Lyman Ntcp Model Analysis

APPLICATION OF THE LYMAN NTCP MODEL INTO CLINICAL TRIALS

The Lyman NTCP Model was first applied into clinical evaluation by Burman et al. in 1991 . The TD50, ‘n’ and ‘m’ were 45Gy, 0.32 and 0.15 respectively. Lawrence and colleagues from the University of Michigan utilised the Lyman model with similar ‘n’ and ‘m’ values but with larger volume effects (n=0.69). This revised model was subsequently used in a dose escalation trial ,

Jackson et al. from University of Michigan modelled liver as a parallel organ. This model used biologic and statistical principles whenever available, to calculate RT tolerance of organs by using dose response behaviour of individual FSU’s. Such models were called as damage injury models (D-I models). The TD 50 was

The methods include: real time tumour tracking, gated RT, abdominal compression, voluntary shallow breathing, voluntary deep inspiration, or voluntary breath-holding at different phases of respiration, and active breath control.

Voluntary breath-holding of at-least 20 seconds is necessary for a reasonably good protocol. The idea of breath-holding is to reduce motion due to respiratory movements to a reasonably low range to facilitate conformal radiation. Breath-holding techniques are based on the assumption that respiratory cycle movements correspond to organ motion. Deep inspiratory holding may be occasionally detrimental for fear of air leak, especially in patients with lung diseases. Verbal instructions to maintain shallow breaths are usually non reliable and may not reduce movements less than 1cm.

Gated radiotherapy and real-time tumour tracking system are techniques which triggers the linear accelerator to irradiate only during specific time intervals of respiratory cycles. Active Breath Control (ABC) is a new promising technology which uses a controlled temporary suspension of breathing in a reproducible phase of the respiratory cycle. The operator uses a computer-controlled valve which allows air inflow upto defined limits, thereby making the patients breathing pattern well controlled. The LINAC is set to irradiate the patient only when a reasonable level

Four dimensional CT scanning for simulation is a novel method to identify the ITV. 4D CT has the advantage of accurate delineation of the movement of the tumour itself within the organ. In a 4D scanner, a series of CT image slices of the area of interest are acquired during different phases of respiration and a maximum intensity projection (MIP) is developed. Contours drawn in all the CT slices and the MIP generated are utilised to generate the ITV.

The movement of dome of diaphragm could also be a useful surrogate of liver motion. Balter et al. analysed the ventillatory movements of liver and the radiographic position of the diaphragm and found that he range of ventillatory movement of different locations within the liver is predicted by diaphragm position to an accuracy that matches or exceeds other existing systems for ventillatory tracking. It suggested that diaphragm is an acceptable anatomic landmark for radiographic estimation of liver movement in anterior-posterior projections for most patients .

Combining TACE with external beam