Evaluation of the use of CT Planning VS orthogonal films in the treatment of rectal carcinoma

To compare field margins generated from CT planned radiotherapy of rectal cancer with traditional orthogonal film field definitions. Thus far, seven CT datasets of patients undergoing preoperative chemo-radiation for rectal cancer have been analysed. The CT scan based fields were generated according to ICRU 50/62 and anatomical principles. The orthogonal film based fields were defined according to traditional bony landmarks as implemented in the department prior to CT planning. The maximum difference in the field margins was measured in the superior-inferior, lateral and anterior-posterior directions. Overall the CT defined volumes were larger than the orthogonal film based volumes. There was little difference in the posterior field margins (average 0.2 cm). However, the CT based margins were greater than the film based margins by an average of 3.2 cm in the anterior direction, 1.5 cm in the superior direction, 1.4 cm in the inferior direction and 1 cm laterally CT based volumes are generally larger than traditional orthogonal film based plans in our radiotherapy department. CT field definitions were mainly based on soft tissue structures. These structures are often poorly visualised and thus, often inadequately covered on simulator films. The exception was the posterior margin that was defined by a bony structure on both sets of plans