Organ dose and total effective dose of whole-body CT in multiple myeloma patients

Multiple myeloma (MM) is the second most common hematological malignancy with an incidence that has risen over the past decades [1, 2]. It is characterized by the abnormal proliferation of monoclonal plasma cells in the bone marrow, which can lead to replacement of normal myelopoiesis, bone destruction and bone marrow failure [3]. Diagnosis of active MM as defined by the International Myeloma Working Group (IMWG) originally relied on histological confirmation of bone marrow plasma cell infiltration with clonal bone marrow plasma cells ≥10% (or extramedullary plasmacytoma) and one of the following features: hypercalcemia, renal insufficiency, anemia and bone lesions [4]. According to the IMWG, the presence of lytic bone lesions is considered one of the criteria to define active disease that requires treatment, even in the absence of clinical symptoms [4]. Consequently, imaging plays an important role in staging and in the follow-up of the disease [3, 5], as well as for the initial work-up of patients with smoldering MM [6].

Skeletal survey using approximately 20 regional radiographs has been used to determine osteolytic lesions or pathological fractures from MM. However, the value of conventional radiographs in detecting osteolytic lesions is limited since at least 30% of trabecular bone must be destroyed to be visible by radiograph [7]. Cross-sectional imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI) and integrated positron emission tomography/CT (PET/CT), have proven to be more accurate in detecting osteolytic and marrow replacing lesions as compared to skeletal surveys using radiographs [8‐14]. As a result, more accurate diagnosis, staging and follow-up of MM is possible [8, 9].

Whole body low-dose CT (WBLDCT) was found to be the most sensitive modality for detecting small osteolytic lesions (<5 mm) [15]. Moreover, WBLDCT provides information for disease monitoring and detection of incidental findings in MM patients, underlining its importance in the management of this patient group [16]. Although advances in CT technology enable low-dose whole-body evaluation, CT is limited by ionizing radiation exposure. While some literature on WBLDCT in MM report radiation dose information [17], current literature lacks dedicated data in larger patient cohorts on the organ dose exposure and effective radiation exposure in the clinical practice of WBLDCT’s in MM patients. Therefore, the objective of our study was primarily to evaluate the organ-specific-radiation dose levels and total effective dose of WBLDCT in a MM patient cohort and to compare them to calculated effective doses of a skeletal survey and representative diagnostic CT scans. We hypothesized that the effective dose of WBLDCT would be lower than that of diagnostic CTs and higher than that of a skeletal survey.

Our study was IRB-approved and complied with HIPAA guidelines with exemption status for individual informed consent. A retrospective search was performed of all WBLDCT’s acquired from July 2017 to February 2019.

In this retrospective study, we analyzed data of 228 patients—108/228 (47.4%) females, mean age 67.9 (SD 10.4) years, mean weight 81.8 (SD 22.4) kg. An overview of the total organ dose and differences between organ dose in males and females is depicted in Table 2. In the total group, the mean organ dose ranged from 3.72 (SD 1.01) mSv for the esophagus up to 13.09 (SD 2.84) mSv for the skeleton. As shown in Table 2, differences were observed between males and females regarding the mean organ dose of the thyroid (10.25 (SD 1.55) mSv vs. 8.27 (SD 1.42) mSv, P < 0.001, respectively), the esophagus (3.58 (SD 0.52) mSv vs. 3.75 (SD 0.58) mSv, P = 0.019, respectively), and the colon (4.06 (SD 0.45) mSv vs. 4.21 (SD 0.63) mSv, P < 0.001, respectively).

