7practical applications.pptVIP

Sec.3 Practical application 临床处方剂量的计算 What we should calculate? Time or mu required to delivery proper dose for tumor Dose distribution at the region of tumor What kind of Parameters we should consider in dose calculation? Field size and shape Depth Beam quality Calibration of output : SSD technique or SAD technique: Radiotherapy institution vary in their treatment techniques and calibration practices. 处方剂量(prescription dose) 为达到一定的靶区（或肿瘤）剂量（DT）所需要的使用射野的射野中心轴上最大剂量点处的剂量，即处方剂量Dm。 Rember :prescription dose is not air dose The main contents Accelerator calculations SSD technique Isocentric technique Cobalt-60 calculations Irregular fields Other practical methods of calculating depth dose distribution SSD technique Percent depth dose is a suitable quantity for calculations involving SSD techniques. Machines are usually calibrated to deliver 1rad(10-2Gy) per monitor unit(MU) at the reference field size 10×10cm and a source-to-calibration point distance of SCD. Assuming that the Sc factors relate to collimator field sizes defined at the SAD, the monitor units necessary to deliver a certain tumor dose(TD) at depth d for a field size r at the surface at any SSD are given by the following. SSD technique SSD technique SSD technique Where rc is the collimator field size, given by It must be remembered that, whereas the field size for the SC factor is defined at the SAD, the Sp factors relate to the field irradiating the patient. Example 1. A 4-MV linear accelerator is calibrated to give 1rad per MU in phantom at a reference depth of maximum dose of 1cm, 100cmSSD, and 10×10cm field size. Determine the MU values to deliver 200rads(cGy) to a patient at 100cm SSD, 10cm depth, and 15×15cm field size, given SC(15×15)=1.020, Sp(15×15)=1.010, PDD(10,15×15,100)=65.1% Answer: Example 2. Determine the MU for the treatment conditions given in Example 1 except that the treatment SSD is 120cm, given SC(12.5×12.5) =1.010 Answer: Isocentric technique TMR is the quantity of c