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ORIGINAL ARTICLE
Year : 2014  |  Volume : 10  |  Issue : 3  |  Page : 611-617

Analysis of patient specific dosimetry quality assurance measurements in intensity modulated radiotherapy: A multi centre study


1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Department of Radiotherapy, P. D. Hinduja Hospital and Medical Research Center, Mumbai, Maharashtra, India
3 Department of Radiotherapy, Basvatarakam Indo American Cancer Hospital and Research Institute, Hyderabad, Andhra Pradesh, India
4 Department of Radiotherapy, Batra Hospital and Medical Research Centre, New Delhi, India, India
5 Department of Medical Physics, Memorial Sloan Kettering Cancer Centre, New York, USA

Date of Web Publication14-Oct-2014

Correspondence Address:
Rajesh Kumar
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS Building, Anushakti Nagar, Mumbai, Maharashtra
India
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Source of Support: Partially supported by the International Atomic Energy Agency (IAEA), Conflict of Interest: None


DOI: 10.4103/0973-1482.137941

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 > Abstract 

Aim of the Study: Statistical analysis of pre-treatment dose verification of intensity-modulated radiotherapy (IMRT) fields to assess the quality of the IMRT practice at different radiotherapy centers.
Materials and Methods: The dose verification data acquired by the institutional physicist of 10 different hospitals for various types of patients were collected and analyzed for mean, median, standard deviation (SD), range, minimum and maximum % deviation. The percentage of cases having positive and negative dose differences as well dose differences within ± 3% were also determined.
Results: The mean values of percentage variation in difference between treatment planning systems calculated dose and difference between measured dose (D TPS and D Meas ) are found to be from – 1.79 to 1.48 and median from – 1.79 to 1.51. The SDs are found to be from 0.76 to 3.70. The range of variation at these centers varies from 3.99 to 16.45 while minimum and maximum values of percentage variation in difference between D TPS and D Meas ranges from – 10.33 to 13.38. The percentage of cases having positive dose difference ranges from 8 to 94 and cases having negative dose difference ranges from 6 to 92. The percentage of cases having dose difference within ± 3% varies from 57 to 100.
Conclusion: IMRT centers are having random and biased (skewed towards over or under dose) distribution of the percentage variation in difference between measured and planned doses. The analysis of results of the IMRT pre-treatment dose verification reveals that there are systematic errors in the chain of IMRT treatment process at a few centers. The dosimetry quality audit prior to commissioning of IMRT may play an important role in avoiding such discrepancies.

Keywords: Dose verification, intensity-modulated radiotherapy, quality assurance, quality audit, statistical analysis


How to cite this article:
Kumar R, Sharma S D, Deshpande S, Sresty NM, Bhatt C P, Amols HI, Chourasiya G, Mayya Y S. Analysis of patient specific dosimetry quality assurance measurements in intensity modulated radiotherapy: A multi centre study. J Can Res Ther 2014;10:611-7

How to cite this URL:
Kumar R, Sharma S D, Deshpande S, Sresty NM, Bhatt C P, Amols HI, Chourasiya G, Mayya Y S. Analysis of patient specific dosimetry quality assurance measurements in intensity modulated radiotherapy: A multi centre study. J Can Res Ther [serial online] 2014 [cited 2020 Jun 4];10:611-7. Available from: http://www.cancerjournal.net/text.asp?2014/10/3/611/137941


 > Introduction Top


Intensity-modulated radiotherapy (IMRT), which is an advance form of three-dimensional conformal radiotherapy is becoming the common mode of curative cancer treatment. In this technique, non-uniform fluence is delivered to the patient from a given direction of the treatment beam to optimize the composite dose distribution. Delivery of intensity-modulated fields relies on the use of computer controlled multileaf collimators (MLCs) equipped on modern linear accelerators. Because of the complex beam intensity-modulation, each IMRT field often includes many small, irregular, off-axis fields resulting in isodose distributions for each IMRT plan that are more conformal to the target volume than those from conventional treatment plans. These features impose requirements for dosimetric verification in IMRT delivery with suitable IMRT phantom. [1],[2],[3],[4],[5] Pre-treatment verification of fluence delivery is an effective method of ensuring the accuracy of IMRT treatments. Pre-treatment dose verification of IMRT fields is performed using ionization chambers, radiographic/radiochromic films, two-dimensional detector array system etc. [6]

