Abstract
OBJECTIVE: To investigate the effect of intermediate field size (fint), volume of reference ionization chamber (IC) and normalization conditions on output factor (OFdet fclin) measurement using small field detectors (SDs) and derive OFdet fclin correction factors kQclin,Qint fclin,fint(SD) for active and passive detectors. METHODS: Measurements were made on Novalis-Tx accelerator for 6 MV photon at 1000 MU min(–1). Six active SDs, high resolution diodes (SFD, EDGE, PTW 60008), CC01 IC, PTW 60003 natural diamond, Exradin W1 plastic scintillator and two passive detectors (Al(2)O(3):C nanoDot-OSLD and Gafchromic EBT3 films) were used for measurement from small circular cones having 4 to 15 mm diameter. Effect of the choice of fint ranging from 20 × 20mm(2) to 40 × 40 mm(2) and reference ICs (FC65, PPC40, CC13) volume on OFdet fclin was investigated for all active SDs. Influence of direct normalization at reference field of 40 × 40mm(2) or 100 × 100 mm(2) and indirect normalization at 100 × 100 mm(2) through fint using Daisy-chain strategy on OFdet fclin was also investigated. kQclin,Qint fclin,fint(SD) was calculated as ratio of the OFdet fclin of each SDs relative to Gafchromic EBT3. RESULTS: For any combination of SDs and reference ICs, selection of fint of 40 × 40 mm(2) will result in OFdet fclinvariation to within ±1.8%. Indirect normalization at 100 × 100 mm(2) through fint of 40 × 40 mm(2) and direct normalization at 40 × 40 mm(2) showed comparable (≤2%) results for EDGE, PTW 60008 and CC01. Whereas, for PTW 60003 and Exradin W1 direct and indirect normalization at 100 × 100 mm(2) agrees within −2%. OFdet fclin measurement by SDs following Daisy-chain strategy shows deviation as large as 23% for 4 mm cone which reduces to <2% for cone size >10 mm except for nanoDot. The calculated kQclin,Qint fclin,fint(SD) is least for SFD (0.992–1.013) and largest for nanoDot (1.047–1.294) which reduces to 0.99–1.023 after correcting for volume averaging effect. The long-term standard uncertainty (k = 2, 95%CI) was largest for nanoDot (2.93%) and least for PTW60003 (0.11%). CONCLUSION: Medium size IC is less sensitive to fint and could be a better alternative to standard volume ICs. Minimum fint of 40 × 40 mm(2) is suggested in Daisy-chain strategy. Different normalization conditions could lead to OFdet fclin variation measured by same SD up to ±5%. kQclin,Qint fclin,fint(SD) varies largely with cone size and detector volume. ADVANCES IN KNOWLEDGE: Comprehensive study of the influencing parameters and conditions on OFdet fclin measurement from single data set and estimation of kQclin,Qint fclin,fint(SD) for newly emerging detectors.