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ORIGINAL ARTICLE
Year : 2017  |  Volume : 13  |  Issue : 1  |  Page : 33-37

Dosimetric verification of dose calculation algorithm in the lung during total marrow irradiation using helical tomotherapy


1 Department of Medical Physics, Greater Poland Cancer Centre, Poznan, Poland
2 Department of Medical Physics, Greater Poland Cancer Centre; Department of Electroradiology, University of Medical Sciences, Poznan, Poland

Date of Web Publication16-May-2017

Correspondence Address:
Anna Kowalik
Department of Medical Physics, Greater Poland Cancer Centre, Poznan
Poland
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_980_15

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 > Abstract 

Introduction: Treatment of proliferative diseases of the hematopoietic system involves, in most cases, chemotherapy combined with radiation therapy, which is intended to provide adequate immunosuppressant. Conventionally, total body irradiation (TBI) was used; however, total marrow irradiation (TMI) performed with helical tomotherapy (HT) has been proposed as an alternative, with the aim of delivering the highest dose in the target area (skeleton bone).
Purpose: The purpose of this study is to evaluate the accuracy of the dose calculation algorithm for the lung in TMI delivered with HT.
Methods: Thermoluminescent detectors (TLD-100 Harshaw) were used to measure delivered doses. Doses were calculated for 95 selected points in the central lung (53 TLDs) and near the rib bones (42 TLDs) in the anthropomorphic phantom. A total of 12 Gy were delivered (6 fractions of 2 Gy/fraction).
Results: HT-TMI technique reduces the dose delivered to the lungs in a phantom model to levels that are much lower than those reported for TBI delivered by a conventional linear accelerator. The mean calculated lung dose was 5.6 Gy versus a mean measured dose of 5.7 ± 2.4 Gy. The maximum and minimum measured doses were, respectively, 11.3 Gy (chest wall) and 2.8 Gy (central lung). At most of the 95 points, the measured dose was lower than the calculated dose, with the largest differences observed in the region located between the target volume and the adjacent lung tissue. The mean measured dose was lower than the calculated dose in both primary locations: −3.7% in the 42 rib-adjacent detectors and −3.0% in the 53 central lung TLDs.
Conclusion: Our study has shown that the measured doses may be lower than those calculated by the HT-TMI calculation algorithm. Although these differences between calculated and measured doses are not clinically relevant, this finding merits further investigation.

Keywords: Helical tomotherapy, lung, thermoluminescent detectors, total marrow irradiation


How to cite this article:
Konstanty E, Malicki J, Łagodowska K, Kowalik A. Dosimetric verification of dose calculation algorithm in the lung during total marrow irradiation using helical tomotherapy. J Can Res Ther 2017;13:33-7

How to cite this URL:
Konstanty E, Malicki J, Łagodowska K, Kowalik A. Dosimetric verification of dose calculation algorithm in the lung during total marrow irradiation using helical tomotherapy. J Can Res Ther [serial online] 2017 [cited 2022 Jun 29];13:33-7. Available from: https://www.cancerjournal.net/text.asp?2017/13/1/33/206241


 > Introduction Top


Proliferative diseases of the hematopoietic or lymphatic systems such as leukemia and multiple myeloma may invade the skeleton and the kidneys, impair immune system function, and lead to hematological abnormalities. Standard treatment for most hematological cancers includes chemotherapy and radiotherapy followed by bone marrow transplantation.[1],[2]

The standard conditioning regimen for bone marrow transplantation includes total body irradiation (TBI) performed with a linear accelerator.[1],[3],[4] However, TBI has certain drawbacks, particularly the potential for secondary cancer induction and severe side effects.[5] Although numerous TBI techniques have been developed to increase the uniformity of the dose delivered to the whole body,[1],[2],[6],[7],[8] it is difficult to maintain dose uniformity throughout the body while also protecting the lung. As a result, the lung is often exposed to large doses of radiation that can lead to interstitial pneumonitis and other severe complications.[1],[4],[9],[10],[11],[12] This treatment-related morbidity has prompted a search for alternatives, one of which is total marrow irradiation (TMI) with helical tomotherapy (HT).[13],[14] In contrast to TBI techniques, the HT-TMI approach restricts the target volume to the skeletal bone marrow alone, thus minimizing morbidity. HT-TMI has accurate dose distribution capabilities,[10],[11],[15],[16] and the imaging system allows for daily verification of the tumor localization before each treatment session, thus helping to increase accuracy further.[17],[18],[19]

Reported results for HT-TMI are promising, and numerous studies have been carried out in recent years to compare TMI to TBI. Several of these studies (Schueng et al.,[4] Schultheiss et al.,[10] Wong et al.,[13],[14] and Wilkie et al.[20]) have found that HT-TMI achieves good results with decreased lung toxicity. However, data on the accuracy of the dose calculation algorithm for HT-TMI are relatively scant, with the notable exception of the study carried out by Hui et al. in a Rando phantom,[21] in which the authors used thermoluminescent detectors (TLDs) to verify doses at seven different anatomical sites. Given the limited evidence on the accuracy of the HT-TMI algorithm – particularly for the lungs, the main site for severe morbidity with this technique – we decided to carry out the study reported here. In this study, we evaluate the accuracy of the HT-TMI dose calculation algorithm for the lung using 95 TLDs inserted into the rib and lung areas of a phantom. To the best of our knowledge, this is the first study to exclusively assess the accuracy of the HT-TMI treatment planning algorithm used to calculate the dose delivered to the lungs.


