Abstract
PurposeEvaluation of dose degradation by anatomic changes for head‐and‐neck cancer (HNC) intensity‐modulated proton therapy (IMPT) relative to intensity‐modulated photon therapy (IMRT) and identification of potential indicators for IMPT treatment plan adaptation.MethodsFor 31 advanced HNC datasets, IMPT and IMRT plans were recalculated on a computed tomography scan (CT) taken after about 4 weeks of therapy. Dose parameter changes were determined for the organs at risk (OARs) spinal cord, brain stem, parotid glands, brachial plexus, and mandible, for the clinical target volume (CTV) and the healthy tissue outside planning target volume (PTV). Correlation of dose degradation with target volume changes and quality of rigid CT matching was investigated.ResultsRecalculated IMPT dose distributions showed stronger degradation than the IMRT doses. OAR analysis revealed significant changes in parotid median dose (IMPT) and near maximum dose (D 1ml) of spinal cord (IMPT, IMRT) and mandible (IMPT). OAR dose parameters remained lower in IMPT cases. CTV coverage (V 95%) and overdose (V 107%) deteriorated for IMPT plans to (93.4 ± 5.4)% and (10.6 ± 12.5)%, while those for IMRT plans remained acceptable. Recalculated plans showed similarly decreased PTV conformity, but considerable hotspots, also outside the PTV, emerged in IMPT cases. Lower CT matching quality was significantly correlated with loss of PTV conformity (IMPT, IMRT), CTV homogeneity and coverage (IMPT). Target shrinkage correlated with increased dose in brachial plexus (IMRT, IMPT), hotspot generation outside the PTV (IMPT) and lower PTV conformity (IMRT).ConclusionsThe study underlines the necessity of precise positioning and monitoring of anatomy changes, especially in IMPT which might require adaptation more often. Since OAR doses remained typically below constraints, IMPT plan adaptation will be indicated by target dose degradations.
Highlights
Tumor-conformal treatment plans with steep dose gradients are required for radiotherapeutic treatment of advanced head-and-neck cancer (HNC)
We found for intensity-modulated radiotherapy (IMRT) that target dose parameters changed significantly but remained mostly within requirements while organs at risk (OARs) dose increase was partly critical, e.g. for parotid glands
Besides indicating the importance of monitoring anatomic changes and performing plan adaptation, we have shown that reasonable effort is required for exact patient positioning, since loss of target coverage, homogeneity and conformity were significantly worse for less accurate computed tomography scan (CT) matching for proton plans
Summary
Tumor-conformal treatment plans with steep dose gradients are required for radiotherapeutic treatment of advanced head-and-neck cancer (HNC). Gradual intratherapy changes in HNC patient anatomy, mainly caused by weight loss, shrinkage of tumor, and shift of close-by structures can be assessed via imaging, e.g. by computed tomography (CT), and are of concern during radiotherapy treatment.[7,8] The dosimetric consequences of such changes, namely the potential underdose of target volumes and overdose in organs at risk (OARs), have been quantified in detail for IMRT plans.[9,10] Treatment plan adaptation can be used to prevent severe dose degradation throughout the fractionated treatment course[11] and is related with lower normal tissue complication probabilities.[12] For IMRT treatment, up to two adaptation steps was reported to be sufficient and is logistically feasible.[13,14,15,16] Adaptive IMRT has been shown to be associated with improved locoregional control,[17] especially for advanced tumor stages.[18]
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