, Italy

Purpose or Objective

The possibility to deliver intensity modulated plans using

most of the 4-pi space, i.e. with extensive use of non-

coplanar beams and complex trajectories for the couch-

gantry-collimator system, has been explored on

stereotactic irradiation in the brain, lungs and prostate

and have shown significantly sharper dose gradients. The

applicability of 4p techniques to large target volumes with

volumetric modulated arc therapy (VMAT) treatments

remains unaddressed for head and neck cancer (HNC). The

aim of this work is to explore the feasibility and

deliverability of multi-isocentric 4-pi VMAT (4pi-VMAT)

plans in comparison with coplanar VMAT (CP-VMAT) plans

for the irradiation of HNC patients characterized by large

targets and the presence of several organs at risk.

Material and Methods

25 previously treated patients of HNC were planned to

achieve the highest dosimetric plan quality with 2 full

coplanar VMAT arcs (CP-VMAT) on 6MV from a Clinac-iX

(Varian), planned with Eclipse version 13.1, calculated

with Acuros. 4pi-VMAT plans were then generated using

same priorities and objectives, using 1 full arc and 4-6

non-coplanar arcs on 2-3 isocenters: typically 1 full arc

with couch at 0°, 2 partial arcs (length of ±210°) with

couch ~±45°, and 2 partial arcs (length of ±250°) with

couch ~±15°. Dose was prescribed on three levels: 70,

60/63, and 56 Gy on targets of median volumes of 720,

492, and 94 cm

, respectively. The following organs at risk

(OAR) were defined and analyzed: parotids, oral cavity,

esophagus, trachea, larynx, pharyngeal constrictor

muscles, mandible, temporomandibular joint, middle ear,

spinal cord and brain stem. Pre-treatment quality

assurance was performed to assess deliverability and

accuracy of the 4pi-VMAT plans.

Results

CP-VMATand 4pi-VMAT plans achieved the same degree of

coverage for all target volumes related to near-to-

minimum and near-to-maximum doses. 4pi-VMAT plans

resulted in an improved sparing of OARs. The average

mean dose reduction to the parotids, larynx, oral cavity

and pharyngeal muscles were 3Gy, 4Gy, 5Gy and 4.3Gy

respectively. The average maximum dose reduction to the

brain stem, spinal cord and oral cavity was 6.0Gy, 3.8Gy

and 2.4Gy respectively. The average MUs were 525±78 and

548±70 for 4pi-VMAT and CP-VMAT, respectively. The

average simulated beam on time for 4pi-VMAT plans

(612±77 s) was 3.7 times higher than that of CP-VMAT

plans (167±30 s). Pre-treatment QA results showed that

plans can be reliably delivered with mean gamma

agreement index of 97.0±1.1% with 3% dose difference and

3% distance to agreement criteria.

Conclusion

4pi-VMATplans significantly decrease dose-volume metrics

for relevant OARs and results are technically feasible and

reliable from a dosimetric standpoint. Early clinical

experience has begun.

PO-0832 The impact of variable RBE and breathing

control in proton radiotherapy of breast cancer

J. Odén

1,2

, K. Eriksson

, A.M. Flejmer

, A. Dasu

, I. Toma-

Dasu

1,5

Stockholm University, Department of Medical Radiation

Physics, Stockholm, Sweden

RaySearch Laboratories, Department of Research,

Stockholm, Sweden

Linköping University, Department of Oncology and

Department of Clinical and Experimental Medicine,

Linköping, Sweden

The Skandion Clinic, Uppsala, Sweden

Karolinska Institutet, Department of Oncology and

Pathology, Stockholm, Sweden

Purpose or Objective

Proton therapy, with or without breathing control

techniques, may be used to reduce the cardiopulmonary

burden in breast cancer radiotherapy. However, such

studies typically assume a constant RBE of 1.1 for protons.

This study aims to assess the impact of using a variable

RBE in breast proton radiotherapy and to evaluate the

sensitivity to respiratory motion when no breathing

control is applied.

Material and Methods

Tangential photon IMRT and 3-fields proton IMPT plans for

breast radiotherapy were generated for twelve patients,

both on a free-breathing (FB) CT and on a CT using breath-

hold-at-inhalation (BHI). 2 Gy(RBE) per fraction in 25

fractions were planned to the whole breast. The physical

proton dose was optimized assuming a constant RBE of 1.1.

Besides the constant RBE of 1.1, the variable RBE-model

by Wedenberg et al. (2013), assuming an α/β of 3.5 Gy for

the CTV/PTV and 3 Gy for the OARs, was used for plan

evaluation. Subsequently, the FB plans were recalculated

on the CT images of the two extreme phases (inhale and

exhale) to evaluate the sensitivity of a treatment delivery

without breathing control.

Results

All photon and constant RBE proton plans met the clinical

goals with similar target coverage. The target conformity

and homogeneity of the proton plans were superior to the

photon plans. The plan quality was generally independent

on whether the FB or BHI CT-scan was used. However, if

the heart was close to the target, the BHI plan lowered

the dose to the left anterior descending (LAD) artery in

most cases. Applying the variable RBE-model resulted in

an average of the mean RBE of 1.18 for the PTV and also

increased the heterogeneity. The predicted RBE values in

the OARs were also substantially higher than 1.1.

However, due to the low physical doses, this is expected

to have a minor impact. The dosimetric parameters for the

BHI plans are shown in Table 1.

The recalculation of the FB plans on the extreme phases

generally resulted in minor differences for the CTV

coverage and OAR doses for the proton plans. Small CTV

volumes may, however, receive a slightly lower dose for

the recalculated photon FB plans. The ranges of

dosimetric parameters for the FB plan for one patient are

shown in Table 2.