Commissioning of a near real-time in vivo film dosimetry system

Accurate in vivo dosimetry is crucial for dose monitoring of cardiac implantable electronic devices (CIED) and for dose verification for special procedures such as total body irradiation (TBI) and total skin electron therapy (TSET).

A new near real-time in vivo dosimetry system using radiochromic films (RCF) is investigated for clinical use in megavoltage external beam radiotherapy.

The Pnt-Dos™ in vivo dosimetry system comprises of a new type of RCF and a dedicated software module. Each Pnt-Dos device is a small piece of RCF individually packed with a unique QR code for identification and record keeping. Different from the traditional film dosimetry workflow, where a film developing time of at least 16 hours is recommended, a near real-time dose readout can be achieved with the Pnt-Dos system using a novel calibration procedure. This involves an automated scanning process at user-specified time intervals, utilizing auto-region of interest (ROI) detection and triple-channel calibration to capture the time-resolved post-irradiation growth. Two standard Epson scanner models (V600/13000XL) were used to cross-validate readouts and accommodate users who may prefer to utilize existing 13000XL scanners rather than acquire an additional V600 for in vivo dosimetry. The dosimetric accuracy was evaluated over a range of 15-400 cGy. Angular dependence was studied in 45° increments over 360°, normalized to the response at 0°, at 250 cGy using a cylindrical phantom. Energy dependence was evaluated for four photon energies (6 MV, 6 MV FFF, 10 MV FFF, 15 MV) and five electron energies (6 MeV, 9 MeV, 12 MeV, 16 MeV, and 20 MeV). Long-term reproducibility/stability were assessed with nine devices with different doses under identical conditions, alongside daily scans of quality control (QC) devices over three months.

The system provides accurate dose measurements across high- and low-dose ranges. All readings were within specification: accuracy was < ± 5 cGy for doses ≤ 80 cGy doses (max discrepancy 6.0 cGy), and < ± 5% for doses > 80 cGy on average (max discrepancy 5.1%). Angular dependence showed a maximum variation of 2.6% ± 2.1% when the beam passed through the posterior oblique side of the device. Daily QC/reproducibility tests confirmed system constancy of 0.1% average day-to-day variation. Energy dependence analysis revealed deviations of up to 4.9% ± 2.3% for all photon and electron energies compared to 6 MV photons, indicating the need for energy correction during commissioning. Film readings were compared with ion chamber measurements at 10 × 10 cm², d_max, 100 cm SAD (photons) or 100 cm SSD (electrons). Both scanners provided comparable readouts, within 1.3 cGy for doses ≤ 80 cGy and 0.6% for doses > 80 cGy. Based on these findings, user guidelines were established to ensure optimal performance and accuracy.

The new film-based in vivo dosimetry system provides an automated workflow that enables consistent, time-independent, and near real-time readout with a user-friendly design that simplifies handling and analysis, thereby streamlining in vivo dosimetry measurements. It also provides a traceable record of the patient dosimetry.