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  Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 57  |  Issue : 4  |  Page : 437-442
 

Determining whether multiple needles are necessary in interstitial brachytherapy for thoracic tumors: A dosimetry analysis


Department of Oncology, The Affiliated Hospital of Southwest Medical University, Luzhou, China

Date of Submission24-Oct-2018
Date of Decision28-Apr-2019
Date of Acceptance04-May-2019
Date of Web Publication17-Oct-2020

Correspondence Address:
Bo Yang
Department of Oncology, The Affiliated Hospital of Southwest Medical University, Luzhou
China
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijc.IJC_693_18

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 » Abstract 


Background: In interstitial brachytherapy, needles must be inserted in a regular, parallel arrangement to ensure a uniform target dose distribution and conformal distribution to the target. It is generally difficult to achieve this in thoracic tumors because of obstruction by the ribs. Furthermore, insertion of multiple needles may cause the patient considerable harm and could expose him/her to additional risks. Thus, we propose the single-dwell-position method, discuss its applicability, and compare it with the actual multiple-needle method using dosimetry. The aim of this study was to evaluate the necessity for multiple needles with irregular alignment in interstitial brachytherapy for thoracic tumors.
Methods: Twelve patients' interstitial brachytherapy plans were reviewed. The single-dwell-position interstitial brachytherapy plans, wherein one needle was hypothetically inserted, were compared with the actual multiple-needle plans. Dose parameters, including clinical target volume (CTV) and volumes of the lung, spinal cord, heart, and ribs, were compared. We also evaluated the correlation between CTV size and dose difference in the lungs. The nonparametric Wilcoxon test was used.
Results: There were no statistically significant differences in the doses achieved with the single-dwell-position plans and actual multiple-needle plans. The correlation between the CTV size and dose difference in the lungs was weak.
Conclusions: Irregularly arranged multiple-needle interstitial brachytherapy does not provide superior doses to the lung, heart, spinal cord, or ribs compared with single-dwell-position plans. If regular arrangement of multiple needles is difficult to achieve, the multiple-needle scheme is not the only viable option.


Keywords: Interstitial brachytherapy, lung cancer, needles, spinal cord


How to cite this article:
Yang B, Sun X, Pang H, Wu J. Determining whether multiple needles are necessary in interstitial brachytherapy for thoracic tumors: A dosimetry analysis. Indian J Cancer 2020;57:437-42

How to cite this URL:
Yang B, Sun X, Pang H, Wu J. Determining whether multiple needles are necessary in interstitial brachytherapy for thoracic tumors: A dosimetry analysis. Indian J Cancer [serial online] 2020 [cited 2020 Oct 24];57:437-42. Available from: https://www.indianjcancer.com/text.asp?2020/57/4/437/297029





 » Introduction Top


Computed tomography (CT)-guided percutaneous interstitial high-dose-rate brachytherapy (PIBT) is an effective alternative for treating lung lesions.[1],[2],[3],[4],[5],[6] To make the high-dose curve more suitable to the target volume, more needles should be used, and the needle alignment should follow the principles proposed in the International Commission on Radiation Units and Measurements (ICRU) Report 58.[3],[7] However, if this arrangement principle cannot be applied, the dose in the lungs might be increased.[8],[9],[10] Two schemes were designed to compare the dose distribution, and the goal of our study was to evaluate the necessity for multiple needles with irregular alignment in interstitial brachytherapy.


