Determination of absorbed dose for Mantle Field with Extended FSD.
K Saravanan, K Reddy, S Vivekanandam, V Parthasarathy, N Vijayaprabhu, S Mourougan
absorbed dose, ebrt, extended fsd, mantle field, supradiaphragmatic
K Saravanan, K Reddy, S Vivekanandam, V Parthasarathy, N Vijayaprabhu, S Mourougan. Determination of absorbed dose for Mantle Field with Extended FSD.. The Internet Journal of Oncology. 2008 Volume 6 Number 2.
Mantle field is used for supradiaphragmatic disease. Determination of absorbed dose for Mantle Field technique by conventional method and direct dosimetry is studied and compared in this paper. For Mantle field, a field size of about or more than 40 X 40cm2 is usually needed. To obtain a bigger field size it is mandatory to extend the FSD. A change of FSD results in a significant change in Percentage Depth Dose (PDD) and beam out-put. Mayneord factor is used to determine the PDD for extended FSD. The behavior of the PDD at various FSD is studied by mathematical as well as by direct dosimetry method. The absorbed dose is obtained by performing dosimetry at extended FSD using TRS-398 protocol. From the study it is concluded that direct dosimetry is one of the precise method to determine absorbed dose for Mantle Field at Extended FSD.
External Beam Radiotherapy (EBRT) is one of the most effective modalities for the treatment of supradiphragmatic disease. The fundamental tent of radiotherapy is to reduce the percentage error in the delivery of a prescribed dose to the tumor volume while limiting the dose to normal tissue. Mantle field is used for supradiaphragmatic disease. The target volume for a mantle field includes the occipital, submental, submandibular, anterior and posterior cervical and supraclavicular nodes. In addition it covers the infraclavicular, axillary, medial-pectoral, paratracheal and mediastinal lymph nodes. Determination of absorbed dose for Mantle Field technique by conventional method and direct dosimetry is studied and compared in this paper.
Material and Method
The PDD curve for FSD 100cm is represented in Fig.1a, similarly the PDD is also obtained for FSD 110cm and 120cm. The values are compared with the calculated values using mayneord factor.
From the study the following observation where made:-
It is found that the dmax migrates towards the surface with increasing FSD. This is because of the degradation of the beam quality with increase in FSD.
The dmax migrates to the surface with increase in Field Size. The secondary electrons produced from the collimator surface contaminates the beam and hence the dmax migrates to the surface with increase in Field Size.
The PDD value increases with increase in Field Size. This is a relative measurement and the behavior of the PDD with field Size is shown in Fig. 1.
The PDD value increases with increase in FSD. Thou the beam out put decreases with increase in FSD the PDD value behaves in a different way. Since PDD values are obtained by relative measurement this can not be compared with the out-put of the beam.
As expected the out put of the beam decreases with increase in FSD and dose not follow inverse square law as the scatter component from the collimator will not be taken into account while applying inverse square law.
In Table 2 the percentage error in comparison of the absorbed dose calculation by direct dosimetry and by applying inverse square law shows that all the values are negative, which clearly indicates that the scatter component will not be taken into account by applying inverse square law.
In Table 2 it can be understood that with increase in Field Size the percentage error decreases. This is because the measurement is carried out along the central axis of the beam, at bigger field size the scatter component from the collimator has to travel a longer distance to reach the central axis of the beam by the time they get degraded.
Thou the conventional method of applying correction factors to calculate absorbed at Extended FSD is within acceptable limits, it will be more appropriate to carry out dosimetry at Extended FSD with the shielding blocks and prescribed field size. This method is a step forward close to precision in dosimetry and is practically achievable.
I thank and acknowledge Dr. K. S. Reddy, Dean, JIPMER, Pondicherry, India. For his guidance and support in my professional carrier and in this publication.