Pictorial review: comparison of portal imaging and megavoltage verification films for conformal pelvic irradiation

Determination of dosimetric leaf gap using amorphous silicon electronic portal imaging device and its influence on intensity modulated radiotherapy dose delivery

As complex treatment techniques such as intensity modulated radiotherapy (IMRT) entail the modeling of rounded leaf-end transmission in the treatment planning system, it is important to accurately determine the dosimetric leaf gap (DLG) value for a precise calculation of dose. The advancements in the application of the electronic portal imaging device (EPID) in quality assurance (QA) and dosimetry have facilitated the determination of DLG in this study. The DLG measurements were performed using both the ionization chamber (DLG_ion) and EPID (DLG_EPID) for sweeping gap fields of different widths. The DLG_ion values were found to be 1.133 mm and 1.120 mm for perpendicular and parallel orientations of the 0.125 cm³ ionization chamber, while the corresponding DLG_EPID values were 0.843 mm and 0.819 mm, respectively. It was found that the DLG was independent of volume and orientation of the ionization chamber, depth, source to surface distance (SSD), and the rate of dose delivery. Since the patient-specific QA tests showed comparable results between the IMRT plans based on the DLG_EPID and DLG_ion, it is concluded that the EPID can be a suitable alternative in the determination of DLG.

Patient-specific daily pretreatment setup protocol using electronic portal imaging for radiation therapy

The purpose of this study was to evaluate electronic portal imaging (EPI) as a means of identifying and correcting field displacement in patients with problematic external beam radiotherapy setups. Fourteen patients with problematic setups were identified for pretreatment daily EPI beam monitoring as part of a physician‐directed therapist intervention protocol. Pretreatment EPIs were used to realign fields as necessary to bring the setup within the physician‐prescribed tolerance level. For comparison, daily EPIs were available for 12 control patients who had no particular setup difficulties and for whom online beam realignment was not made. Anatomy‐matching software was used to measure setup variation along medial‐lateral, superior‐inferior, and anterior‐posterior axes. Online field realignment yielded a significant (p=0.001) improvement when comparing initial and final setup variations. The mean standard deviation of setup displacement averaged over three axes was reduced from 6.4 mm to 3.1 mm after realignment. The final variation of protocol patients was comparable to that of control patients. In conclusion, EPI provided effective means to perform online beam realignment in a group of difficult‐to‐position patients. This procedure resulted in a reduction in setup displacement that was statistically significant, clinically relevant, and approached that of a more typical patient group.

PACS number: 87.53.Oq

Portal imaging

Portal imaging is the acquisition of images with a radiotherapy beam. Imaging theory suggests that the quality of portal images could be much higher if the efficiency of the imaging media in detecting radiation could be improved. Introduction of new media (films and electronic portal imaging devices) has confirmed this by markedly increasing the quality of portal images. Images from these devices can then be used to verify a patient's treatment. Geometric verification requires the portal image to be registered with a reference image. Dosimetric verification requires the portal imager to be calibrated for dose. This review gives a brief overview of the current areas of interest in portal imaging: imaging theory; imaging media, film and electronic portal imaging devices; image registration; and dosimetry using these devices.

Guide to clinical use of electronic portal imaging

The Electronic Portal Imaging Device (EPID) provides localization quality images and computer-aided analysis, which should in principal, replace portal film imaging. Modern EPIDs deliver superior image quality and an array of analysis tools that improve clinical decision making. It has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments. However, in many radiation therapy clinics EPID technology is not in routine clinical use. This low utilization suggests that the capability and potential of the technology alone do not guarantee its full adoption. This paper addresses basic considerations required to facilitate clinical implementation of the EPID technology and gives specific examples of successful implementations.

A randomized study of the use of a customized immobilization system in the treatment of prostate cancer with conformal radiotherapy

PURPOSE

To evaluate the impact of a customized immobilisation system on field placement accuracy, simulation and treatment delivery time, radiographer convenience and patient acceptability.

PATIENTS AND METHODS

Thirty men receiving radical radiotherapy for prostate cancer were randomised using a cross over trial design to have radiotherapy planning and treatment given either in a conventional treatment position (CTP) or using an immobilisation system (IMS). The randomisation was to have either the CTP or IMS for the initial 3 weeks of radiotherapy after which patients were replanned and changed to the alternative treatment set-up. Treatment accuracy was measured using an electronic portal imaging device. Radiographers and patients completed weekly questionnaires.

RESULTS

Median simulation time was 22.5 min (range 20-30 min) in the CTP and 25 min (range 15-40 min) for the IMS (P < 0.001). Median treatment time was 9 min for CTP (range 8-10 min), and 10 min (range 8.5-13.5 min) for IMS (P < 0.001). Median isocentre displacement for anterior fields was 1.7 mm from the simulated isocentre for the CTP compared to 2.0 mm for IMS (P = 0.07). For left lateral fields values were 1.8 and 1.8 mm (P = 0.98), and for right lateral fields 2.1 and 1.7 mm (P = 0.06), respectively. No clinically significant reduction in either systematic or random field placement errors was demonstrated. Radiographers reported that patients found the IMS more comfortable than CTP (P < 0.001), but when using the IMS, they noticed greater difficulty in patient positioning (P < 0.001), and alignment to skin tattoos (P < 0.001).

