Background and purpose: The use of intensity-modulated radiotherapy (IMRT) is now widely advocated for the treatment of head-and-neck cancers, to increase the therapeutic ratio of radiotherapy used as sole modality of treatment or in combination with chemotherapy. This report aims to summarize the technical and dosimetric factors to be taken into consideration to assess the respective advantages of the various high conformality treatments in radiotherapy, especially in the framework of quality assurance procedures.
Materials and methods: Twenty-six head-and-neck cancer patients were irradiated following a feasibility internal protocol with IMRT. Treatments were performed with either the static step-and-shoot (20) or the dynamic sliding window (6) techniques on a 6 MV Varian Clinac equipped with a multileaf collimator with 80 leaves. Dose plans were computed using commercial treatment planning systems: MDS-Nordion Helax-TMS for static cases and Varian Eclipse for dynamic cases. Dose plans were evaluated in terms of physical quantities based on dose-volume histograms and isodose distributions. Each IMRT plan was also compared to a reference 3D conformal therapy plan (3DCRT).
Results: Elective target volumes ranged from 530 to 1151 cm(3) with a mean of 780 +/- 141 cm(3). Boost volumes ranged from 248 to 832 cm(3) with a mean of 537 +/- 165 cm(3). Thirty-two dose plans were generated with static technique and 10 with dynamic. In the static mode, 6.8 +/- 3.4 fields were applied on average with 12.5 +/- 1.3 segments per field. In the static mode, 264 +/- 56 MU per Gy were erogated, whereas in the dynamic mode, 387 +/- 126 MU per Gy were erogated, to be compared to 147 +/- 20 computed for reference 3DCRT plans. For all target volumes in general, conformity was improved compared to 3DCRT (e.g. V(95) increased from 85% to 93% with p < 0.001, or equivalent uniform dose normalized to prescribed dose increased from 0.86 to 0.96 with p = 0.002). Irradiation of parotid glands or spinal cord improved, as well: For parotids, D(2/3V) reduced from 59 Gy to 41 Gy (p < 0.001). For spinal cord, D(max) reduced from about 40 Gy to about 30 Gy (p < 0.001).