Session Item

The primary aim of the study is to evaluate if the metabolic tumor volume (MTV) in anal cancers is stable during fractionated radiotherapy and how volumetric variations translate into dosimetric variations following dose painting of MTV based on pre-therapy PET/CT.  Secondary aims are to assess doses to organs at risk (OARs) and use of adaptive dose painting.

This study included 11 patients with squamous cell carcinoma of the anal canal who received radical fractionated radiotherapy. Ethical approval and written informed consent from all patients was obtained. 18F-FDG PET/CT images were acquired at pre- and mid-treatment (around the 10^th fraction). Conventional target volumes and OARs were contoured manually on both images series. The MTV was generated from the PET images by adaptive thresholding. Treatment plans were created retrospectively based on both image series, and optimized in the Eclipse RT planning system (Varian Medical Systems) using VMAT technique with two full arcs and 6 MV photons. Standard plans consisted of 48.6 Gy, 54 Gy and 57.5 Gy in 27 fractions to elective lymph nodes, lymph node metastases and primary tumor, respectively. Dose painting plans included an extra dose level of 65 Gy to the MTV. Pre-treatment plans were also transferred and re-calculated in the mid-treatment basis. Overlap fraction and DICE similarity between MTV_mid and MTV_pre was calculated. Wilcoxon signed-rank test was used to evaluate differences.

MTV decreased from pre- to mid-treatment in 10 of the 11 patients, and on average 71 % of MTV_mid overlapped with MTV_pre (Table 1). Dose statistics for MTV in case of original, transferred and re-optimized (adapted) plans for standard and escalated regimes are shown in Figure 1A. The median (D50%) and mean dose (Dmean) to the MTV were robust against anatomical changes from pre- to mid-treatment for both the standard and dose painting plans. The transferred dose painting plans had significantly lower D98% values compared to original and re-optimized plans plans (p=0.047).  Figure 1B shows relevant dose statistics of for OARs. No significant differences were found between standard and dose painting plans, with the exception of V45Gy to the intestinal cavity for transferred plans (p = 0.042), in which dose painting gave a slightly lower population-based dose.

Despite volumetric changes in the MTV, adequate dose coverage was observed in the dose painting plans apart from underdosage of a small part of the volume. The findings indicate little or no need for adapted dose painting, although greater changes may occur at later time points in the fractionated schedule. Dose painting appears to be a safe treatment alternative with non-inferior dose sparing of OARs compared to a standard regime.