Stereotactic body radiation therapy (SBRT) is an increasingly important treatment option for delivering large localised radiation doses to non-brain legions. Non-conventional fractionation effects are a subject of radiobiological research; including issues such as non-LQ dose responses and the reduction of beneficial reoxygenation (ROx) effects. As such, clinical trials (e.g. RTOG phase III 1112 and phase II 0618) are being pursued and the technique is being applied at many centres to treat diseases such as lung, liver and head and neck squamous cell carcinoma (HNSCC) metastases....

Methods in the HYP-RT MC Code Starting from 1 clonogenic tumour cell, 108 cell tumours were generated. Tumours were based on HNSCC cell proliferative hierarchy, cell kinetic and radiation response parameters (α=0.3 Gy-1,β=0.03 Gy-2)2,4-5 [FIGURE 1]. To simulate radiative cell death, the code analysed each tumour cell individually for each dose fraction and calculated a probability of death based on the LQ (or LQC equation), the oxygen enhancement ratio (OER) of the cell (dependent on pO2) and a cell phase factor or quiescence factor (quiescent...

Oxic Tumours Oxic virtual tumour results, shown in FIGURE 4, illustrate the total doses required to kill all clonogens or proliferative cells and the effects of the LQC “DL”. The model predicts tumour control when delivering 73 Gy (all proliferative) or 67 Gy (all clonogens) ±5 Gy for conventional fractionation, validating the model against typical clinical EBRT conventional prescriptions. Applying a DL value of 10 and/or 18 increased the total dose required for tumour control (for the largest two d/#) by 11 to 12 (±8)...

Discussion Virtual well oxygenated tumours were controlled with mean EQD2’s of 69±11Gy, an expected and uniform dose value for all SBRT schemes analysed. BED conversions also had uniform outcomes(83±11 Gy) across the schedules. Assessing total cell death compared to having 1 or 5 cells remain, bordered on significance and required approx. 5 Gy less dose. These cells may be controlled by natural immune function and hence tumour control still achieved, however this hypothesis is beyond the scope of this work. Hypoxic SBRT simulations required, on...

Contact information W. Harriss-Phillips: [email protected] Department of Medical Physics, Royal Adelaide Hospital, South Australia (08) 8222 2460

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