Dosimetric evaluation of synthetic CT relative to bulk density assignment-based magnetic resonance-only approaches for prostate radiotherapy.

BACKGROUND: Magnetic resonance imaging (MRI) has been incorporated as an adjunct to CT to take advantage of its excellent soft tissue contrast for contouring. MR-only treatment planning approaches have been developed to avoid errors introduced during the MR-CT registration process. The purpose of this study is to evaluate calculated dose distributions after incorporating a novel synthetic CT (synCT) derived from magnetic resonance simulation images into prostate cancer treatment planning and to compare dose distributions calculated using three previously published MR-only treatment planning methodologies. METHODS: An IRB-approved retrospective study evaluated 15 prostate cancer patients that underwent IMRT (n = 11) or arc therapy (n = 4) to a total dose of 70.2-79.2 Gy. Original treatment plans were derived from CT simulation images (CT-SIM). T1-weighted, T2-weighted, and balanced turbo field echo images were acquired on a 1.0 T high field open MR simulator with patients immobilized in treatment position. Four MR-derived images were studied: bulk density assignment (10 HU) to water (MRW), bulk density assignments to water and bone with pelvic bone values derived either from literature (491 HU, MRW+B491) or from CT-SIM population average bone values (300 HU, MRW+B300), and synCTs. Plans were recalculated using fixed monitor units, plan dosimetry was evaluated, and local dose differences were characterized using gamma analysis (1 %/1 mm dose difference/distance to agreement). RESULTS: While synCT provided closest agreement to CT-SIM for D95, D99, and mean dose (<0.7 Gy (1 %)) compared to MRW, MRW+B491, and MRW+B300, pairwise comparisons showed differences were not significant (p < 0.05). Significant improvements were observed for synCT in the bladder, but not for rectum or penile bulb. SynCT gamma analysis pass rates (97.2 %) evaluated at 1 %/1 mm exceeded those from MRW (94.7 %), MRW+B300 (94.0 %), or MRW+B491 (90.4 %). One subject's synCT gamma (1 %/1 mm) results (89.9 %) were lower than MRW (98.7 %) and MRW+B300 (96.7 %) due to increased rectal gas during MR-simulation that did not affect bulk density assignment-based calculations but was reflected in higher rectal doses for synCT. CONCLUSIONS: SynCT values provided closest dosimetric and gamma analysis agreement to CT-SIM compared to bulk density assignment-based CT surrogates. SynCTs may provide additional clinical value in treatment sites with greater air-to-soft tissue ratio.

How often are previously undetected radiographic abnormalities detected at the time of CT simulation for breast cancer patients?

OBJECTIVES: In most institutions, planning computed tomography (CT) scans are not interpreted by diagnostic radiologists. The purpose of this analysis was to determine the percentage of cases in which a previously undetected radiographic finding was found on review of CT simulation images by diagnostic radiology. METHODS: At the Henry Ford West Bloomfield Center, CT simulations are prospectively interpreted by diagnostic radiologists and a formal report is generated. CT simulation scan reports of 332 consecutive breast cancer patients from 2000 to 2006 were reviewed. The percentage of these reports in which a previously undetected abnormality was noted on the planning CT was determined. Prior and subsequent diagnostic CT scans were also reviewed to determine the clinical relevance of these diagnostic abnormalities. RESULTS: Of 332 patients with CT simulations for breast cancer treatment planning, 52 patients (16%) had a newly detected abnormality noted. Of these, 31 patients (or 60% of the abnormal findings) were deemed by diagnostic radiology to have potentially significant findings (e.g., "can not exclude metastatic disease"), and a follow-up CT or magnetic resonance imaging scan was recommended. Abnormalities in this category included previously undetected lung nodules, liver lesions, kidney/adrenal lesions, and sclerotic bony lesions. On follow-up, however, to date, these findings have demonstrated no clinical significance, although further follow-up is needed in many patients. CONCLUSIONS: In this study, a significant proportion of breast cancer patients undergoing CT planning studies were diagnosed with potential abnormalities for which follow-up was recommended by diagnostic radiology. To date, these findings have not been clinically relevant, though further follow-up is needed in many of the patients. Thus, in cases of clinical uncertainty, a diagnostic radiologist should be consulted and follow-up imaging obtained if necessary.