Over-response correction of SNC 350p Roos and PTW Markus chambers in surface dose and build up region dosimetry of 6MV photon beam

Document Type : Original Paper


1 Department of Medical Physics,State Cancer Institute, Indira Gandhi Institute of Medical Sciences,Patna, Bihar.

2 Indira Gandhi Institute Of Medical Sciences ,Sheikhpura ,Patna

3 BALCO Medical Center, Vedanta Research Foundation, New Raipur, atal Nagar, Chhattisgarh 493661

4 Department of Radiation Oncology, Kiran Multi Super Speciality Hospital, Surat-395004, Gujarat.

5 Department of Radiation Oncology, STARLIT Cancer Center, Ahmadabad-382350, Gujarat.


Introduction: The construction and design of an ionization chamber is an important component for accurate measurement of Percentage Surface Dose (PSD, dose at 0.5 mm depth) and Percentage Depth Dose (PDD) in the build-up region. Fixed separation parallel plate chamber is a reasonable choice in the clinical set-up, both Markus and Roos chambers have preferable characteristics for measurements of PSD and PDD in the Build-up region. The purpose of this study was to evaluate the over-response correction in PSD and PDD in Build-up regions for various field sizes due to the construction and design effect of Markus and Roos fixed separation parallel-plate ionization chambers.

Method & Material: The Sun Nuclear Corporation SNC 350P Roos and PTW Markus parallel plate ionization chambers were used to measure the PDD along the central axis in the Build-up region of the 6 MV photon beam. The experiment was performed on a Solid Water® HE phantom with a density of 1.032 g/cm3. The Measurements were taken at Source to Surface distance (SSD) of 100 cm with Field sizes of 10x10cm2,20x20cm2, and 30x30cm2 and at variable depths.

Results: For field sizes of 10x10 cm2, 20x20 cm2, and 30x30 cm2 at a source-to-axis distance (SAD) of 100 cm, the PSD values obtained with the Markus chamber, before and after applying Rawlinson's equation for over-response correction, were as follows: 35.11%, 42.92%, 50.72% (before correction) and 28.37%, 36.19%, 44.05% (after correction). Similarly, the PSD values measured using the Roos chamber, before and after applying over-response correction, were as follows: 41.61%, 50.50%, 58.04% (before correction), and 33.96%, 42.87%, 50.38% (after correction).

Conclusion: The application of over-response correction using Rawlinson's equation improved the accuracy of measurements for both chambers, emphasizing the significance of employing appropriate correction techniques to ensure precise dose measurements in the build-up region using ionization chambers.


Main Subjects

Articles in Press, Accepted Manuscript
Available Online from 11 August 2023
  • Receive Date: 31 March 2023
  • Revise Date: 01 August 2023
  • Accept Date: 11 August 2023