 » Methods Top


Patient characteristics

The interstitial brachytherapy plans of 12 patients with lung tumors who underwent PIBT performed using an192 Ir high-dose-rate afterloader (microSelectron-HDR, Elekta, the Netherlands) between January 2015 and May 2017 were included in this analysis. The procedures were conducted in accordance with the ethical standards of the institutional ethical committee and with the Helsinki Declaration of 1975, as revised in 2000, and written informed consent was obtained from all patients before treatment. The enrolled patients were 18- to 80-year old with peripheral, histopathologically confirmed T1-3 N1-2 M0 non-small-cell lung carcinoma (NSCLC). In these cases, more than two needles with course intersections were implanted in the interspace of the ribs under computed tomography (CT) guidance (LightSpeedPlus 4, General Electric Company, Boston, MA, USA) by hand, by the senior radiation oncologist [Table 1].
Table 1: Patient and tumor characteristics (Original Data)

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When the diameter of the clinical target volume (CTV) was <3.0 cm, one needle passing through the center of the CTV was preplanned. However, in some cases with a tumor diameter <3.0 cm, if the actual result of the needle implant was off-center inside the CTV, an additional feasible needle was implanted into the target area to compensate for the insufficient dose of the CTV; thus, in our study, the minimum volume of CTV was 6.9 cc. The cases of one needle passing through the center of the CTV were not included in this study.

For the cases with a tumor diameter larger than 3.0 cm, the “center needle” was not considered, and the scheme of multiple needles with 1.0- to 1.5-cm spacing between needles (nonparallel) was adopted in preplanning, using needles with course intersections.[6]

Multiple-needle scheme and single-dwell-position scheme

On the basis of the actual needles, the interstitial brachytherapy plan was designed on the Oncentra 4.1 three-dimensional (3D) treatment planning system (TPS) (Elekta, Sweden); the active source dwell positions were selected along the actual needle, and the step size was 0.25–0.75 cm. This is referred to as the actual multiple-needle interstitial brachytherapy plan (AMBP) [Figure 1].
Figure 1: Two actual needles inserted into the clinical target volume (CTV) through the rib interval at an ~30° angle are visible. The CTV was contoured with the dotted red line. Solid lines denote isodose curves. (Original Image)

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We designed a virtual single-dwell-position interstitial brachytherapy plan (VSBP), wherein one needle was hypothetically inserted into the interspace of the ribs adjacent to the tumor and one active source dwell position was located at the center of the CTV [Figure 2]. For achieving these plans in our study, we first designed a plan where one needle passes through the center of the CTV on the CT image in which CTV size reaches the maximum, followed by positioning of one dwell point selected along this virtual needle in the center of the shape of the CTV in the CT image.
Figure 2: A single needle with a single dwell position. Only the virtual needle was hypothetically inserted, and one virtual active source dwell position was located in the center of the clinical target volume; the isodose curves are concentric circles. (Original Image)

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In the TPS, the insertion needles must be identified manually. Generally speaking, one insertion needle is determined by at least two points on the CT images. In the multiple-needle scheme, the points depicted are based on the CT image of actual needles.

For the single-dwell-position scheme, one point was depicted at the center of the CTV, while the other was depicted at the interspace of the ribs, through which an actual needle passes. The straight line connecting the two points represented the virtual insertion needle. In theory, the virtual needle could be successfully implemented.

In all plans, the prescription dose (PD) was 100 Gy, and the radioactivity obtained was ~5.74 Ci using192 Ir source afterloading equipment. In the TPS, plans were optimized using the graphics optimization tool. Plans were accepted after meeting the following constraints: 95% coverage of the CTV by at least 100% of the PD (D95 ≥ PD) and minimization of the percentage of the normal ipsilateral lung volume that received a dose of ≥5 Gy.[8]

Statistical analyses

The differences in D95, D90, D80, D50, the maximum dose (Dmax) of the ribs, the Dmax of the spinal cord, the D50 of the heart, the percentage of lung volume receiving a dose of ≥5 Gy compared with the whole lung volume (V5), V20, V30, the mean lung dose (MLD), and the irradiation time were tested for statistical significance using a non-parametric Wilcoxon test. A P value <0.05 was considered statistically significant.

With SPSS version 19.0 software, the relationship between the CTV and △V5 was tested using Pearson's analysis, with △V5 equal to the value of V5 in VSBP minus the value of V5 in AMBP.


