Absolute point dose measurement of stereotactic radiotherapy plan for ultra-small tumors

Abstract

A real-time ion-chamber cross-calibration method was proposed to achieve accurate absolute point dose measurements to plan stereotactic radiotherapy (SRT) for ultra-small volume tumors. The clinical SRT radiotherapy plans were first replicated onto the CT of a stereotactic dose verification phantom with a micro-chamber, and the theoretical value of the absolute point dose was calculated via the planning system (TPS). The micro-chamber was then cross-calibrated with a classic farmer chamber prior to each batch of measurements to derive a real-time cross-calibration factor. Finally, measurements were performed with the micro-chamber, and the measured values of the point dose and absolute point dose error were calculated using real-time cross-calibration factors and the calibration factors of the farmer chamber. The absolute point dose errors were then retrospectively recalculated using the average of all cross-calibration factors and compared with the previous results. A total of 10 batches of measurements were carried out, one for each stereotactic body radiation therapy plan. The results showed that the absolute point dose error calculated using the real-time cross-calibration factors were all within ±3% tolerance, with 8/10 within ±2%. In contrast, 2/10 of the results calculated using the average cross-calibration factor exceeded ±3%, and only 5/10 were within ±2%. The real-time cross-calibration method was found to be more robust when using the micro ion chamber for absolute point dose measurements for ultra-small tumors.

Keywords

• stereotactic radiotherapy
• ultra-small volume tumor
• micro ion-chamber
• absolute point dose measurement
• cross-calibration