Accuracy of Using In-Vivo Dose Verification with Diodes for Different Sites

In radiotherapy, treatment success depends on accurate dose delivery to the target, and should be adequately taken into consideration. One of the effective methods for ensuring accurate dose delivery and as a quality check tool is in-vivo dosimetry using diodes during radiotherapy. In-vivo dosimetry measurements were performed with diode detectors calibrated against a reference ionization chamber for 6 MV and 15 MV for photon beams and 6 MeV, 10 MeV and 15 MeV for electron beams. A total of 101 cancer patients were considered for this study, with 420 field measurements made with low and high energy diode detectors for prostate, cervix, breast and head and neck cancer cases. All patients had their treatments planned with an Ocentra Masterplan Treatment Planning System v. 4.1 and 4.3, and treated with either 6MV and 15MV or a mixture of both energies using an Elekta Synergy platform Linear accelerator. The diodes measured the entrance doses in each field in the Dmax depth and were compared with the calculated dose from the TPS at Dmax depth for every single beam. All treatment cases including the wedged fields were considered appropriately and assigned to a 10 % action level. Approximately 95.59 % of the 136 measurements for the head and neck treatments were within the SGMC ±10 % action level. In the case of the cervical cancer treatments, 97.87 % of 47 measurements made were within the ±10 % action level, whereas the breast and prostate treatments produced 89.06 % (64 measurements) and 96.53 % (173 measurements) respectively within the set action level. For the Head and Neck treatment, the average discrepancy, δavg for 136 measurements (N) was 2.16 %, while the corresponding standard deviation (σ) was 4.61%.The results for prostate treatments (N=173) indicated an average discrepancy, δavg of 4.57 % and a corresponding σ of 3.65%. For cervix (N=47), an average discrepancy, δavg of 4.69 % was obtained with a corresponding value σ of 2.58 %, while 5.02 % and 5.19 % was obtained for δavg and  respectively for the breast cancer patients. A greater percentage of the observed discrepancies were well within the set tolerance level. It is however recommended that the positioning of diode on patient’s skin, and the angular sensitivity of the diodes should be reconsidered. It is also recommended that, a more accurate calculation of expected diode values be done, especially for fields which pass through the table. Further studies are being carried out to find the existence of a cluster of larger deviations at a particular gantry angular range. By these, several efforts would then be made to achieve action levels of ±5 %.