ESTRO 36 Abstract Book

S980

ESTRO 36 2017

_______________________________________________________________________________________________

K. Koiwai

, A. Shinoda

, T. Ozawa

, H. Matsushita

, A.

Fukazawa

, K. Sakai

, M. Kadoya

Shinshu University School of Med., Radiology,

Matsumoto, Japan

Purpose or Objective

In radiotherapy for head and neck cancers, postural

reproducibility is sometimes deteriorated due to the non-

rigidity of the head and neck. Image-guided corrections

cannot completely compensate for the deterioration. We

evaluated whether aligning the chest with a treatment

couch just before immobilisation improved the postural

reproducibility in radiotherapy for head and neck cancers.

Material and Methods

Fifty patients with head and neck cancers who underwent

radiotherapy from February 2015 to January 2016 were

selected for this study. Twenty five patients were treated

with the “aligning-the-chest” method, and others not.

When we acquired planning CT images of the patients with

the aligning-the-chest method, we drew marks on the skin

of their anterior chest in line with the fixed marks on the

both sides of the couch of the CT scanner which were

geometrically the same as on the treatment couch. On the

set-up for the treatment, the marks on their chest were

aligned with the fixed marks on the both sides of the

treatment couch just before immobilisation. For all

patients, 2 oblique X-ray images (image 1 and image 2)

were obtained on the first session of the treatment. We

generated digital reconstructed radiographies (DRRs)

representing an ideal posture from the planning CT images

with the maximal similarity to the corresponding X-ray

images with 6 degree-of-freedom registration. On the both

X-ray images and DRRs, we measured the mandibuler-

spinal angle that was defined as the angle formed by the

line projected parallel to the base of the mandible and the

line projected parallel to the alignment of the lower

cervical spine. As an index for postural reproducibility, the

difference of the mandibuler-spinal angle between the X-

ray image and the DRR (DMSA) was calculated in each

patient. The raw and absolute DMSA were analysed.

Results

In the mean value of the raw DMSA, there was no

difference between the patients with the aligning-the-

chest method and without the method (image 1 and 2, -

0.5° vs. 1.7° and 1.2° vs. 3.0°, p = 0.1713 and p = 0.1072,

respectively). The mean value of the absolute DMSA in the

patients with the aligning-the-chest method was

significantly smaller than that in the patients without the

method (image 1 and 2, 1.3° vs. 4.7° and 2.0° vs. 5.7°, p

= 0.0004 and p < 0.0001, respectively).

Conclusion

Aligning the chest with a treatment couch before

immobilisation improved postural reproducibility in

radiotherapy for head and neck cancers.

EP-1812 A study on the patient positioning accuracy for

stereotactic radiotherapy of brain lesions

O. Pashkovskaya

, E. Polovnikov

NSRICP, Oncology and radiotherapy, Novosibirsk,

Russian Federation

Purpose or Objective

The purpose of this study is an investigation of the patient

positioning accuracy (inter-fraction and intra-fraction

shifts) for stereotactic radiation treatment of the brain

lesions using invasive Lexsell Coordinate Frame G®, non-

invasive vacuum-activated head frame system HeadFix®

and individual thermoplastic masks, and comparison

position errors of these systems.

Material and Methods

A total of 532 patients, which were treated for benign and

malignant brain tumors in our center, represented study

population. The invasive Lexsell Coordinate Frame G®

(LFG), vacuum-activated head frame system HeadFix®

(HF) and individual thermoplastic masks (TM) were used to

immobilize patients during pre-radiation preparation and

radiation treatment. Dose distributions were typically

planned with a volumetric modulated arc therapy

technique and noncomplanar arcs if needed for optimal

dose distributions. Positioning accuracy was estimated

using linac-mounted cone beam computer tomography

(CBCT) scan system. The target intra-fraction offsets were

determined by comparison target positions before and

after the irradiation procedures with planning positions.

To determine inter-fraction shifts data from daily CBCT

scans were used. Statistical samples of target offsets in six

coordinates were collected and statistical analysis was

carried out.

Results

For 386 patients TMs were used, for 31 LFG and for others

115 HF. Analysis of inter-fraction shifts, based on

scaterplots, showed no significant correlations and

selected

systematic

errors

patient’s

positioning. Quantitative estimations of mean values and

standard deviations also demonstrated acceptable

accuracy. It was concluded that it’s necessary to do daily

CBCT control, if shifts at least in one direction are greater

or close to 5 mm during first four procedures. This is due

to patient’s specific characteristic and TM doesn’t provide

sufficient immobilization. Mask remaking does not always

solve the problem. As the result of intra-fraction offset

estimations, the position errors for LFG and HF were less

than 0.3 mm in coordinates and less than 0.3 degrees in

rotation.

Conclusion

Study of inter-fraction shifts showed that TMs provide

acceptable setup reproducibility, as consistent with PTV.

With respect to intra-fraction shifts, due to the slight

differences of the offset error results, the choice between

LFG and HF should be determined by the individual patient

characteristics and does not affect on the accuracy of the

delivered

dose.

EP-1813 The investigation of the immobilization

devices and localization methods for brain cancer in P-

SRS.

R.D. Lin

, J. Gao

, P. Chen

Chang Gung medical hospital, Proton and radiation

therapy center, Taoyuan, Taiwan

Purpose or Objective

There are two aims of this study: one is to assess the

accuracy of the Base of Skull (BoS) immobilization cast and

BRW headring for proton stereotactic radiosurgery (P-

SRS). The other one is to investigate the accuracy of

imaging-guided localization method by using bony

structure and fiducial markers as different landmarks.

Material and Methods

To assess the accuracy of the immobilization devices, we

immobilized the CIRS Radiosurgery Head Phantom with

hidden target by BoS cast and headring, respectively. Then

we implemented the imaging guidance by using bony

structure. Compare to the localization method, the CIRS

Radiosurgery Head Phantom with hidden target was

immobilized by headring followed by implementation of

the imaging guided process using bony structure and

fiducial markers, respectively. The overall couch shifts

were performed by the radiation therapists, the deviation

with respect to the proton isocenter was measured by

using the hidden target with paired t-test and 3D vector

Results

For BoS cast and headring, the deviations (mm) were

0.54±0.68 and 0.48±0.28 (p

＝

0.412) in right-left (RL),

0.22±0.28 and -0.11±0.34 (p

＝

0.186) in superior-inferior

(SI), -0.30±0.11 and -0.15±0.22 (p

＝

0.167) in anterior-

posterior (AP) directions, and 0.85±0.42 and 0.60±0.32 (p

＝

0.017) in 3D vector, respectively. For the imaging-

guided localization method, the deviations

(mm) of bony