ESTRO QUASIMODO used a horseshoe-shaped planning target volume (PTV) surrounded by a cylindrical organ at risk (OAR) along with ionization chamber measurement and radiographic film in a pelvic phantom to access the quality of IMRT treatment delivery in a few hospitals. [7] The irradiated films, the results of the ionization chamber measurements and the computed dose distributions were collected and analyzed at a nodal center that compared the measured and computed dose distributions with the gamma method and composite dose-area histograms. American Association of Physicists in Medicine Task Group 119 (AAPM TG 119) developed nine test cases to assess the goodness of IMRT commissioning in nine hospitals. These nine hospitals included in the study had passed the Radiological Physics Center credentialing tests for IMRT. The degree of agreement has been quantified using the concept of "confidence limit" which is defined as I mean deviation I + 1.96 σ. The agreement between the planned and measured doses was determined using ionization chamber and films. [8]

In a survey among Indian radiotherapy centers performing IMRT, it was observed that each institution uses its own specific equipment and method for planning, delivery and quality assurance (QA). [9] As per this survey, almost all the hospitals have the program of pre-treatment dose verification using calibrated ionization chambers of sensitive volumes in the range of 0.01-0.65 cc. It was also observed in the survey that majority of the hospitals perform the single point dose verification with a single gantry angle of 0° using slab phantoms.

Considering wide variability in delivery, planning, QA and pre-treatment dose verification methods; dose verification data acquired by the hospitals as part of their institutional pre-treatment dose verification program in IMRT were collected from 10 different hospitals in the country. The statistical analysis of these data was conducted to assess the quality of the IMRT practice at these institutions. This paper describes the results of this statistical analysis of point dose verification with a single gantry angle using the slab phantom.


 > Materials and Methods Top


The dose verification data acquired by the institutional physicist of 10 different hospitals for various types of patients were included in the statistical analysis. The sites treated using IMRT at these centers include head and neck, breast, cervix, prostate, lung, etc., Randomly selected dose verification data of these centers which includes different types of cases were collected for the analysis. The beam delivery devices used at these centers were Varian Clinac 2300 CD, 6EX and Trilogy equipped with Varian 120 leaves millennium MLC (Varian Oncology System, USA); Elekta Synergy equipped with Elekta 80 leaves MLC (Elekta, UK) and Siemens Oncor equipped with Siemens 80 leaves MLC (Siemens Healthcare, Germany). The radiotherapy treatment planning systems used by the hospitals were Varian Eclipse v6.0, CMS Xio v2.33.02, Elekta Precise Plan 2.6.9 v11 and Siemens KonRad v2.2. The hospitals have been identified here as H1 to H10 and the vendors of the beam delivery systems have been identified as vendor 1 to vendor 3 so that their exact identity could not be disclosed.

Pre-treatment dose verifications at these centers were performed by measuring the point dose using ionization chamber in a slab phantom. The 0.125/0.6 cc (PTW Freiburg, Germany) and 0.13/0.65 cc (IBA Sweden) ionization chambers were used by these hospitals for dose verification measurements. The computed tomography (CT) images of the 30 cm 3 × 30 cm 3 × 20 cm 3 slab phantom containing ionization chamber at 5 cm depth from the anterior surface of the phantom was acquired. Surface plot of the CT data with chamber in the place is shown in [Figure 1]. The active volume of the ion-chamber was contoured as a region of interest on CT images so that mean dose to the chamber volume can be calculated by the treatment planning system. A verification plan with the same fluence maps as in the treatment plan was generated on CT images of the phantom in the treatment planning system (TPS) by resetting the planned gantry, collimator and couch angle to 0° angles. The dose to the ionization chamber location was determined and referred here as difference between treatment planning systems calculated dose (D TPS ). The generated plans were transferred through the record and verify system of the hospitals to the linear accelerator for execution on the phantom. The cumulative reading of IMRT delivery of each patient at the point of measurement was recorded. The reading of the ionization chamber was converted into absorbed dose to water, which is referred here as difference between measured dose (D Meas ) using methodology described in the International Atomic Energy Agency TRS 398. [10] The D Meas and D TPS was calculated using the following formula: [11]
Figure 1: Histogram of the measured and planned dose difference of Group A hospitals (H1 and H2) using medical electron linear accelerator of vendor 1 for intensity-modulated radiotherapy delivery

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In this study, difference between individual field doses were not considered. Collected data were analyzed for mean, median, standard deviation (SD), range, minimum and maximum % deviation using Statistica 6.0 (StatSoft Inc., OK, USA). The percentage of cases having positive and negative dose differences as well dose differences within ± 3% were also determined.


 > Results Top


The results of statistical analysis of difference between measured and planned doses of different hospitals having medical electron linear accelerator of vendor 1 are shown in [Table 1]. Table contains mean, median, SD, range, minimum and maximum of percentage variation in difference between D TPS and D Meas . In addition, the table also presents percentage of cases with positive (+ve) and negative (−ve) dose difference along with percentage of cases having dose difference within ± 3%. The mean values of percentage variation in difference between D TPS and D Meas of the six hospitals are found to be from − 1.79 to 1.48 and median from − 1.79 to 1.48. The SDs of these hospitals are found to be from 0.076 to 2.91. The range of variation at these centers varies from 3.99 to 15.4 while minimum and maximum values of percentage variation in difference between D TPS and D Meas ranges from − 9.41 to 7.9. The percentage of cases having positive dose difference ranges from 8 to 94 while the percentage of cases having negative dose difference ranges from 6 to 92. The percentage of cases having dose difference within ± 3% varies from 74 to 100.

