RESUMEN
BACKGROUND: Interest in boosting the dose to the tumour during neo-adjuvant radiochemotherapy for rectal cancer is ever increasing, especially within the frame of adaptive radiotherapy. Rectal motion remains a potentially important obstacle to the full exploitation of this approach and needs to be carefully investigated. MATERIAL AND METHODS: The main purposes of this work were to: a) quantify rectal motion on all fractions of a treatment course; and b) assess margins for adaptive boosting in the second part of the treatment in order to benefit of tumour reduction during treatment. Ten consecutive patients treated with image-guided tomotherapy (41.4 Gy, 18 fractions) were selected. The cranial half of the rectum (subject to motion) was contoured by a single observer on daily MVCTs. The variations of rectal volume and of the envelope of rectum positions were investigated (169 MVCTs). The impact of applying different margins to the rectum in including all its possible positions was also investigated when considering the planning kVCT, the first fraction MVCT, the half-treatment MVCT or the median rectal contours of the whole or second half of treatment as reference volumes. RESULTS: Rectal volume reduced during treatment in all patients, with a significant time-trend in 6/10 patients. The median values of the envelope volumes were 129 cm(3) and 87 cm(3) in the first and second half of the treatment, respectively. On average, 95% of the rectal envelope was included by an isotropic expansion of 12 mm and 5 mm of the median contours when considering the whole or the second half of the treatment, respectively. CONCLUSION: A significant reduction of rectal volume was found in the second part of the treatment where rectal mobility was limited. As a consequence, relatively small margins may be used around the residual tumour volume when adaptive boost is delivered in the second half of the treatment.