title: Characterization of volume and shape modifications of PET-positive nodes during Tomotherapy for head and neck cancer as assessed by MVCTs.
abstract: OBJECTIVE
  Characterizing the changes of PET-positive lymphnodes (PNs) of head-neck cancer patients during image-guided Tomotherapy in order to verify if our clinical margin for PTV(boost) are adequate.
  
  
  METHODS
  Weekly MVCTs of 30 patients were matched with the planning kVCT (kVCT_pl) on bony anatomy: 42 visible PNs were contoured on kVCT_pl/MVCTs. Intra/inter-observer and inter-modality variability in contouring PNs was evaluated by blind re-delineation. Shrinkage of PNs and center-of-mass (CM) shifts were measured and Van Herk margins for the residual error were estimated. In addition, due to the PNs' shrinkage during therapy, probability coverage maps were considered to estimate the fraction of the high probability contours missed by the clinical PTV (5 mm margin); larger margins were tried for PNs showing some missing.
  
  
  RESULTS
  MVCTs were adequate for PNs' delineation (DICE=0.85; range=0.79-0.91). Twenty-seven PNs showed a significant volume shrinkage at the end of therapy (median: 71%, range: 27-94%, ρ=-0.93). Time-trend of 3D-CM shift was significant for 38% of PNs (median: 5.1 mm at the end of treatment, range: 1.0-8.9). The clinical PTV included 95% of the 90%/100% probability contours in 40/36 (95%/86%) PNs respectively. Van Herk margins (not considering shrinkage) were approximately 7 mm for all three main axes. The clinical PTV included 95% of the 90%/100% probability contours in 40/36 (95%/86%) PNs respectively.
  
  
  CONCLUSIONS
  The residual error relative to PNs after bone match is relatively small; the impact of CM shifts is partially counterbalanced by shrinkage. Our results do not seem to support an extensive use of adaptive re-planning to avoid the missing of PNs in dose-escalated protocols, although more information about the dosimetry impact of the reported changes is warranted.
authors: Medical Physics, S. Raffaele Scientific Institute, Milano, Italy. Electronic address: belli.marialuisa@gmail.com.
authors: Maria Luisa
authors: ML
authors: Belli
authors: Radiotherapy, S. Raffaele Scientific Institute, Milano, Italy.
authors: Flavia
authors: F
authors: Zerbetto
authors: Medical Physics, S. Raffaele Scientific Institute, Milano, Italy.
authors: Roberta
authors: R
authors: Raso
authors: Radiotherapy, S. Raffaele Scientific Institute, Milano, Italy.
authors: Anna
authors: A
authors: Chiara
authors: Medical Physics, S. Raffaele Scientific Institute, Milano, Italy.
authors: Giovanni Mauro
authors: GM
authors: Cattaneo
authors: Radiotherapy, S. Raffaele Scientific Institute, Milano, Italy.
authors: Nadia
authors: N
authors: Di Muzio
authors: Medical Physics, S. Raffaele Scientific Institute, Milano, Italy.
authors: Riccardo
authors: R
authors: Calandrino
authors: Medical Physics, S. Raffaele Scientific Institute, Milano, Italy.
authors: Claudio
authors: C
authors: Fiorino
authors: Radiotherapy, S. Raffaele Scientific Institute, Milano, Italy.
authors: Italo
authors: I
authors: Dell'Oca
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology; D006801:Humans; D008207:Lymphatic Metastasis; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography; D014057:Tomography, X-Ray Computed
keywords: Adaptive radiotherapy; Head-and-neck cancer; IGRT; Lymphnodes