ESTRO 36 Abstract Book

S991

ESTRO 36 2017

_______________________________________________________________________________________________

Material and Methods

Three dimensional conformal radiotherapy plans (3D-CRT)

for patients treated in our hospital were analysed. The

analysis and assessment of differences in calculations

obtained with two TPS were performed for 160 and 130

patients treated with 6 MV and 15 MV X-rays, respectively.

3D-CRT treatment plans were prepared in the Eclipse TPS

(Analytical Anisotropic Algorithm, version 10).

Subsequently the DICOM RT data were transferred to the

Oncentra MasterPlan TPS (Collapsed Cone Convolution,

version 3.3) and dose distribution was calculated. The

same number of monitor units were kept. The differences

between mean dose in the PTV and dose in the isocenter

were compared. For statistical analyses, the Wilcoxon

matched-pair signed rank test was used. In addition, the

calculations of both TPS were compared with

measurements performed in the inhomogeneous phantom.

Results

The statistically significant (p < 0.001) systematic shift

between systems, both in the mean dose in the PTV and

dose in isocenter was observed. The doses calculated in

the Oncentra MasterPlan TPS were always lower than

doses calculated in the Eclipse TPS. The average

difference of mean dose delivered to the PTV was 1.4% ±

1.0% for 6 MV X-rays and 2.5% ± 0.6% for 15 MV X-rays. The

average difference in dose in the isocenter was 1.2% ±

3.5% and 2.4% ± 2.7% for X6 MV and X15 MV, respectively.

In most cases the largest differences were caused by air

cavities and bone structures. The significant influence of

inhomogeneities on dose calculations was confirmed by

measurements.

Conclusion

Estimated differences of dose calculations in most cases

did not exceed the action level of 3% recommended by the

Knöös et al. publications (2006 Phys. Med. Biol., 51: 5785–

5807). Based on this study the independent verification of

the Eclispe TPS calculations was introduced in our

hospital.

EP-1836 Study of changes in bowel gas in pelvic

radiotherapy

A. Horne

, M. Zahra

, W. Keough

NHS Lothian, Clinical Oncology, Edinburgh, United

Kingdom

Purpose or Objective

To assess volumetric changes in pelvic bowel gas using

serial CT scans in patients receiving pelvic radiotherapy.

Material and Methods

We performed a retrospective analysis of bowel gas

volume and location using planning CT scans for 22

patients. All patients had an initial plan scan (CT1) with a

full bladder . In group 1 (n=9) this was compared with a

planning scan(CT2) done after about 30 minutes of CT1

with an empty bladder . In group 2 (n=15) CT1 was

compared with a CT scan (CT3) done for a phase 2

boost 4 weeks after CT1.

Eclipse

planning software was used to contour

the luminal bowel gas volume within a conventional pelvic

radiotherapy field (see figure 1). For each of the 2 groups

the total volume and the volume in each quadrant was

measured and compared. Significance testing was carried

out using a paired t-test with two-tailed significance

levels

at 0.05.

Results

The results have been summarised in the table below:

Conclusion

In group 1 no significant difference was identified due to

changes in bladder volume over a short period of time. In

group 2 there was a tendency towards an increase in bowel

volume over the 4 week period of radiotherapy more so in

the upper quadrants reflecting changes mainly in the colon

rather than in the rectum. This is possibly due to the

radiotherapy toxicity and associated diet changes.

The volume of bowel gas does not appear to be consistent

during a course of pelvic radiotherapy and this should be

taken into consideration when dosimetry changes are

made to the initial planning scan to compensate for bowel

gas.

Electronic Poster: RTT track: Image guided radiotherapy

and verification protocols

EP-1837 Dosimetric effect by rotational error in VMAT

on Brain tumor patients

H. Park

Yonsei University, Radiation Oncology, SEOUL, Korea

Republic of

Purpose or Objective

This study is to evaluate the dose effects on whether

Hexapod Couch is applied for patient positioning errors to

be corrected in the course of VMAT on brain tumor

patients.

Material and Methods

For this study, a total of 1129 cases of CBCT acquired from

46 brain tumor patients with VMAT and Hexapod used are

comparatively analyzed and difference on dose effects is

evaluated by dosimetric quality

assurance(DQA) and