SU‐E‐T‐201: Issues Encountered in Film‐Based IMRT QA

Abstract

Purpose: To investigate characteristics of EDR2 film‐based IMRT QA with an eye towards moving from isodose line comparisons to more objective gamma statistical analysis. Methods: IMRT QA was performed on Siemens Linacs using a traditional ion‐chamber‐and‐film phantom setup for step‐and‐shoot IMRT plans. All beams were delivered at their actual gantry angles and to the same film. Fiducial beams were used to fix film location for comparison with calculated dose. Dose was calculated on the Pinnacle planning system (8.0m). The EDR2 films were developed using a Kodak PP‐XOMAT film processor that has been remanufactured by Picker. Films were scanned with a VIDAR VXR‐16 Dosimetry Pro film digitizer using RIT 113 v5.2 software. Calibration films were created using crossed step wedges with doses calculated by Pinnacle. Gamma was calculated using the standard software settings of 3% / 4 mm. Results: For each QA film, the normalization doses (target to measured), mean gamma, and percentage of points failing the gamma limit (gamma < 1) were recorded. These percentages were found to be considerably higher in many cases than the often‐used limit of 5%. This problem was reduced when a new calibration film was used every week and developed together with the QA films; however, the mean fraction of points failing the gamma criterion was still 7%, with 45% of films failing. The mean gamma was 0.4. The product of (target dose) * (mean gamma) * (3%/100%) was relatively constant and averaged 2.3 cGy. Conclusions: Various factors were found to be relevant to the deviations observed, especially film calibration and the target dose (which sets the dose scale for gamma). In many cases, the problem was due to a poorly functioning film processor, resulting in films with streaks and artifacts; this problem was solved by service but recurs on occasion.

Radiation dose verification using real tissue phantom in modern radiotherapy techniques

“…The density correlation with dose and comparison is done by commercially available film QA dosimetry systems, for example, OmniPro-I’MRT. [ 6 7 ] Reference dosimetry is also done using electronic portal imaging device (EPID), where planned dose is delivered by LA and recorded by EPID and then it can be compared with the planned dose distribution. [ 8 9 ]…”

A comparative study on patient specific absolute dosimetry using slab phantom, acrylic body phantom and goat head phantom

“…film dosimetry by using commercially available film quality assurance (QA) dosimetry system OmniPro-I'MRT, dosimetry using elec-tronic portal imaging device (EPID) dosimetry etc. [8][9][10][11] For absolute dosimetry most of people are using small volume ion chamber and homogeneous phantoms. 12,13 The plan already done for patient on TPS is exported on phantom CT-images and dose is re-calculated without changing the monitor units (MU).…”