 » Results Top


Schematic diagrams of the dose distributions in the CT image are shown in [Figure 1] and [Figure 2]. Two actual needles were inserted into the CTV through the rib interval at an approximately 30° angle and the active source dwell positions at 0.25-cm intervals [Figure 1]. One virtual needle was hypothetically inserted, and one virtual active source dwell position was located in the center of the CTV [Figure 2].

There were no statistically significant differences in the doses for either VSBP or AMBP administered to the whole lungs, ipsilateral lung, heart, and spinal cord [Table 2]. The mean irradiation time associated with the VSBP (6080.00 ± 5375.16 seconds) was 166.26 seconds longer than that for AMBP (5913.74 ± 5536.23 seconds). The relationship between the CTV and △V5 is shown in [Figure 3] (Pearson's correlation coefficient was 0.05 and P = 0.87; △V5 equaled the value of V5 in the VSBP minus the value of V5 in the AMBP). The positive value of △V5 indicates that the lung was exposed to more radiation doses in VSBP.
Table 2: Dose parameters for comparing the median of the two methods (Original Data)

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Figure 3: The scatter diagram for the clinical target volume and △V5. Pearson correlation coefficient was 0.05 and P = 0.87; the positive value of △V5 means the lung was exposed to more radiation doses in virtual single-dwell-position interstitial brachytherapy plan. (Original Image)

Click here to view



 » Discussion Top


For implementing the dose gradient inside the target area or to make the high-dose curve more suitable to the target volume, the number of needles used should be based on the size of the target area, and needle alignment should be parallel, with equal intervals.[7],[8] However, because of the influence of rib and respiratory movements, a regular alignment of needles by hand is difficult to achieve for a lung tumor. In this situation, the use of needles with course intersections is common. However, this may not only hamper the desired dose result but also increase the operative risk. Thus, it is important, but difficult, to decide on the procedure to implant needles and the number of needles to be used, as scarce literature is available on the topic.

In clinical practice, experts limit the application of a single needle to target volume and size. Ricke et al.[11] studied 15 patients with 30 lesions. The minimum diameter of the tumors was 5.5 cm for multiple needles, and nine needles were used for a lesion that was 11.0 cm in diameter. Similarly, Peters et al.[12] studied 30 patients with 83 lesions and used one needle for a lesion ≤4.0 cm. Sharma et al.[13] used a single needle for lesions up to 4.0 cm and two needles for lesions up to 6.0 cm in diameter. Tselis et al.[5] collected data on 55 cases and used multiple needles in larger or nonspherical tumors. They proposed the concept of using a single central needle for spherical lesions up to 3.0 cm in diameter, but they did not report on how to locate the active source of the dwell position. Whether the actual result of multiple-needle arrangement strictly followed the rules, was not specified by the experts.

In the CT-guided PIBT for thoracic tumors, each implantation of the needle mainly includes three steps:[6] Step 1, perform the first CT scan in order to preset the direction, depth, and angle of the implanted needle; step 2, needle implantation; step 3, perform CT scans again to check whether the preset of the needle position is met (if the needle position needs to be adjusted, another CT scan is required, and then step 3 might be repeated). Therefore, in clinical practice, multiple needle implantation has the following shortcomings: It carries higher operational risks, causes more radiation damage in the patient because of more CT scanning, lengthens implantation time, and increases patient suffering time.

In our study, the dose differences were not statistically significant between the AMBP and the VSBP; this suggests that irregularly arranged multiple-needle interstitial brachytherapy does not provide superior doses to the lung, heart, spinal cord, or ribs, when compared with single-dwell-position plans. Especially for spherical tumors, the dose advantages of the VSBP were obvious because the dose distribution at a single point was spherical, which is in accordance with the tumor volume shape. Therefore, we think that, in this case, multiple needles are not a key factor to reduce the dose of lung exposure for large CTV. As shown in [Figure 3], a positive value of △V5 indicated that the lung was exposed to more radiation doses in VSBP, and the low correlation between the CTV and △V5 might mean that, with the increase in CTV, multiple needles may not always reduce the dose of lung exposure as compared with the single-needle scheme. Correspondingly, the large volume target area might not be a forbidden area for the single-needle scheme.