The data of hospital 1 (H1) and hospital 2 (H2); hospital 3 (H3) and hospital 4 (H4); and hospital 5 (H5) and hospital 6 (H6) show the similar trend and hence they were grouped as Group A (H1 and H2), Group B (H3 and H4) and Group C (H5 and H6) hospitals, respectively. [Figure 1] presents the histogram of the dose difference between measured and planned dose values of Group A hospitals. It can be observed from this figure that the dose differences of these hospitals are skewed toward negative side. [Figure 2] presents the histogram of dose difference between measured and planned dose values of Group C hospitals. It can be seen from this figure that the dose difference data are skewed towards positive side. [Figure 3] presents the histogram of dose difference between measured and planned dose values of Group B hospitals. It can be seen from this figure that the dose difference data of these hospitals are randomly distributed.
Figure 2: Histogram of the measured and planned dose difference of Group B hospitals (H3 and H4) using medical electron linear accelerator of vendor 1 for intensity-modulated radiotherapy delivery

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Figure 3: Histogram of the measured and planned dose difference of Group C hospitals (H5 and H6) using medical electron linear accelerator of vendor 1 for intensity-modulated radiotherapy delivery

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The results of statistical analysis of dose difference data of hospital 7 (H7) and hospital 8 (H8) equipped with medical electron linear accelerator of vendor 2 are given in [Table 2]. It can be observed from the data in this table that the mean values of percentage variation in difference between D TPS and D Meas of these hospitals are found to be − 0.30 and 1.51; median values are − 0.12 and 1.57. The SDs of these data are found to be 0.94 and 3.7. The range of variation in dose difference of these centers is 5.73 and 9 while minimum and maximum values of percentage variation in difference between D TPS and D Meas ranges from − 10.33 to 13.38. The percentage of cases having positive dose differences of these hospitals are 31 and 43. The percentage of cases having negative dose difference are 57 and 69. The percentage of cases having dose difference within ± 3% varies from 57 to 100. [Figure 4] presents the histogram of the dose difference between measured and planned dose values of H7 and H8. It can be seen from this figure that the dose difference data of these hospitals are almost randomly distributed.
Figure 4: Histogram of the measured and planned dose difference of hospitals H7 and H8 using medical electron linear accelerator of vendor 2 for intensity-modulated radiotherapy delivery

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[Table 3] shows the results of statistical analysis of dose difference data of hospital 9 (H9) and hospital 10 (H10) equipped with medical electron linear accelerator of vendor 3. The mean, median and SD of percentage variation in difference between D TPS and D Meas of these hospitals are found to be from − 0.43 to 1.15, from − 0.43 to − 1.01 and from 1.76 to 2.52, respectively. The range in dose difference of these centers varies from 10.82 to 16.45 while minimum and maximum values of percentage variation in difference between D TPS and D Meas ranges from − 6.89 to − 8.97 and 3.93 to 7.48, respectively. The percentage of cases having positive and negative dose difference ranges from 31 to 62 and from 58 to 69, respectively. The percentage of cases having dose difference within ± 3% varies from 78 to 93. [Figure 5] presents the histogram of the dose difference between measured and planned dose values of H9 and H10. It can be seen from this figure that the dose difference data of these hospitals are almost randomly distributed.
Figure 5: Histogram of the measured and planned dose difference of hospitals H9 and H10 using medical electron linear accelerator of vendor 3 for intensity-modulated radiotherapy delivery

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 > Discussion Top


Results of intercomparison of IMRT dose verification studies have been reported by a number of investigators. [7],[12],[13],[14],[15],[16],[17],[18] ESTRO QUASIMODO study revealed maximum local deviation of less than 3.5% in the mean planned and measured dose values for the PTV and mean local deviation of 1.4%. However, local deviations in planned and measured dose values for the OAR were up to 5.8%. [7] In case of IMRT dose verification by ionization chamber, AAPM TG 119 reported 4.5% difference between measured and planned doses in the target region for 95% of the test cases. [8] PARSPORT Trial Management Group have reported for two-dimensional dose comparison, 94% passing rate in gamma criteria of 3%/3 mm for individual fields and 75% in gamma criteria of 4%/3 mm for combined fields were proposed in multi-center head and neck IMRT trials. [12],[19]