CONCLUSIONS

Although IMS may have been more comfortable, treatment accuracy was not improved compared to the CTP in our department. In addition, treatment took longer and patient set-up was more difficult.

Assessment of accuracy of daily set-ups in prostate radiotherapy using electronic imaging

The purpose of the study was to assess the accuracy, using electronic portal imaging, of daily set-ups, in patients undergoing radiotherapy for prostate carcinoma. We used a scanning liquid ion chamber to assess the accuracy of set-ups in 25 consecutive patients undergoing a 6 1/2 week course of radiotherapy to the prostate. Electronic images (EPIs) were collected during 33 treatments to each of four ports. The positions of anatomical structures on the EPIs were compared with the same structures seen on digitally reconstructed radiographs (DRRs) made after CT simulation before beginning radiotherapy. Displacements of the EPIs compared with the DRRs were computer-calculated in millimetres in lateral, longitudinal and rotational directions for each port. 11 patients had ports moved because of discrepancies between the EPIs and the DRRs; eight required moves in the first five treatments to correct systematic (simulator) errors. In the right-left and anterior-posterior directions, nearly 95% of the EPIs were within 5 mm of the simulated port position. In the superior-inferior direction, 98% of the ports were within 5 mm of the simulated port position. Two patients had in-plane rotational errors on the lateral ports (8 degrees and 10 degrees respectively). It was concluded that daily electronic imaging is an effective technique for assessing the accuracy of set-ups in prostate radiotherapy.

Linear accelerator output variations and their consequences for megavoltage imaging

An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of +/- 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during are therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.

Portal imaging protocol for radical dose-escalated radiotherapy treatment of prostate cancer

PURPOSE

The use of escalated radiation doses to improve local control in conformal radiotherapy of prostatic cancer is becoming the focus of many centers. There are, however, increased side effects associated with increased radiotherapy doses that are believed to be dependent on the volume of normal tissue irradiated. For this reason, accurate patient positioning, CT planning with 3D reconstruction of volumes of interest, clear definition of treatment margins and verification of treatment fields are necessary components of the quality control for these procedures. In this study electronic portal images are used to (a) evaluate the magnitude and effect of the setup errors encountered in patient positioning techniques, and (b) verify the multileaf collimator (MLC) field patterns for each of the treatment fields.

METHODS AND MATERIALS

The Phase I volume, with a planning target volume (PTV) composed of the gross tumour volume (GTV) plus a 1.5 cm margin is treated conformally with a three-field plan (usually an anterior field and two lateral or oblique fields). A Phase II, with no margin around the GTV, is treated using two lateral and four oblique fields. Portal images are acquired and compared to digitally reconstructed radiographs (DRR) and/or simulator films during Phase I to assess the systematic (CT planning or simulator to treatment error) and the daily random errors. The match results from these images are used to correct for the systematic errors, if necessary, and to monitor the time trends and effectiveness of patient imobilization systems used during the Phase I treatment course. For the Phase II, portal images of an anterior and lateral field (larger than the treatment fields) matched to DRRs (or simulator images) are used to verify the isocenter position 1 week before start of Phase II. The Portal images are acquired for all the treatment fields on the first day to verify the MLC field patterns and archived for records. The final distribution of the setup errors was used to calculate modified dose-volume histograms (DVHs). This procedure was carried out on 36 prostate cancer patients, 12 with vacuum-molded (VacFix) bags for immobilization and 24 with no immobilization.

RESULTS

The systematic errors can be visualized and corrected for before the doses are increased above the conventional levels. The requirement for correction of these errors (e.g., 2.5 mm AP shift) was demonstrated, using DVHs, in the observed 10% increase in rectal volume receiving at least 60 Gy. The random (daily) errors observed showed the need for patient fixation devices when treating with reduced margins. The percentage of fields with displacements of < or = 5.0 mm increased from 82 to 96% with the use of VacFix bags. The rotation of the pelvis is also minimized when the bags are used, with over 95% of the fields with rotations of < or = 2.0 degrees compared to 85% without. Currently, a combination of VacFix and thermoplastic casts is being investigated.

CONCLUSION

The systematic errors can easily be identified and corrected for in the early stages of the Phase I treatment course. The time trends observed during the course of Phase I in conjunction with the isocenter verification at the start of Phase II give good prediction of the accuracy of the setup during Phase II, where visibility of identifiable structures is reduced in the small fields. The acquisition and inspection of the portal images for the small Phase I fields has been found to be an effective way of keeping a record of the MLC field patterns used. Incorporation of the distribution of the setup errors into the planning system also gives a clearer picture of how the prescribed dose was delivered. This information can be useful in dose-escalation studies in determining the relationship between the local control or morbidity rates and prescribed dose.