Another side effect of bone radiation is marrow suppression, in babies all the bones make blood but in adults most of the active marrow is in the pelvis and spine (68%), after low dose radiation that part of the bone marrow may take months to start making a normal production of blood and after high doses of radiation (40- 50Gy) that marrow may never make blood again
 

Distribution of Active Bone Marrow in the Adult
Site % Total Red Marrow
Pelvis               40.0
  Pelvic Bones 22.3
  Sacrum 13.9
  Femoral Head/Neck  3.8
Thoracic Spine              14.1
Lumbar Spine              10.9
Head              13.1
Upper Limb Girdle                8.3
  Scapulae 4.8
  Humeri 1.9
  Clavicles 1.5
Ribs                7.9
Cervical Spine                3.4
Sternum                2.3

Ellis RE. Phys Med Biol 1961 Jan 5:255-9

Distribution of Proliferating Bone Marrow in Adult Cancer Patients Determined Using FLT-PET Imaging   ijrobp.2009.11.040

Given that proliferating hematopoietic stem cells are especially radiosensitive, the bone marrow is a potential organ at risk, particularly with the use of concurrent chemotherapy and radiotherapy. Existing data on bone marrow distribution have been determined from the weight and visual appearance of the marrow in cadavers. 18F-fluoro-l-deoxythymidine concentrates in bone marrow, and we used its intensity on positron emission tomography imaging to quantify the location of the proliferating bone marrow.

The hematopoietic stem cells in the bone marrow that are continually replacing circulating peripheral blood cells are among the most radiosensitive cells in the body. Data exist suggesting that radiation doses as low as 2–4 Gy delivered within 1–3 days can cause a significant reduction in bone marrow cellularity and proliferation , and doses of 30–40 Gy in conventional fractionation can lead to complete ablation of the bone marrow. In addition to the radiation dose, the volume of bone marrow irradiated is an important factor. As the volume of actively proliferating bone marrow irradiated increases, the total hematopoietic output decreases precipitously. Taken to its extreme, only 4.5 Gy of total body irradiation is estimated to be necessary to cause death in 50% of humans. Depending on the dose delivered and volume treated, it can also take a considerable period for the bone marrow to recover. In a study of Hodgkin's patients treated with total nodal irradiation, Rubin found that bone marrow suppression frequently lasted for ≥1 year.

Although many larger volume RT techniques have recently fallen out of favor (e.g., total nodal irradiation, hemibody irradiation, whole abdominal radiation), newer techniques such as intensity-modulated RT (IMRT) are delivering lower radiation doses to ever-enlarging volumes of adjacent normal tissues, including the bone marrow. In addition, myelosuppressive chemotherapy is now frequently delivered concurrently with RT, which, when given together, can place greater stress on the bone marrow than either alone.

The available data regarding the distribution of actively proliferating bone marrow in adults might not be entirely accurate. Most of these data were studied >50 years ago and were determined by weighing bones from cadavers before and after heating and visual inspection of the color of the marrow/ Although data have been published using radionuclides such as 59Fe and 52Fe to estimate the erythroid component and 99mTc-labeled nanocolloids to image the distribution of reticuloendothelial cells, most of these data either provided no quantitative information regarding the distribution of the bone marrow or reflected only a component of hematopoietic marrow function.

To date, most positron emission tomography (PET) in oncology has used 18F-flurodeoxyglucose (FDG), which is taken up more readily by metabolically active tissues, including most malignancies. FDG probably does reflect marrow proliferative activity to some extent, as demonstrated by the increased uptake in response to growth factor stimulation, including the use of granulocyte colony-stimulating factor. However, areas known to harbor proliferating bone marrow generally have relatively low FDG uptake, limiting its ability to quantify the bone marrow distribution.

In addition to an increase in the range of radiotracers available for use during PET scanning, advances have also been made in how the PET data are collected. In particular, most current PET scans are now obtained with computed tomography (CT) (i.e., PET/CT). This advance has allowed, not only for better PET images as a result of improved tissue attenuation correction, but also, and equally important, the ability to link radiotracer uptake to specific anatomic structures, including bone marrow cavities.

The 18F-fluoro-l-deoxythymidine positron emission/computed tomography scans performed at the Peter MacCallum Cancer Centre between 2006 and 2009 on adult cancer patients were analyzed. At a minimum, the scans included the mid-skull through the proximal femurs. A software program developed at our institution was used to calculate the percentage of administered activity in 11 separately defined bony regions.

Results

The mean percentage of proliferating bone marrow by anatomic site was 2.9%  at the skull, 1.9% proximal humeri, 2.9% sternum, 8.8% ribs/ clavicles, 3.8% scapul, 19.9% thoracic spine, 16.6% lumbar spine, 9.2% sacrum, 25.3% pelvis, and 4.5% proximal femurs.