Concurrent boost radiotherapy as preoperative treatment for locally advanced rectal carcinoma: a new beam arrangement.

Purpose: To describe a new beam arrangement for preoperative concurrent boost radiotherapy in locally advanced rectal carcinoma. Material and methods: Three different volumes, le posterior pelvis, total mesorectal space, and gross tumor volume plus 2 cm, are selected to receive radiation doses of 47 Gy, 51 Gy, and 54 Gy, respectively, in 24 fractions. There are two prerequisites for the use of such a radiotherapy schedule: complete displacement of the small bowel outside the boost volume, and horizontal positioning of the rectal long axis. Both conditions can be attained by patient positioning on a new device, the "Up-Down Table" (UDT). The dose gradient between the three volumes Is realized with two dally arc rotation fields with an isocenter that is different from the three additional multileaf collimator pelvic fields (postero-anterior + 2 latero-lateral). Results: The treatment data are reported according to the ICRU 62 criteria. A comparison was made between concurrent arc rotation and concomitant static boost techniques. Conclusion: The new beam arrangement, with the use of the UDT, allows to administer different radiation doses to three volumes with different tumor cell density in order to obtain the same probability of local response in all target volumes without increasing the toxicity.