Head and Neck Cancer

PV QA 2 - Poster Viewing Q&A 2

MO_40_2526 - The difference between planned and delivered dose for head and neck cancer, and the consequences for normal tissue toxicity probability

Monday, October 22
10:45 AM - 12:15 PM
Location: Innovation Hub, Exhibit Hall 3

The difference between planned and delivered dose for head and neck cancer, and the consequences for normal tissue toxicity probability
J. Heukelom1, M. E. Kantor2, A. S. Mohamed2, H. Elhalawani3, E. Kocak2,4, T. Lin2, R. T. Williamson2, C. R. Rasch5, C. D. Fuller2, and J. J. Sonke1; 1The Netherlands Cancer Institute (NKI-AVL), Department of Radiation Oncology, Amsterdam, Netherlands, 2The University of Texas MD Anderson Cancer Center, Houston, TX, 3Department of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, 4Medipol University, Department of Radiation Oncology, Istanbul, Turkey, 5Amsterdam Medical Center, Amsterdam, Netherlands

Purpose/Objective(s): Anatomical changes during radiation for head and neck cancer (HNC) induce differences between the planned and delivered dose. To account for this, an adapted treatment plan can be designed. The decision to do this is based on expert opinion after examination of inroom images. The aims of this study were to 1. quantify the difference between planned and delivered dose in HNC patients. 2. assess the consequential difference in normal tissue complication probability (ΔNTCP). 3. Assess the positive predictive value (PPV) for the allocation of adaptive radiotherapy (ART) of clinical judgement vs. NTCP based on dose differences at fraction 10 (F10) and 15 (F15).

Materials/Methods: Daily CT scans in treatment position for 56 patients were retrospectively analyzed. Delivered dose was calculated based on the anatomy of the day and subsequently mapped back to the planning CT to allow dose accumulation. The difference between planned and delivered dose was analyzed for CTVs and 9 organs at risk (OAR). ΔNTCP was calculated for xerostomia, dysphagia, parotid gland dysfunction and tube feeding dependency at 6 months post treatment (Tube6m) using previously published NTCP models. ART was deemed necessary if ΔNTCP was >5%. To predict ΔNTCP at the end of treatment using the dose difference at F10 or F15, the dose on that day was extrapolated to the full treatment length prior to the NTCP calculation. The PPV was calculated for identification of ART-patients by clinical judgement, ΔNTCP at F10 and F15.

Results: For the primary CTV, median differences between planned and delivered dose (D95, D99 and D1) were ≤1.2 Gy. For OAR, considerable dose differences were observed (table). ΔNTCP >5% was seen most for dysphagia (n=6), followed by xerostomia (n=3), parotid gland dysfunction (n=3) and Tube6m (n=2). Only 7/11 patients with any ΔNTCP >5% clinically received ART, although ART had been done for 16 patients (PPV: 0.44). PPV was 0.89 and 0.80 for dose at F10 and F15 respectively using a ΔNTCP cut-off for the allocation of ART of 5%. Using other ΔNTCP cut-offs did not significantly improve PPV. With this cut-off the negative predictive value was 0.90, 0.94 and 0.93 for clinical judgement, F10 and F15.

Conclusion: Clinical judgement poorly identifies patients who have a >5% ΔNTCP between planned and delivered dose. To identify them more accurately, NTCP models based on the dose differences between planned and delivered dose at day ten could be used; thus allocating ART to patients who are most likely to benefit.
OAR Dose difference median; (lowest difference; highest difference)
Brainstem* 1.05 (-4.72; 10.59)
Esophagus -0.26 (-3.76; 3.71)
Larynx 1.83 (-0.65; 10.93)
Mandible -0.60 (-6.22; 2.93)
Contralateral Parotid -0.21 (-5.42; 7.05)
Ipsilateral Parotid 0.03 (-6.90; 5.63)
Spinal Cord* 1.24 (-3.20; 6.21)
Contralateral submandibular 0.11 (-2.48; 3.43)
Ipsilateral submandibular -0.16 (-2.37; 3.27)
Difference mean planned vs mean delivered dose for OAR, except for * D1

Author Disclosure: J. Heukelom: Research Grant; KWF, Rene Vogels Stichting. M.E. Kantor: None. A.S. Mohamed: None. H. Elhalawani: Employee; University of Texas M.D. Anderson Cancer Center. E. Kocak: Employee; Medipol University, Department of Radiation Oncology, Istanbul, Turkey. T. Lin: None. R.T. Williamson: None. C.R. Rasch: chair; Dutch Society for Radiation Oncology. Board member; Stichting Oncologische Samenwerking. C.D. Fuller: Research Grant; National Institutes of Health, National Science Foundation, Elekta AB. Grant funding; Elekta AB. Honoraria; Nederlandse Organisatie voor Wetenschappelijk Onde. Consultant; Elekta AB, Nederlandse Organisatie voor Wetenschappelijk Onde. Travel Expenses; Elekta AB, Nederlandse Organisatie voor Wetenschappelijk Onde. Reviewer; Radiological Society of North America. Associate Editor; Radiographics. Data Management Task Force Committee Member; MR-LinAc Consortium. Member; National Cancer Institute. Task Group Member; American Association of Physicists in Medicine. J. Sonke: Employee; Antoni van Leeuwenhoek. Research Grant; Elekta Oncology Systems Ltd. international advisory board member; PMB. associate editor; medical physics.

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