In film dosimetry the scanner is the measuring instrument and the film is only the detector. What to buy, how to lock the settings down, and the handling discipline that makes a calibration mean something a month later.
A physicist troubleshooting an IMRT verification that failed gamma analysis will usually check the plan, the delivery and the film. The scanner is checked last, if at all, and it is the component most likely to be responsible. In radiochromic film dosimetry the film is the detector and the scanner is the instrument that reads it, which makes the scanner a piece of dosimetry equipment that happens to look like office hardware.
Treating it as office hardware is where the errors come from.
The requirements are short and non negotiable.
Ask, before purchase, whether the model is available in Bangladesh with local service and whether the transparency unit is a separately supplied part. A dosimetry scanner that has to be replaced by a different model in three years means recalibrating everything, so continuity of supply is a real selection criterion.
Three effects dominate, and the protocol exists to control them rather than to correct them afterwards.
Lateral response. The same film read at the centre of the scanner bed and read near the side gives different values, because of the scattering geometry of the light path. The practical answer is a positioning jig or template that puts every film, including calibration films, in the same place on the platen every time. If films must be scanned across the bed, a lateral correction has to be characterised and applied, which is more work than using a template.
Orientation. Radiochromic film's active layer contains elongated particles that align during manufacture, which makes the reading depend on whether the film is scanned portrait or landscape. Choose one, mark the orientation on every sheet before it is cut, and never deviate. Marking has to happen before cutting, because once a piece is separated there is no way to recover which way it lay in the sheet.
Warm up and drift. The lamp output changes over the first minutes of use. Run several throwaway scans with the film in place before recording anything, and run the same number of throwaway scans every session so the state of the machine is the same each time.
None of this survives careless handling.
A calibration relates scanner response to dose for one film batch, on one scanner, with one protocol. Change any of those and it is no longer valid. Buy film in batches large enough to cover a sensible period, record the lot number with the calibration, and recalibrate when the batch changes.
Calibration points should span the dose range you are actually measuring, with more points where the response curve bends. Deliver them on a known geometry with a calibrated reference, scan them in the same session and the same orientation and position as the measurement films, and keep an unirradiated piece from the same batch to characterise the background.
Match the film to the dose range rather than using one type for everything. Ashland's Gafchromic range is specified for this: EBT3 is stated for 0.2 to 10 Gy and covers IMRT, VMAT, brachytherapy, Star Shot, picket fence and flatness and symmetry work; EBT-XD is stated for 0.4 to 40 Gy and is the one for stereotactic radiosurgery, small field dosimetry and hypofractionated treatments; RTQA2 is stated for 0.2 to 10 Gy for routine machine QA including light and radiation field alignment; MD-V3 is stated up to 100 Gy and HD-V2, which is unlaminated so the active chemistry is exposed, up to 1000 Gy for high dose and research work. Measuring an SRS plan on a film specified to 10 Gy puts the measurement into saturation and produces a confident wrong answer.
Single channel analysis, typically using the red channel, is sensitive to anything that changes the amount of material the light passes through, including film thickness variation, dust and scratches. Multi channel methods use all three colour channels to separate the dose dependent signal from the disturbances that affect the channels differently, which suppresses film and scanner artefacts rather than averaging them in.
Ashland's FilmQA Pro implements this, along with a one scan protocol that combines calibration and plan verification in a single scan. That is more than a convenience feature. Scanning the calibration and the measurement together removes any drift between the two scans, which is one of the larger error sources in a busy department where the calibration was done last month.
Whatever software you use, be clear about what a gamma pass rate means. It is a comparison between a measured dose distribution and a calculated one, under stated distance and dose difference criteria, over a stated region and above a stated dose threshold. Two departments quoting different pass rates may simply be using different thresholds. Record the criteria alongside the result every time, and set the criteria before you look at the answer.
The last step is administrative and it is the one that keeps the whole thing valid. Dedicate the computer to the scanner and stop it updating drivers. Save the scan settings as a named profile and write them out on a laminated card next to the machine: resolution, colour depth, orientation, film position on the platen, warm up scan count, and every correction that must remain switched off. Add the fixed post exposure interval and the current film lot number.
Then test it. Rescan an archived irradiated film every month and compare the reading against its original value. If the scanner has drifted or someone has changed a setting, that single test finds it before a patient plan does.