ESTRO 36 Abstract Book

S190

ESTRO 36

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and the radiotherapy management system, as the two

systems are not linked.

Results

In total, 10 patients were treated with the long treatment

schedule, and 10 with the short treatment schedule,

resulting in 300 plan selections. Margin sets of 25 mm, 15

mm, 0 mm were created for 6 patients, and margin sets of

15 mm, 0 mm, -15 mm for 13 patients. One patient had a

set of only two margins available (0 mm, 15 mm), due to

insufficient time at treatment planning. Overall, the -15

mm, 0 mm, 15 mm and 25 mm plans were selected in 2%,

45%, 39% and 14% of fractions, respectively. For

distributions per patient, see figure 2. The largest

available margin was always sufficient. Treatment was

delayed a total of 7 times (of which 5 times in 1 patient)

to obtain a more favorable anatomy in case of a very full

rectum, usually caused by gas pockets. Evaluation of the

post-treatment CBCT scans showed for 1 fraction the

selected plan was no longer suitable due to a moving gas

pocket. The weekly review showed that a plan with a

smaller margin could have been selected in 20% of

fractions, and a larger margin in 2% of fractions. No

inconsistencies were found in selected plans between the

imaging system and radiotherapy management system.

Conclusion

A plan selection strategy for rectum cancer patients was

successfully and safely implemented. Next we will

quantify the dosimetric impact of plan selection to the

dose of the organs at risk in this dataset.

Proffered Papers: Physics Dosimetry

OC-0357 Treatment planning dosimetry accuracy in

192Ir HDR brachytherapy of lip carcinoma

P. Papagiannis

, V. Peppa

, T. Major

National and Kapodistrian University of Athens, Medical

Physics Lab.- Medical School, Athens, Greece

National Institute of Oncology, Department of

Radiotherapy, Budapest, Hungary

Purpose or Objective

Advanced dose calculation algorithms h ave become

clinically available for

192

Ir HDR brachytherapy y to

account for the effects disregarded by TG43 based

dosimetry algorithms (heterogeneities, applicators and

patient specific scatter conditions).

The aim of this work is to study the effect of improved

dosimetric accuracy in HDR brachytherapy of squamous

carcinoma of the lip.

Material and Methods

Three anonimized patient cases were studied (treatments

using the

192

Ir microSelectron-HDR v2 source, 27 Gy

planning aim delivered in 3 Gy fractions b.d.).

The plans were imported to OncentraBrachy v4.5 and

dosimetry was repeated using both the TG43 and the

Advanced Collapsed Cone Engine (ACE) TPS algorithms.

The same TRAK was used with both algorithms for the

same patient case. ACE calculations were performed using

the high accuracy option taking into account individual

voxel densities and assuming the elemental composition

of water, average skin and cortical bone for the PTV and

soft tissue, the skin, and the mandible, respectively. The

spatial resolution of TPS dosimetry results was 1 mm,

isotropic.

Corresponding reference data were obtained from patient

specific Monte Carlo (MC) simulations using the MCNP6

code with input files prepared from the parsing of dicom

RT data with the BrachyGuide software tool.

The TPS HU calibration was imported to BrachyGuide to

ensure identical density input to ACE and MC. Dose was

approximated by collision Kerma and kerma to medium in

medium was scored using the F6 tally.

BrachyGuide was also used for the comparison of the three

RT dose files for each patient case (TG43, ACE, and MC).

Results

TG43 clearly overestimates results for all cases as shown

in the left side of Figure 1 for an indicative case. This

cannot be attributed solely to the difference between

patient scatter conditions and TG43 assumptions since

large differences are also observed close to the source

dwell positions. The corresponding comparison between

ACE and MC (right side of figure 1) shows agreement within

MC type A uncertainty up to 5 cm from the implant. While

ACE improves dosimetric accuracy, considerable

differences are still observed close to the source dwell

positions.

Results from the comparison of median DVH parameters in

the Table show large differences between TPS calculations

and MC for high dose PTV volumes (V150 και V200) in

accordance with the above findings. Large differences

between TPS calculations and MC are also observed for

OAR parameters. These differences however correspond