The acceptance criteria of pre-treatment patient specific IMRT QA for point dose at the most of the radiotherapy centers is ± 3% and for gamma index it is 3%/3 mm. [9] It can be inferred from the results presented in of [Table 1], [Table 2], [Table 3] that percentage variation in difference between D TPS and D Meas is more than the acceptable limit except H6. In case the dose difference is more than the acceptance limit, the doubt goes to the limitation of point dose measurement method and if the point of measurement is within the high dose gradient zone, the measured dose may differ by more than ± 3%. In such cases ideally a point of low dose gradient should be identified and dosimetry measurements should be repeated. However, this is not a common practice for all the radiotherapy centers in India. Because, some of the individuals assume that the error may be due to the high dose gradient at the point of measurement and they do not repeat the dosimetry measurements by identifying a suitable measurement point of relatively low dose gradient.
Table 1: Results of statistical analysis of difference between measured and planned doses of different hospitals having medical electron linear accelerator of vendor 1

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Table 2: Results of statistical analysis of difference between measured and planned doses of different hospitals having medical electron linear accelerator of vendor 2


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Table 3: Results of statistical analysis of difference between measured and planned doses of different hospitals having medical electron linear accelerator of vendor 3


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The sources of error between measured and planned doses are broadly attributed to positioning errors of MLC, insufficient dosimetric data of MLC in TPS, inaccurate handling of small field dosimetry, human errors and inaccurate measurement devices for IMRT QA procedures. [20],[21],[22],[23],[24] The other reported source of error in TPS can be the tongue and-groove effect that often results in the systematic under-dosage. [25],[26] It is also reported that, highly-modulated treatment plans are more sensitive to accuracies of the above sources of errors than a mildly-modulated plans. [27]

Chung et al. [11] in their study have reported that in case of dynamic IMRT, the tongue and grove effect could be a reason for systematic under-dosage where treatment planning has been done using the older version of the Eclipse treatment planning system. They have also reported that with upgraded version of the TPS there were no noticeable systematic biases.

Sarkar et al. [28] have reported a biased variation in difference between D TPS and D Meas where they have used the Elekta precise as treatment delivery machine and Nucletron Plato sunrise as the treatment planning system and they have address, variation is biased even for the open field dosimetry and pointed out systematic error in the TPS commissioning as one of the reasons.

We have observed that there are IMRT centers having random and biased (skewed towards over or under dose) distribution of the percentage variation in difference between D TPS and D Meas , while they are having the TPS and beam delivery systems of the same vendor [Figure 1], [Figure 2] and [Figure 3]. Similarly, the dose difference data of H8 [Figure 4] and H10 [Figure 5] are more biased while the dose difference data of hospitals H7 [Figure 4] and H9 [Figure 5] are random in nature.

Budgell et al. [15] have carried out dosimetric audit of IMRT implementation in over 90% of radiotherapy centers in UK. Their audit result shows that IMRT TPS modeling and delivery is accurate, suggesting that the implementation of IMRT has been carried out safely. They have also reported a histogram of percentage variation in difference between the ion chamber measurements relative to predicted doses which is random in nature. In addition, they have reported that percentage error is ranging from − 14% to 20% with the mean difference of 0.02% and SD of 3.1%. However, we observed that dose difference is having biased distribution for the hospitals included in this study.

This study also reveals that there are systematic errors involved in dosimetry and planning and delivery at these radiotherapy centers. The sources of error are not common in nature. This analysis suggests that in implementations of IMRT, some parameters in the chain have not been properly tuned. Though the magnitude of discrepancy is not alarming but certainly need correction. This work suggests a strong justification for a third party verification of the commissioning of treatment delivery and planning system before commissioning of IMRT treatments. If there would have been a third party mechanism for verification in place such unpredicted variation would not have been observed. It is important to identify the actual cause of discrepancy at these radiotherapy centers through a systematic dosimetry approach.


 > Conclusion Top


Dose verification data acquired by the hospitals as part of their institutional pre-treatment dose verification program in IMRT were collected, and the statistical analysis of these data was conducted to assess the quality of the IMRT practice. This study reveals that IMRT centers are having random and biased (skewed towards over or under dose) distribution of the percentage variation in difference between measured and planned doses, while they are using the TPS and beam delivery systems of the same vendor. The analysis of results of the IMRT pre-treatment dose verification also reveals that there are systematic errors in the chain of IMRT treatment process at a few centers included in this study. The dosimetry quality audit prior to commissioning of IMRT may play an important role in avoiding such discrepancies.

 
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28.
Sarkar B, Ghosh B, Sriramprasath, Mahendramohan S, Basu A, Goswami J, et al. Optimized point dose measurement for monitor unit verification in intensity modulated radiation therapy using 6 MV photons by three different methodologies with different detector-phantom combinations: A comparative study. J Med Phys 2010;35:144-50.  Back to cited text no. 28
[PUBMED]  Medknow Journal  


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