Wong T P, Metcalfe P E, Kron T, Emeleus T G
Medical Physics Department, Illawarra Cancer Care Centre, Wollongong, Australia.
Australas Phys Eng Sci Med. 1992 Sep;15(3):138-46.
Radiation oncologists are particularly concerned about tumours growing on the surface of air cavities in the head and neck regions, which involve treatment with small x-ray fields. An inhomogeneous dose distribution exists within and beyond the cavity. This is caused by the loss of electron equilibrium and the attenuation of both the primary and scattered photons is altered. The scatter function photon beam models for tissue inhomogeneity, such as the ETAR correction algorithm, currently implemented in commercial treatment planning systems do not predict the dose distribution accurately in many situations where lateral electron equilibrium does not exist. Using a Markus ionization chamber and different solid water slabs to simulate different air cavities, it is found that internal body cavities, depending upon their sizes, experience underdose or overdose on the distal surfaces of the cavities when compared with the results predicted by an ETAR correction method for 6 MV and 18 MV x-ray beams. For an infinitely long air passage of dimensions 2 cm x 2 cm, the error in the ETAR correction for a 6 MV x-ray beam is 4.8%, 0.5% and 1.1% for the field size of 5 cm x 5 cm, 7 cm x 7 cm and 10 cm x 10 cm respectively. The ETAR correction is accurate to within 1.6% for a 6 MV x-ray beam provided that the field size is 5 cm across the cavity and greater than 7 cm along it.
放射肿瘤学家特别关注头颈部气腔表面生长的肿瘤，这类肿瘤涉及小X射线野治疗。在气腔内及气腔外均存在剂量分布不均匀的情况。这是由电子平衡的丧失导致的，并且原发射线光子和散射光子的衰减都发生了改变。商业治疗计划系统中目前实施的用于组织不均匀性的散射函数光子束模型，如ETAR校正算法，在许多不存在侧向电子平衡的情况下并不能准确预测剂量分布。使用马克斯电离室和不同的固体水模体来模拟不同的气腔，结果发现，与ETAR校正方法对6兆伏和18兆伏X射线束预测的结果相比，体内气腔根据其大小在气腔远端面会出现剂量不足或剂量过量的情况。对于尺寸为2厘米×2厘米的无限长气道，对于5厘米×5厘米、7厘米×7厘米和10厘米×10厘米的射野尺寸，6兆伏X射线束的ETAR校正误差分别为4.8%、0.5%和1.1%。如果射野在气腔上的尺寸为5厘米且沿气腔方向大于7厘米，6兆伏X射线束的ETAR校正精度在1.6%以内。
