Abstract
Many head‐and‐neck cancer (HNC) patients treated with radiotherapy suffer significant anatomical changes due to tumor shrinkage or weight loss. The purpose of this study was to assess dose changes over target volumes and organs at risk during intensity‐modulated radiotherapy for HNC patients. Sixteen HNC IMRT patients, all requiring bilateral neck irradiation, were enrolled in the study. A CTplan was performed and the initial dose distribution was calculated. During the treatment, two subsequent CTs at the 15th (CT15) and 25th (CT25) fractions were acquired. The initial plan was calculated on the CT15 and CT25, and dose‐volume differences related to the CTplan were assessed. For target volumes, mean values of nearmaximun absorbed dose (D2%) increased at the 25th fraction, and doses covering 95% and 98% of volume decreased significantly at the 15th fraction. Contralateral and ipsilateral parotid gland mean doses increased by 6.1% (range: ‐5.4, 23.5%) and 4.7% (range: ‐9.1, 22.3%), respectively, at CT25. The D2% in the spinal cord increased by 1.8 Gy at CT15. Mean absorbed dose increases at CT15 and CT25 were observed in: the lips, 3.8% and 5.3%; the oral cavity, 3.5% and 2.5%; and lower middle neck structure, 1.9% and 1.6%. Anatomical changes during treatment of HNC patients affect dose distribution and induce a loss of dose coverage to target volumes and an overdosage to critical structures. Appropriate organs at risk have to be contoured and monitored in order to know if the initial plan remains suitable during the course of the treatment. Reported dosimetric data can help to identify patients who could benefit from adaptive radiotherapy.PACS numbers: 87.53.Kn, 87.55.Dk
Highlights
102 Beltran et al.: Dose variations in H&N intensity-modulated radiation therapy (IMRT) accuracy is to reduce geometrical errors
Patient characteristics Between November 2008 and March 2010, a total of 16 patients with bilateral neck and supraclavicular nodes treated with IMRT were included
The study shows that head-and-neck cancer (HNC) patients treated with IMRT undergo weight changes correlated with limited external volume variations, suggesting that patient weight may be a reliable parameter to detect changes in irradiated body areas
Summary
102 Beltran et al.: Dose variations in H&N IMRT accuracy is to reduce geometrical errors. Rigid errors, such as setup, have been extensively studied. Mechalakos et al[6] for instance evaluated the interfraction and intrafraction errors in treatments of HNC and compared their results with previous studies from others authors. Margins are added to clinical volumes in order to take into account geometrical uncertainties. These planning margins are commonly calculated from measured systematic and random geometrical errors.[7]. Several scheduled rescanning studies have evaluated these volumetric changes in both target volumes and normal tissues,(8-11) mostly on the parotid glands and their consequent effects on dose distribution.[12,13,14,15] The information obtained from these studies indicates that anatomical changes during the treatment can cause deviations between the planned and delivered dose, reducing dosage to target volumes whilst increasing dosage to critical structures
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