Sharma Shamurailatpam Dayananda, Upreti Ritu Raj, Deshpande Deepak Dattatray
Department of Medical Physics, Tata Memorial Hospital, Dr Ernest Borg's Marg, Parel, Mumbai 400 012, India.
Phys Med Biol. 2006 Jun 7;51(11):2987-95. doi: 10.1088/0031-9155/51/11/020. Epub 2006 May 24.
Peripheral doses (PD) from uniform dynamic multileaf collimation (DMLC) fields were measured for 6 MV x-rays on a Varian linear accelerator using a 0.6 cc ionization chamber inserted at 5 cm depth into a 35 x 35 x 105 cm3 plastic water phantom. PD measurements were also carried out under identical conditions for seven patients treated for head and neck and cervical cancer employing sliding window intensity-modulated radiotherapy (IMRT). The measured PD from these patient-specific intensity-modulated beams (IMBs) were compared with the corresponding data from uniform DMLC fields having similar jaws setting. The measured PD per monitor unit (PD/MU) decreases almost exponentially with out-of-field distance for all uniform DMLC and static fields. For the same strip field width of 1.2 cm, uniform DMLC fields with a larger size of 14 x 22 cm2 deliver an average of 3.51 (SD = 0.51) times higher PD/MU at all out-of-field distances compared to 6 x 6 cm2. Similar to uniform DMLC fields, PD/MU measured from different patient-specific IMBs was found to decrease almost exponentially with out-of-field distance and increase with increase in field dimension. PD per MU from uniform DMLC fields and patient-specific IMBs having similar jaws setting shows good agreement (+/-7%) except at the most proximal distance, where a variation of more than 10% (maximum 15%) was observed. Our study shows that PD data generated from uniform DMLC fields can be used as baseline data to estimate out-of-field critical organ or whole-body dose in patients treated employing sliding window IMRT if an appropriate correction factor for field dimension is applied. The whole-body dose information can be used to estimate the possible increase in risk of fatal secondary malignancy in patients treated employing sliding window IMRT.
在瓦里安直线加速器上，使用一个0.6立方厘米的电离室插入到35×35×105立方厘米的塑料水模体中5厘米深处，测量了6兆伏X射线在均匀动态多叶准直（DMLC）野中的外周剂量（PD）。对于7例接受头颈癌和宫颈癌滑动窗口调强放射治疗（IMRT）的患者，也在相同条件下进行了PD测量。将这些患者特异性调强射束（IMB）测得的PD与具有相似准直器设置的均匀DMLC野的相应数据进行比较。对于所有均匀DMLC野和静态野，每监测单位的测量PD（PD/MU）几乎随野外距离呈指数下降。对于相同的1.2厘米条带野宽度，与6×6平方厘米相比，尺寸为14×22平方厘米的较大均匀DMLC野在所有野外距离处的平均PD/MU高3.51（标准差=0.51）倍。与均匀DMLC野类似，发现从不同患者特异性IMB测得的PD/MU几乎随野外距离呈指数下降，并随野尺寸增加而增加。具有相似准直器设置的均匀DMLC野和患者特异性IMB的每MU的PD显示出良好的一致性（±7%），但在最近距离处除外，在该距离观察到超过10%（最大15%）的变化。我们的研究表明，如果应用适当的野尺寸校正因子，均匀DMLC野产生的PD数据可用作基线数据，以估计接受滑动窗口IMRT治疗患者的野外关键器官或全身剂量。全身剂量信息可用于估计接受滑动窗口IMRT治疗患者致命性二次恶性肿瘤风险的可能增加。
