|
|
| Year : 1994 | Volume
: 19
| Issue : 3 | Page : 98-103 |
|
|
Specifications For Photon Beam Quality From Linacs
Anil Kumar Sharma, Sanjay S Supe, K Subbarangaiah
Correspondence Address:
Anil Kumar Sharma
 Source of Support: None, Conflict of Interest: None  | Check |
|
|
|
The photon beam quality is normally determined along the central axis of a symmetrical beam of a defined field size. Today when there are at least three different methods for field flattening, giving significantly different energy distributions over the field area, it would certainly be important to have some more detailed
information about the off-axis photon energy. The treatment techniques are further becoming more and more complex. Irregular and heavily off-centered beams, dynamic wedges etc. are often used in clinical practice today. Another drawback with the commonly used methods for beam quality specifications, TPR20/10and D20 / D10, is the large change in stopping power ratio relative to TPR20/10 when used for ionization chamber dosimetry. Further, it was recently shown that high energy electrons may affect these ratios in high energy photon beams. To obtain more detailed information about the energy distribution over the whole field, a method based on narrow beam penetration of water (HVL in water) was used to study the beam quality of a dual photon energy linac (Clinac - -1800) employing a simple and reliable device specially constructed for these measurements. Both the photon beams (6 and 18 MV) showed marked softening away from the beam central axis. It is very important to apply corrections for the observed beam quality change in the cross-section of the beam, otherwise it may result in erroneous patient dose while using large or off-centered fields. |
|
|
|
| [PDF]* |
|
 |
|
