Change of Dose Distribution on the Beam Axis of 60Co g Ray and 10MV X-ay with Part Thickness

Kang,  Wee Saing; Koh,  Kyoung Hwan; Ha,  Sung Whan; Park,  Charn Il

J Korean Soc Ther Radiol > Volume 1(1); 1983 > Article

Original Article

Journal of the Korean Society for Therapeutic Radiology 1983;1(1): 21-24.

Change of Dose Distribution on the Beam Axis of 60Co g Ray and 10MV X-ay with Part Thickness

Wee Saing Kang, Kyoung Hwan Koh, Sung Whan Ha, Charn Il Park

Department of Radiology, College of Medicine, Seoul National University, Korea.

ABSTRACT

The thickness of the part being irradiated is finite. Percent depth dose tables being used routinely are generally obtained from dosimetry in a phantom much thickner than usual patient. At or close to exit surface, the dose should be less than that obtained from the percent depth dose tables, because of insufficient volume for backscattering. To know the difference between the true absorbed dose and the dose obtained from percent depth dose table, the doses at or close to the exit surface were measured with plate type ionization chamber with volume of 0.5ml. The results are as follows; 1. In the case of 60Co, percent depth dose at a given depth increases with underlying phantom thickness up to the 5cm. 2. In the case of 60Co, the dose correction factor at exit surface which is less than 1, increases with part thickness and decreases with field size. 3. Exposure time may not be corrected when the part above 10cm in thickness is treated by 60Co. 4. In the case of 10MV x-ay, the dose correction factor is nearly 1 and constant for the underlying phantom thickness and field size, so the correction of monitor unit is not necessary for part thickness.