Setting up an in vivo dosimetry (IVD) program at the William Buckland Radiotherapy Centre (WBRC): Preliminary Clinical Results Sabeena Beveridge, Jennifer Bull, Alison Beal, Leigh Smith, Trevor Ackerly William Buckland Radiotherapy Centre, Melbourne, Australia In vivo dosimetry (IVD) is in use in external beam radiotherapy (EBRT) to detect major errors, to assess clinically relevant differences between planned and delivered dose, to record dose received by individual patients, and to fulfil legal requirements. AIM: To establish an IVD program for all EBRT patients receiving treatment at the WBRC using the Sun Nuclear IVD2 QED diode system (see Figure 1). 6-12MV 15-25MV Electrons Figure 1: Three types of diodes have been commissioned for the beam energies used for treatment at the WBRC – 6MV, 18MV, and five Electron energies (6MeV, 9MeV, 12MeV, 15MeV, 18MeV). Each diode is about the size of a 5cent piece. Figure 4: An example of a treatment plan (Eclipse planning system) where an IVD point is placed and the expected dose is calculated at this point. The diode dose, which is measured at treatment, is compared to this expected dose and is required to be within +5%. Figure 2: An Allied Health Assistant (AHA) informs the patient of the IVD procedure typically at the second fraction of treatment. Once the patient has been set up and positioned correctly for their treatment, the AHA places the diodes for each field being measured. Radiation therapy is delivered using a Linear Accelerator. Limitations of the system included calculating the expected dose in or around air, which is a known planning calculation issue (using the AAA algorithm), particularly with exit dosimetry and dose measurements in or around bowel and intestinal organs. IVD points had to be at least 2cm from a beam edge and fields had to be larger than 4x4cm2, which excluded IMRT and stereotactic patients from the IVD program. The dose perturbation from the electron diodes required clinical restrictions for IVD, where a “dose shadow” of up to 26% was seen with 6MeV. However, it was determined that the dose shadow only affected a small area underneath the diode and did not significantly affect the patient’s treatment. METHODS: All patients treated with conformal and palliative EBRT at the WBRC between May and September 2014 had IVD measurements taken on one or more of their treatment fields (see Figures 2 and 3). Measurements were compared to the dose delivered at dmax (the depth of tissue where the dose is at a maximum) which was calculated from the Eclipse treatment planning system (see Figure 4). Commissioning for both entrance and exit dosimetry for photons was performed. Corrections for source-to-skin distance, field size, directional response, off-axis placement, and wedge insertion were modelled within an in-house software program. IVD passed if the diode dose was within +5% of the expected dose calculated from the treatment plan. Figure 3: A yellow diode (6MV photons) placed on a mock brain cast patient. The diodes are simply held in place with surgical tape. The IVD2 system is not connected to any other clinical systems, which makes it an independent check of the treatment. RESULTS: A total of 468 fields from 206 patients were analysed. Figure 5 shows the %Difference calculated for each field measured over all of the energies. The WBRC achieved a pass rate of 97.6%, where the clinical acceptance is 90%. There were 11 failures, all of which could be explained and justified and had %Differences of less than 7.5%. The most common error was diode placement, where recalculation of the IVD point in the treatment planning system or a repeat (27 repeats performed) of the measurement was required to fix the error. Figure 5: A graph showing the %Difference between the expected dose (calculated by the treatment planning system) and the measured dose of the diode for the 468 fields measured between May and September 2014. The blue lines represent the pass area (+5%) so any point outside of these lines is considered a failure. The 11 failures could all be explained, where the majority of reasons were due to air bubbles in the gut that caused minor discrepancies in the dose. CONCLUSION: IVD has verified the safety and accuracy of EBRT treatments at the WBRC. Diode measurements through air cavities were more accurate than dose calculations performed by the treatment planning system – as a result of this study, a more robust dose calculation algorithm has been purchased.