Standard off-cord lung oblique fields do not include the entire mediastinum - A computed tomography simulator study

The routinely recommended target volume for off-cord lung oblique fields in the treatment of postoperative bronchogenic carcinoma includes the entire mediastinum, as defined by coverage of the contralateral mainstem bronchus and subcarinal space. However, this may be difficult to accomplish with the field angles of 20 degrees to 40 degrees, recommended in the recently comp leted Intergroup Trial (Radiation Therapy Oncology Group 91-05). This proje ct was undertaken to define the oblique angle necessary to encompass the en tire mediastinum as determined by computerized tomography simulator verific ation. Axial computerized tomography simulation images of 25 patients with non-small-cell lung cancer were used in this study. Ten patients had prior lobectomy or pneumonectomy as part of their management. The contralateral m ainstem bronchus, subcarinal space (SS), and the spinal cord were each cont oured as separate volumes. The length of the contralateral mainstem bronchu s was defined as extending from the carina to the bifurcation of the lobar bronchi. The subcarinal space was defined as a triangular space (in a coron al plane) with the carina at the apex, the mainstem bronchi superiorly, and a horizontal line 5 cm below the carina as the base of the triangle. The m inimal angle to encompass the contralateral mainstem bronchus and subcarina l space, and to exclude the spinal cord was determined for each patient. Th e contoured volumes did not have additional margin added. The position of t he carina was scored as "midline" if located in the midsagittal plane, or " off-midline" if deviated to either side from midline. Midline deviation was determined at the level of the carina to evaluate possible anatomical dist ortion relating to the tumor or prior surgery, and its effect on the minima l angle was assessed. The median minimal angle measured was 45 degrees (ran ge: 28-65 degrees) for the entire group, and in 64% of those evaluated, thi s oblique angle was significantly greater than the 40 degrees recommended i n Radiation Therapy Oncology Group guidelines (p = 0.017). In patients with out midline deviation (n = 17), the median minimal angle was 45 degrees (ra nge: 28-60 degrees), and in patients with midline deviation (n = 8), it was determined to be 44 degrees (range: 27-65 degrees), with no statistical di fference noted between the two groups (p = NS). Although midline deviation was present in 4 of 10 patients previously resected, the above relationship remained unchanged. Based on computerized tomography simulation verificati on, off-cord oblique field angles of 20 degrees to 40 degrees do not adequa tely cover the entire mediastinum in most patients. To adequately encompass the entire mediastinum as defined in the Intergroup Trial (Radiation Thera py Oncology Group 91-05) with off-cord oblique fields, treatment angles gre ater than 40 degrees are necessary. Whether the potential increase in lung volume exposed to radiation from these larger angles results in a poorer th erapeutic ratio requires further investigation.