 > Methods Top


The female Alderson Radiation Therapy Phantom was used to perform this study (height: 155 cm; weight: 50 kg; syntactic foam lungs; density: 0.30 g/cc) (Radiology Support Devices Inc., California, USA).

This anthropomorphic phantom is composed of a series of 2.5 cm thick slices located adjacently. Each of the patches has openings (recesses), in which tissue-equivalent inserts (i.e., bone-equivalent, soft-tissue-equivalent, and lung-tissue equivalent pins). For the purposes of this study, TLDs were inserted into these recesses [Figure 1].
Figure 1: A single slice of the phantom and method of dosimetric mapping points

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The HT treatment planning system (TPS) was used to calculate the estimated doses to the phantom's lungs. Actual doses delivered were measured with TLDs (Harshaw, TLD-100, size of 3.2 mm × 3.2 mm × 0.38 mm; Thermo Electron Corporation, Ohio, USA). The benefit of TLDs is that they provide reliable information about dose distribution inside the body without requiring recalculation.[21],[22],[23],[24],[25]

Individual coefficients were assigned to each TLD for calibration of the TomoTherapy Hi-Art machine at 6 MV photon energy using a plexiglass phantom with a mean density of 1010 g/cm 3 [Figure 2]. Calibration was performed with a 1 Gy dose for an exposure time of 2.289 MU calculated with the tomotherapy TPS. A Harshaw 3500 TLD Reader was used. The calibration procedure was repeated 10 times to ensure the stability of the selected detectors.
Figure 2: Calculated dose distribution for calibration of thermoluminescent detector

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Target definition and dose distribution optimization

Target volume and critical organ definition were performed using CT scans from an Alderson phantom. We used a separation of 8 mm between successive layers (cross-sections) to ensure sufficient quality of the three-dimensional (3D) reconstruction of the irradiated target and critical organs. A radiation oncologist contoured the volume to be irradiated and the critical organs (chest, heart, esophagus, thyroid, and lungs) on the Somavision unit (Varian Systems, Palo Alto, CA, USA). The target dose was 12 Gy to the skeletal bone over 6 fractions (2 Gy/fraction). Optimization and calculation of dose distributions were performed using the TPS, and the entire process took approximately 8 h. Maximal and minimal doses and dose volume histograms for each organ were also calculated. Parameters for the HT treatment plan were as follows: field = 5.0 cm; pitch = 0.287; given modulation factor = 3.000 (real, 1.955); speed of table movement = 0.085 cm/s; duration of application of one fraction = 1061.7 s.

Dose measurements

The entire lung was included within seven cross-sections of the phantom. TLDs were placed at 95 selected points of the phantom [Figure 3]. Dose measurements were repeated three times. Calibration factors derived previously were applied for each detector reading.
Figure 3: Computed tomography scans with numbers of the individual thermoluminescent detectors

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Statistical analysis

The Shapiro–Wilk normality test was used to evaluate the measured dose distribution. Data which did not follow a normal distribution were analyzed by nonparametric tests. Two groups of independent data were analyzed by the Student's t-test. Dependent data (i.e., calculated dose vs. measured dose) were assessed by the Wilcoxon test since data did not follow a normal distribution. A significance level of α = 0.05 was used for all analyses. Calculations were performed using the Statistica 10 statistical package (StatSoft Inc., Tulsa, OK, USA).


 > Results Top


Calculated dose distributions

At the prescribed dose of 12 Gy to the target volume, the lung wall received doses ranging from 5 to 10 Gy. This steep-dose gradient was due to the 5 Gy limit established for doses inside the lung. The HT-TPS was used to calculate doses for all 95 locations. The obtained dose distributions are presented in [Figure 4].
Figure 4: Dose distribution obtained in the Hi-Art treatment planning system

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Data collected from the CT scanner allows us to determine the volume of the area of irradiation and critical organs. Dividing the volume of such a vital organ into small parts (through the TPS) allows us to calculate the value of the doses of the individual elements and the results of the calculations. These calculations are shown in [Figure 5], which presents calculated dose distribution histogram for dose frequency obtained in the present study.
Figure 5: Calculated dose distribution histogram. On the X-axis of the histogram is accumulated dose value on the ordinate and percent the volume under consideration

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In this study, the average dose calculated in the lungs was 5.61 Gy. The maximum dose of 12.44 Gy and the minimum 2.99 Gy.

