ESTRO 36 Abstract Book

S901

ESTRO 36

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Conclusion

This work shows that the dosimetric effect of weight loss

does not cause any clinically significant changes in the

presence of a magnetic field, as the difference between

pCT and rCT for 0T and 1.5T are similar. Therefore,

current off-line strategies for adaptive planning for head

and neck patients are valid for use on the MR-Linac.

EP-1659 Quantitative triggering of plan adaptation:

monitoring plan quality by recalculation on CBCT scans

R. Canters

, M. Wendling

, M. Kusters

, R. Monshouwer

Radboud University Medical Center, Radiation oncology,

Nijmegen, The Netherlands

Purpose or Objective

Since the introduction of 3D imaging on the linac,

anatomical changes observed on CBCT scans regularly lead

to plan adaptation. However, adaptation is often triggered

by qualitatively assessing anatomical changes between

CBCT and planning CT. This regularly leads to unnecessary

replanning, disrupting the regular workflow in the clinic.

In this study, we created an automated evaluation tool,

that recalculates the treatment plan on recorded CBCT

scans to indicate if a replanning may be necessary. The

aim of this work is to assess its potential for regular

clinical use.

Material and Methods

The recalculation tool imports planning CT a nd CBCT

scan, after which the treatment plan is transferred to the

CBCT scan. Subsequently, the plan is recalculated on the

CBCT using Pinnacle, and DVH’s are compared (Figure 1).

The CT-CBCT match is derived from the CBCT match at the

linac. Since Hounsfield units (HU) of the CBCT are not

calibrated, a CT to CBCT HU conversion table was created

to obtain a reliable CBCT based dose calculation. To

validate this approach, we evaluated 11 CT-CBCT

registrations of the head with no visible deformations, and

compared plan calculations on both scans. To assess the

potential to monitor planned dose on the CBCT, 22

patients receiving postoperative head and neck irradiation

with 2 or 3 dose levels were evaluated retrospectively for

a total of 265 CBCT scans. 5 Patients received a new CT

and a replanning during the treatment course. All dose

distributions were evaluated on V95% of the PTV, mean

dose on parotid glands, mandible, oral cavity, larynx,

maximum dose on myelum, and low dose volume (<5Gy).

Results

Validation on 11 patients of the dose calculation showed

an average deviation between planning CT and CBCT scans

of less than 1% on all evaluated dose metrics (Figure 2a).

Evaluation of 22 patients shows deviations of <5% in PTV

coverage in 20 patients over the course of the treatment

(Figure 2b). Two patients showed a higher deviation.

Patient 14 showed anatomical variation that was not

detected during treatment. Patient 18 had a relevant

reduction in PTV coverage during treatment course due to

weight loss and received a new plan. Four other patients

received a replanning because of other considerations,

e.g. a deteriorating condition or treatment side effects.

In the evaluated OAR’s, variations in evaluated metrics of

<5% were observed.

Conclusion

The automated evaluation tool in this study provides a

reliable prediction of delivered dose for the daily patient

anatomy. Evaluation of a series of fractions shows that it

is can detect dose deviations and trigger plan adaptation,

with an action level of approximately 5% deviation in

V95%. Inclusion of deformable image registration is

expected to further increase the reliability of the DVH

predictions.