THE ROLE OF 3-DIMENSIONAL FUNCTIONAL LUNG IMAGING IN RADIATION TREATMENT PLANNING - THE FUNCTIONAL DOSE VOLUME HISTOGRAM

Purpose: During thoracic irradiation (XRT), treatment fields are usual ly designed to minimize the volume of nontumor-containing lung include d. Generally, functional heterogeneities within the lung are not consi dered. The three dimensional (3D) functional information provided by s ingle photon emission computed tomography (SPECT) Lung perfusion scans might be useful in designing beams that minimize incidental irradiati on of functioning lung tissue. We herein review the pretreatment SPECT scans in 86 patients (56 with lung cancer) to determine which are lik ely to benefit from this technology. Methods and Materials: Prior to t horacic XRT, SPECT lung perfusion scans were obtained following the in travenous injection of approximate to 4 mCi of Tc-99m-labeled macro-ag gregated albumin. The presence of areas of decreased perfusion, their location relative to the tumor, and the potential clinical usefulness of their recognition, were scored. Patients were grouped and compared (two-tailed chi-square) based on clinical factors. Conventional dose-v olume histograms (DVHs) and functional DVHs (DV(F)Hs) are calculated b ased on the dose distribution throughout the computed tomography (CT)- defined lung and SPECT-defined perfused lung, respectively. Results: A mong 56 lung cancer patients, decreases in perfusion were observed at the tumor, adjacent to the tumor, and separate from the tumor in 94%, 74%, and 42% of patients, respectively. Perfusion defects adjacent to the tumor were often large with centrally placed tumors. Hypoperfusion in regions separate from the tumor were statistically most common in patients with relatively poor pulmonary function and chronic obstructi ve pulmonary disease (COPD). Considering all SPECT defects adjacent to and separate from the tumor, corresponding CT abnormalities were seen in only approximate to 50% and 20% of patients, respectively, and wer e generally not as impressive. Following XRT, hypoperfusion at and sep arate from the tumor persisted, while defects adjacent to the tumor im proved in several patients. In four patients who achieved a complete r esponse scored by CT with chemotherapy prior to XRT, persistent hypope rfusion was present at and adjacent to the tumor site in three. Among 30 patients with cancers not arising in the lung (14 breast, 12 lympho ma, 4 others), perfusion defects were seen in only 4 (2 adjacent and 2 apart). Recognition of decreases in perfusion mainly impacted on trea tment planning for a few patients with poor pulmonary function and lim ited target volumes. DV(F)Hs have been useful in beam selection for pa tients with marked perfusion heterogeneities. Conclusions: Lung perfus ion scans provide functional information not provided by CT scans that can be useful in designing radiation treatment beams that minimize in cidental irradiation of the functional regions of the lung. This appro ach appears to be most helpful in patients with gross intrathoracic lu ng cancer, especially those with small targets and relatively poor pul monary function. One limitation of this approach is that some of the d efects adjacent to the tumor site reperfuse following treatment, indic ating that these scans identify perfusion rather than potential perfus ion. Three dimensional functional data can be used to generate DVFHs t hat may be more predictive of the physiological consequences of the ra diation than conventional DVHs. Additional work is currently underway to test this hypothesis.