Radiation and Cancer Physics

PD 12 - Physics 6 - Poster Discussion - Adaptive Planning/Delivery and Motion

1105 - Comparison of Epid Based In-Vivo Dosimetry and CBCT Dose Recalculation for Triggering of Treatment Adaptation in Lung Cancer Irradiation

Tuesday, October 23
3:27 PM - 3:33 PM
Location: Room 217 C/D

Comparison of Epid Based In-Vivo Dosimetry and CBCT Dose Recalculation for Triggering of Treatment Adaptation in Lung Cancer Irradiation
R. Canters, K. Zegers, W. van Elmpt, and B. Nijsten; Department of Radiation Oncology (MAASTRO), GROW – School for Oncology and Developmental Biology, Maastricht University Medical Centre (MUMC), Maastricht, Netherlands

Purpose/Objective(s): In vivo EPID dosimetry is regularly performed in our institution, resulting in 3D delivered dose based on the planning CT scan. The main cause for treatment adaptation is changing patient anatomy during the course of treatment, hence the use of kV CBCT imaging. In this study, we have investigated the usefulness of both planning CT scan based 3D in-vivo EPID dosimetry and 3D dose recalculation on kV CBCT scans for triggering of treatment adaptation.

Materials/Methods: For 40 lung cancer patients, daily CBCT’s were acquired. 20 Patients in the study group underwent one or more treatment adaptations. Contours were propagated to the daily CBCT’s using deformable registration, after which dose was recalculated using the original plan in the daily anatomy, using a monte carlo algorithm (XVMC). To obtain a meaningful comparison, the same algorithm was used to recalculate the treatment plan on the planning CT. Both DVH criteria(one or more OAR’s exceeding clinical constraints) and 3D gamma criteria (in-field gamma 5% and 3mm, mean gamma value >0.5) were used to quantify dose differences between planning CT and CBCT. In vivo EPID dosimetry was performed, projecting measured dose back on the planning CT (using XVMC). Gamma criteria (5% and 3mm, one or more structures have a pass rate <95%) and DVH criteria (one or more OAR’s exceeding clinical constraints) were used to assess each treatment fraction. For both methods, the number of fractions flagged for violating the criteria were counted to assess the potential for triggering adaptation.

Results: Based on gamma criteria, our in-vivo dosimetry system on average flagged 34% of the fractions in non-adapted patients, while 24% of the fractions in adapted patients were flagged (p=0.8). Based on DVH criteria respectively 25% and 31% of the fractions were flagged (p=0.3). In CBCT recalculation, evaluation of gamma criteria flagged 20% of the fractions in adapted patients, while 10% of the fractions were flagged in non-adapted patients (p=0.1). Evaluation of DVH-based parameters on CBCT flagged respectively 17% and 6% of the fractions (p=0.003).

Conclusion: In vivo EPID dosimetry based on dose reconstruction using planning CT scans appears not to be able to detect differences between patients needing adaptation and patients not needing adaptation. On the other hand, DVH based evaluation of recalculated doses on CBCT’s shows a clear difference between adapted and non-adapted patients. Therefore, it is useful to further investigate the exact triggering levels and protocols in CBCT dose recalculation to effectuate a dose based treatment evaluation and adaptation protocol. Table 1: Example of DVH based evaluation of clinical constraints on CBCT in a patient that underwent adaptation vs. a non-adapted patient (red = one or more constraints outside limits, green = all constraints within limits)
Adapted Patient
Fraction 1 4 7 10 16 adaptation 17 20 23
Outside limits N N N Y Y N N Y
Non-adapted patient
Fraction 1 5 9 13 17 21 25 29 32
Outside limits N N N N N N N N N

Author Disclosure: R. Canters: None. K. Zegers: None. W. van Elmpt: None.

Richard Canters, PhD

Disclosure:
No relationships to disclose.

Presentation(s):

Send Email for Richard Canters


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