The radiation exposure from PET/CT studies is considerable, especially for those children undergoing regular follow-up exams. Radiation doses vary significantly depending on the number of PET/CT studies performed. The results from this study demonstrate that the largest portion of the radiation dose comes from the CT portion of the examination. The effective doses from the CT studies ranged from 2.7 to 54.2 mSv and nearly half of the studies exceeded 20 mSv. These studies all involved “full-body” CT scans, which ranged from mid-brain to mid-thigh and thus irradiated essentially all of the radiosensitive organs. The techniques for these scans were usually adjusted for patient size with kVp and/or mAs being reduced for smaller/younger patients. For those studies exceeding 20 mSv, a typical set of technical parameters was 130 kVp, 100 to 130 mA, 1-s rotation time, 2
5-mm beam collimation and pitch 1. After performing the age adjustment described above, this would often result in effective doses that exceeded 20 mSv. This effective dose is actually reasonably consistent with the effective doses received by adults undergoing similar anatomic coverage, even though the technical factors (e.g., mA) would be increased due to the increased size of the adult patient. It should also be noted that the average age of our patient population was 13.1 years at the time of the study, that 77% of our patient population was >10 years old and that average weights were 112 pounds with 64% exceeding 100 pounds.
The long-term effects of the treatment of pediatric malignancies are numerous and substantial. These include development of cardiomyopathy, avascular necrosis of the hip, cognitive delay, early onset of heart disease, pulmonary fibrosis and increased risk of secondary malignancies. Children who receive alkylating chemotherapeutic agents and radiation as part of their treatment regimens are at particular risk for developing secondary malignancies. The increasing use of PET/CT in the management of pediatric malignancies raises the important issue of radiation exposure in children, particularly when it is reported that children have an increased risk of developing secondary malignancies from radiation exposure compared with adults. In fact, by extrapolating data from atomic bomb survivors, this increased risk is by an order of magnitude greater than that of adults [14
]. It has been estimated, but not proved, that the lifetime cancer risk of a 1-year-old who has received an abdominal CT is 1 in 550 [17
]. At doses of 100 mSv or greater (21% of the patients reported), the cancer risk has been estimated at 1 in 100 individuals [18
]. Judicious use of imaging is fundamental for the management of these patients, using tailored protocols that follow the ALARA principle (as low as reasonably achievable) to obtain diagnostic information.
The significance of incremental radiation exposure from serial PET/CT scans for children who have received high-dose radiation therapy as part of their treatment is not known. The significance of the incremental exposure may also depend on the location of radiation; for example, one could compare patients who receive mantle radiation versus limited radiation to a distal extremity. We suggest that the cumulative radiation dose to organs such as the lung, heart, thyroid, gonads, etc., is significant.
We have estimated the dose of radiation from serial PET/CT scans in pediatric patients, but it is important to note that the cumulative radiation from other imaging modalities has not been included. These imaging modalities include the technetium 99 bone scans, 123Iodine MIBG (metaiodobenzylguanidine) scans, interval CT studies for acute events and fluoroscopy, which also add to the radiation dose.
It is necessary to apply the ALARA principle in the radiation exposure of PET/CT without sacrificing diagnostic information. The greatest contributor to overall radiation exposure in PET/CT is the whole-body diagnostic CT. It is widely accepted that by decreasing the tube current (mA) value on the CT portion the radiation dose can be reduced substantially. It is clear that until the standards of frequency, interval and number of needed PET/CT scans is established in the management of pediatric malignancies, that the use of PET/CT in children should be used judiciously on a case-by-case basis with particular emphasis on the risk, benefit and cumulative radiation dose to children. We have reviewed the CT protocols for all pediatric studies at our institution and decreased doses following “image gently” guidelines, according to patient weight. Doses of FDG for PET have also been reduced to 0.14 mCi/Kg, as recommended in the literature. We are now monitoring all pediatric CTs and PET/CT studies for dose (Table ).
Current parameters for CT chest and abdomen at our institution
Alternative approaches to the use of whole-body PET/CT include routine CT or PET/CT limited to the area of interest, intercalated by periodic whole-body PET/CT, and whole-body PET followed by limited CT in areas of PET-positive lesions (albeit with some reduced sensitivity).
Whole-body PET/CT continues to be an important noninvasive diagnostic/staging modality for certain malignancies such as Hodgkin and non-Hodgkin lymphomas, and it is thought that alternative or more cautious approaches should be weighed against the unequivocal benefit provided. In order to reduce the radiation exposure from imaging studies in children with cancer we suggest the following guidelines:
- Strict adherence to the ALARA principle. This may involve decreasing the CT tube (mA) current and avoidance of whole-body scans when only a limited scan is sufficient.
- Use of pediatric protocols and, when possible, imaging at pediatric centers where radiologists and technicians are acutely aware of the need to reduce radiation exposure.
- Establishment of formal guidelines for the interval, number, and frequency of PET/CT scans for each pediatric malignancy.
- When appropriate, use of modalities such as MR or US to reduce radiation exposure.
- Overall attention to the cumulative radiation exposure for each child.