Measured dose distributions

The phantom was irradiated up to 2 Gy (equivalent to one fraction) for 15,510 MU (monitor units) at a dose rate of 879 MU/min. Before irradiation, an MVCT scan was performed at 94 MU and dose rate of 19 MU/min.

The MVCT doses were measured for each dosimetric point (the mean MVCT dose was 0.03 Gy). The measured doses were corrected by the MVCT dose obtained during phantom alignment. Given that the steep-dose gradient along the ribs could pose greater difficulties in measuring the doses, we decided to group the dosimetric points according to location, with all 42 points located near the ribs placed in one group (which we denominated “rib-adjacent TLDs”) and the remaining 53 points (i.e., those in the middle region of the lungs) into another group (“central lung-TLDs”).

Comparison of the measured and calculated dose distributions

For each dosimetric point, the measured value was averaged and corrected by the MVCT scan value and converted according to the calibration factors. The mean difference between the measured and calculated dose for the rib-adjacent TLDs was −3.7% (with a range of −32% to +32% for the individual detectors included in this group), indicating that the measured dose was lower than the calculated dose. For the central lung-TLD group of detectors, the corresponding difference was −3.0% (range, −36% to +16%) [Figure 6]. The calculated dose was higher than the measured dose in both groups: 7.8 Gy vs. 7.4 Gy in the rib-adjacent TLD group, and 4.5 Gy vs. 4.3 Gy in the central lung-TLD group. However, these intragroup differences between calculated and measured doses (i.e., −3.7% vs. −3.0%) were not significant (P = 0.7902).
Figure 6: Percentage difference between measured and calculated doses for two groups of dosimetric points: thermoluminescent detectors placed near ribs (bones) versus thermoluminescent detectors located in the middle of the phantom lungs

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 > Discussion Top


Compared to conventional TBI, HT-TMI offers numerous advantages: it is more accurate, easier to perform, and more comfortable for the patient. In addition, this technique allows for an accurately shaped dose distribution restricted to the bony structures only, thus allowing for dose escalation while sparing organs at risk (OARs).[16],[26] Although Hui et al. performed a study with some similarities to ours, those authors evaluated seven different anatomical sites while we focused exclusively on the lung.[26] As a result, ours is the first study to verify the accuracy of the algorithm used to calculate the dose delivered throughout the lungs with HT-TMI.

The largest differences between calculated and measures doses were observed in the area located along the border of the target volume (the rib bones). This finding was not unexpected given the steep-dose gradient (12 Gy to the ribs vs. 5 Gy maximum to the adjacent lungs) required to protect the lungs from excessive irradiation. The proximity of organic materials that differ not only in density but also in elemental composition affects the physical interaction and consequently, the dose. In addition, given that the target dose must have a rapid drop-off to protect critical tissues, we can expect a greater inaccuracy in both calculation and measurement. The dose gradient directly affects measurement although in our study, the small size of the TLDs (2–3 mm) helps to minimize possible errors. However, it is important to keep in mind that the presence of TLDs affects the scattering effect.

One of the attractions of using TMI versus TBI is the ability to precisely deliver radiation so that the OARs - particularly the lungs - can be spared the toxicity typically associated with TBI. The mean measured dose to the lungs in our study was only 5.7 Gy whereas most studies of TBI have reported a range of 8–10 Gy.[9] Volpe et al. reported a median lung dose of 9.4 Gy with TBI and more importantly, these authors also observed a direct correlation between higher doses and pulmonary complications.[27] If our findings are correct, this would suggest that lung toxicity should be much less with TMI compared to TBI.

In recent years, a number of studies have investigated the feasibility of using TMI instead of TBI to reduce toxicity to OARs, including the lung. Schueng et al. found that HT-TMI decreased the dose delivered to the chest by 21%–51% compared to TBI similarly,[28] Schultheiss et al. reported that the median dose to OARs was 51% lower when HT-TMI was used versus TBI.[16] Wong et al. found that TMI reduced lung irradiation by a factor of 1.4–1.7 when compared to TBI.[20] Finally, in a study that used a method that was quite similar to ours (i.e., a Rando phantom with TLDs), Wilkie et al. reported that TMI delivered with intensity-modulated radiation therapy reduced doses to critical structures by 29%–65% versus conventional TBI.[29]





Our study has shown that the measured doses may be lower than those calculated by the HT-TMI calculation algorithm. Although these differences between calculated and measured doses are not clinically relevant, this finding merits further investigation.

Our results show that HT-TMI reduces the dose delivered to the lungs in a phantom model to levels that are much lower than those reported for TBI delivered by a conventional linear accelerator. We believe that these findings provide additional support for the use of TMI in conditioning regimens for bone marrow transplantation.

Acknowledgments

We would like to recognize that the Polish Ministry of Science for providing funding (grant ref. # NN 402352338) for this research. We also thank Bradley Londres for improving English in this paper.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]


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