Ostwald P M, Kron T, Hamilton C S
Department of Radiation Oncology, Newcastle Mater Misericordiae Hospital, Waratah, NSW, Australia.
Int J Radiat Oncol Biol Phys. 1996 Aug 1;36(1):181-7. doi: 10.1016/s0360-3016(96)00281-7.
Mucosal underdosing as a result of electron disequilibrium at the air cavity may affect local recurrence rates for T1 and T2 larynx cancers. Secondary build-up properties of high-energy beams have been demonstrated in a slab phantom. It was the aim of this investigation to determine whether significant surface underdosing exists for the mucosa under clinical conditions.
Measurements were made using a thermoluminescent dosimetry (TLD) extrapolation technique in an anatomic larynx phantom. The larynx phantom was constructed using tissue and cartilage equivalent material, based on patient cross-sectional anatomy. Three different thicknesses of LiF ribbons, 0.14, 0.39, and 0.89 mm, were placed reproducibly at 12 different positions at the anterior, posterior, and lateral walls on the endolarynx surface. Measured doses were plotted and an extrapolation was made back to the mucosal depth to obtain the dose received at each of the positions. Results were obtained for two different field configurations, opposed laterals and oblique fields, for 6-MV X rays and opposed lateral fields from a telecesium unit. In addition, the larynx surface doses of field sizes from 4 x 6 cm2 to 7 x 6 cm2 were investigated.
Surface underdosing was observed owing to the secondary build-up and build-down effect of the air cavity, and the dose measured for the three extrapolation TLDs at any position varied by up to 18%. An average variation of 6% was observed. The surface underdosing was most apparent for the 6-MV opposed lateral beam technique, where mucosa doses down to 76% of the prescribed dose were observed. Mucosal underdosing at the measurement positions was less marked with oblique techniques, telecesium treatment, and increasing field size.
Because of underdosing, some surface positions receive < 80% of the prescribed dose. This may contribute to the potential for higher recurrence rates observed with high-energy photons.
气腔处电子不平衡导致的黏膜剂量不足可能会影响T1和T2期喉癌的局部复发率。高能束的二次建成特性已在平板模型中得到证实。本研究的目的是确定在临床条件下黏膜是否存在显著的表面剂量不足。
在解剖学喉模型中使用热释光剂量测定（TLD）外推技术进行测量。喉模型基于患者横断面解剖结构，使用组织和软骨等效材料构建。将三种不同厚度（0.14、0.39和0.89毫米）的氟化锂带可重复地放置在内喉表面的前壁、后壁和侧壁的12个不同位置。绘制测量剂量并外推至黏膜深度，以获得每个位置所接受的剂量。针对两种不同的射野配置（对侧野和斜野）、6兆伏X射线以及远距离铯源的对侧野，获得了结果。此外，还研究了4×6平方厘米至7×6平方厘米射野大小的喉表面剂量。
由于气腔的二次建成和剂量衰减效应，观察到表面剂量不足，在任何位置测量的三种外推TLD的剂量变化高达18%。观察到平均变化为6%。表面剂量不足在6兆伏对侧野束技术中最为明显，在该技术中观察到黏膜剂量低至处方剂量的76%。在测量位置，黏膜剂量不足在斜野技术、远距离铯源治疗和射野增大时不太明显。
由于剂量不足，一些表面位置接受的剂量小于处方剂量的80%。这可能导致观察到高能光子治疗时较高复发率的可能性增加。