The majority of the WBLDCT scans were obtained using the Revolution CT, GE Medical Systems (198 [86.8%)], followed by the SOMATOM Definition Edge, Siemens [12 (5.3%)], SOMATOM Definition Flash, Siemens [7 (3.1%)], IQon - Spectral CT, Philips [7 (3.1%)], Discovery CT750 HD, GE Medical Systems [3 (1.3%)), and the SOMATOM Force, Siemens [1 (0.4%)]. The effective dose, CTDI_vol and DLP per used device are depicted in Table 3. The mean effective dose ranged from 4.34 mSv (SOMATOM Force, Siemens) up to 8.37 mSv (Discovery CT750 HD, GE Medical Systems).

Table 4 shows the calculated effective doses of a skeletal survey and representative commonly used diagnostic CT scans at our institution. The median effective dose of our skeletal survey (including 22 radiographs) was 2.04 mSv. The calculated median effective dose of representative diagnostic CT scans at our institution ranged from 1.2 mSv for a low-dose chest CT up to 9.0 mSv for a contrast-enhanced CT of the abdomen/pelvis.

Our study showed that WBLDCT in MM patients has a higher effective dose than that of a radiographic skeletal survey, but a lower effective dose than that of diagnostic CTs of the lumbar spine, abdomen and pelvis. Within the past decades the survival of MM patients has improved because of the availability of novel treatment options [21‐23]. The presence of these treatment options had underlined the importance of early diagnosis and accurate staging [8]. According to the most recent IMWG diagnostic criteria, lesions measuring 5 mm on CT or MRI are now included in the diagnostic criteria of MM [4], and it is of clinical importance to detect bone involvement in MM and its precursors. Moreover, in MM patients, bone involvement is an important cause of morbidity and mortality and a key indicator of prognosis [4].

Although skeletal surveys using radiographs have been traditionally used to assess for bone involvement in MM patients, its limitations are well known and have been documented previously [8]. Therefore, more sensitive imaging techniques for the diagnosis and follow-up of MM have been described, such as WBLDCT, MRI and PET/CT. Whole-body MRI and 18F-fluorodeoxyglucose-PET/CT have been described to be more accurate in detecting bone lesions in MM as compared to skeletal surveys [8‐14], and whole-body MRI have been found to detect significantly more MM bone involvement compared to PET/CT [12]. Whole-body MRI and PET/CT are, however, limited in their availability and are relatively costly. Additionally, FDG-PET/CT results in a relatively high radiation dose to the patient.

The widespread availability of multi-detector CT scanners has contributed to the increased use of WBLDCT for the diagnosis and follow-up of MM. The total effective dose of the WBLDCT in our study is comparable to those reported in recent literature [24‐27]. Moreover, our measured organ specific dose is comparable to the organ dose described by Kröpil et al. [28]. In our study, the effective dose of WBLDCT was, as expected, higher than that of a radiographic skeletal survey; however, the total effective dose of WBLDCT proved to be lower than that of commonly used diagnostic CTs of the lumbar spine, abdomen and pelvis. Moreover, WBLDCT has been shown to provide important information for disease monitoring and detection of incidental findings [16], thereby improving the management of MM patients.

In recent literature, WBLDCT protocols with estimated effective doses comparable to or lower than radiographic skeletal surveys have been described, using for instance hybrid iterative reconstruction techniques [10] or spectral shaping [24]. Although these studies show the potential for further decreasing radiation exposure of WBLDCT in MM, the radiation doses in our study reflect those that result from current daily practice.

Our study was limited by the retrospective study design. Secondly, over 85% of our WBLDCT scans were performed using one scanner, the Revolution CT, GE Medical Systems. This resulted in a relative underrepresentation of the other systems used. Strengths of our study include the large number of patients scanned using a uniform clinical protocol and the detailed assessment of radiation dose.

In conclusion, WBLDCT in MM has a higher effective dose than a radiographic skeletal survey, but a lower effective dose than diagnostic CTs of the lumbar spine, abdomen and pelvis. This underlines the broad applicability of WBLDCT in the management of MM patients. The additional diagnostic value of WBLDCT in the management of MM patients outweighs the relatively limited additional radiation dose as compared to a radiographic skeletal survey.