18. 由于极端病例的数量相对较少，造成了统计精确度较低，因此在确定某些部位的癌症风险特征方面仍然存在一些问题。例如，这可能会限制对与性别、受照年龄和受照以来的时间有关的风险趋势进行估计的能力。另外，有时会缺乏数据，或者所公布的数据不够详细，使得无法对不同人群之间的风险变化进行评估。但乳腺癌是一个例外，就这种癌症对日本原子弹爆炸幸存者数据和关于北美接受医疗照射的妇女的数据进行的比较表明，不同人群之间的风险估计转移有一个所谓的“绝对”模式。对有些癌症部位而言，并无证据表明癌症与辐射有关，另一些部位患癌的超高风险则只与非常高剂量的（放射治疗）照射有关。虽然在若干研究中对淋巴瘤的风险评估由于病例数量较少而受到影响，但这些结果应与在许多人口中发现的辐射和白血病（这也是一种罕见的疾病）患病风险之间的明显联系相对照。
18. There are still problems in characterizing cancer risks for some sites, owing to the low statistical precision associated with relatively small numbers of excess cases. This can limit, for example, the ability to estimate trends in risk in relation to factors such as sex, age at exposure and time since exposure. Furthermore, data are sometimes lacking or have not been published in a format that is detailed enough to allow an assessment of how risks vary among populations. An exception is breast cancer, for which a comparison of data on the Japanese atomic bombing survivors and on medically exposed women in North America points to a so-called “absolute” model for the transfer of risk estimates between populations. There are some cancer sites for which there is no evidence for an association with radiation and others where excess risks have only been seen following very high dose (radiotherapeutic) exposures. While the risk evaluations for lymphomas are affected by the small numbers of cases in several studies, these results should be contrasted with the clear relation found in many populations between radiation and the risk of leukaemia, which is also a rare disease.