Although the irradiation time for AMBP is shorter, the multiple-needle scheme might take more time for preparation and operation. Moreover, the duration of patient treatment is extended, and the risk of complications, such as hemopneumothorax, might increase because of the operation. The irradiation time is directly proportional to the PD and source strength. Thus, high-intensity source strength and lower prescribed doses allow for a shorter irradiation time. In clinical reports, <30 Gy in one fraction was generally adopted as the PD.[14],[15],[16] Therefore, if 30 Gy is used as the PD, the actual increase in the irradiation time seems to be unimportant, as expected.

In this study, the prescribed dose was 100 Gy mainly on the basis of two considerations: 1) if 30 Gy was used as the PD, V30 could not be easily measured to facilitate the comparison of parameters; 2) relevant literature indicates that the local recurrence rate can be reduced by increasing the radiation dose to the target area—when the biological dose exceeds 100 Gy, the local control rate reportedly reaches 80%.[17],[18],[19],[20] A Radiation Therapy Oncology Group gradient test for the radiation dose in 3D conformal radiotherapy showed that the dose in the target area should be >83.8 Gy.[21] Therefore, with the development of radiotherapy technology, the PD for the target area might be further increased depending on normal tissue tolerance.[22]

To achieve a uniform target dose distribution (reduced high dose) and a conformal distribution to the target, multiple needles are recommended in the ICRU report 58. However, the outcome of the expectation is not obvious, owing to the irregular arrangement of multiple needles; the statistical difference of D50, which represents the high-dose volume, is not observed. In addition, the high dose always surrounds each source positioned in the target area; thus, a completely flat dose distribution without dose gradients is impossible. Therefore, the high-dose area (hot volume) near the center of solid tumors could be acceptable.

As the regular alignment of needles is important for interstitial brachytherapy, experts are seeking the optimal method for arrangement of multiple needles. In125 I-seed implants, a 3D printing template was adopted for guiding the implantation of needles. With the help of the 3D printing template, parallel arrangement of multiple needles and the exact localization of the implantation could be achieved.[23],[24] However, no successful technology has been reported for the Ir-based interstitial brachytherapy for thoracic tumors.

It should be noted that the single-needle scheme used in this study does not refer to an actual single needle. When implementing the single-needle scheme by hand, the interference of ribs and motion still exists. However, it is worth a try, because the available rib interspace for applying the single-needle scheme would be more than that for the multiple-needle scheme. Moreover, if the target volume is spherical, a successful single-needle scheme might reduce the operative risks, and the dose distribution might be superior to that of the irregular multiple-needle scheme. The extent of the effect of irregular needle arrangement on dose distribution requires further research with more cases.

Based on our experience with cases involving one needle pass through the center of the CTV, which were not included in this study, we have the following suggestions: When the needle must be inserted at an angle, a surgical assistant who specializes in needle insertion should be present; the surgical assistant should provide feedback to the surgeon regarding the direction of the needle during implementation. In addition, when the insertion needle approaches the tumor surface, the patient should be allowed to breathe smoothly, and we recommend that the tumor be pierced quickly. However, research on the practical application of the single-needle scheme is beyond the scope of the current study and will be tested in future studies.


 » Conclusion Top


The irregularly arranged multiple-needle scheme for interstitial brachytherapy does not provide superior doses to the lung, heart, spinal cord, or ribs compared with the single-dwell-position plans. Whether multiple needles should be applied might not be based on the target volume and size; rather, it is based on whether regular alignment of needles could be achieved. If achieving a regular arrangement of multiple needles is difficult, the possibility of implementing a single-needle scheme should be considered, regardless of tumor size.

Acknowledgements

We would like to express our gratitude to the Research Foundation of Health and Family Planning Commission of Sichuan (the grant number: 18PJ202) and Medical Research Projects of Sichuan (the grant number: S19007).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

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    Tables

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