Dosimetric effects of the supine and prone positions in proton therapy for prostate cancer

Takahiro Kato; Masato Kato; Kimihiro Takemasa; Masao Murakami

doi:10.4103/jmp.jmp_85_22



ORIGINAL ARTICLE

Year : 2022 \| Volume : 47 \| Issue : 4 \| Page : 331-335

Dosimetric effects of the supine and prone positions in proton therapy for prostate cancer

Takahiro Kato¹, Masato Kato², Kimihiro Takemasa², Masao Murakami³
¹ Department of Radiation Physics and Technology, Southern Tohoku Proton Therapy Center; Department of Radiological Sciences, School of Health Sciences, Fukushima Medical University, Fukushima, Japan
² Department of Radiation Physics and Technology, Southern Tohoku Proton Therapy Center, Fukushima, Japan
³ Department of Radiation Oncology, Southern Tohoku Proton Therapy Center, Fukushima, Japan

Date of Submission	18-Sep-2022
Date of Decision	06-Nov-2022
Date of Acceptance	07-Nov-2022
Date of Web Publication	10-Jan-2023

Correspondence Address:
Dr. Takahiro Kato
Department of Radiation Physics and Technology, Southern Tohoku Proton Therapy Center, 172 Yatsuyamada 7 Chome, Koriyama, Fukushima 963-8563
Japan

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jmp.jmp_85_22

Abstract

Purpose: To quantitatively evaluate how much the doses to organs at risk are affected in the prone position compared to the supine position in the proton therapy (PT) for prostate cancer. Materials and Methods: Fifteen consecutive patients with clinically localized prostate cancer underwent treatment planning computed tomography scans in both the supine and prone positions. The clinical target volume (CTV) consisted of the prostate gland plus the seminal vesicles. The PT plans were designed using the standard lateral opposed fields with passively scattered proton beams for both treatment positions. The prescribed dose for each plan was set to 78 Gy (Relative biological effectiveness)/39 fractions to 50% of the planning target volume. Dose-volume metrics of the rectum and bladder in the two treatment positions were analyzed. Results: It was confirmed that all the parameters of D₀₅, D₁₀, D₂₀, D₃₀, D_mean, and V₉₀ examined in the rectum were significantly reduced in the prone position. There was no significant difference between the two positions in the bladder dose except for D_mean. The distance between the CTV and the rectum tended to increase with the patient in the prone position; at the prostate level, however, the maximum change was approximately 5 mm, and there was significant variation between cases. Conclusions: We confirmed that the rectal doses were significantly lower in the prone compared with the supine position in PT. Although uncertain, the prone position could be an effective method to reduce the rectal dose in PT.

Keywords: Prone position, prostate cancer, proton therapy, supine position, treatment position

How to cite this article:
Kato T, Kato M, Takemasa K, Murakami M. Dosimetric effects of the supine and prone positions in proton therapy for prostate cancer. J Med Phys 2022;47:331-5

How to cite this URL:
Kato T, Kato M, Takemasa K, Murakami M. Dosimetric effects of the supine and prone positions in proton therapy for prostate cancer. J Med Phys [serial online] 2022 [cited 2023 Mar 24];47:331-5. Available from: https://www.jmp.org.in/text.asp?2022/47/4/331/367431

Introduction

External beam radiation therapy for prostate cancer is recognized as a standard treatment and is widely employed.^[1] The dose must be concentrated on the target while also reducing the dose to organs at risks (OARs), such as the rectum and bladder. Reducing the rectal dose has been an issue, and various methods have been proposed to achieve this. High-precision irradiation techniques such as intensity-modulated radiation therapy and proton therapy (PT) are typical methods; however, no matter which irradiation technique is used, some compromise is required if the target and the OAR are in close proximity. The most effective means to solve this issue is to separate them as much as possible. In recent years, a hydrogel spacer insertion technique has become a commonly used method for achieving this.^{[2],[3],[4],[5]} This method places hydrogel between the prostate and the rectum, and its use is rapidly increasing because the distance between the prostate and rectum can thus be securely maintained.

In the field of photon radiation therapy; however, treatment in the prone position has been adopted as a standard by some institutions as a means of increasing the distance between the prostate and the rectum.^[6],[7],[8] Prone position irradiation is considered to be highly versatile because it can be easily and noninvasively performed. Hydrogel spacer insertion has restrictions, such as nonindication in cases with extracapsular infiltration, whereas there are few such clinical restrictions in the prone position. In addition, since there are cases in which the hydrogel spacer technique itself is refused by the patients and problems associated with the procedure are not completely eliminated,^[9] prone position irradiation is worth considering. However, there have been no reports to date on prone position irradiation in PT, and there remain many unclear points about the specific effect. The purpose of this study was therefore to quantitatively evaluate how much the dose to the OARs is affected in the prone position compared with the supine position in PT for prostate cancer.

Materials and Methods

Patients and structure delineation

Fifteen consecutive patients with clinically localized prostate cancer underwent treatment planning computed tomography (CT) scans in both the supine and prone positions. The mean age was 72 years (range: 60–89 years). Our institutional review board approved this study. Thirty to sixty minutes before the treatment planning CT scan, the patients were instructed to drink 200 ml water to ensure that the bladder was comfortably full. The patients were also instructed to defecate and exhaust gas as much as possible before the examination. An Aquilion LB (Canon Medical Systems, Otawara, Japan) was used for the CT scans, and images were taken in 2-mm slices. As a patient immobilization device, a vacuum cushion was used to immobilize the patients' legs in both treatment positions. The supine scan was performed initially, with the prone scan following immediately. The CT images were transferred to the XiO-M treatment planning system (Hitachi, Kashiwa, Japan), where the clinical target volume (CTV) and OARs, such as the rectum and bladder, were delineated by a single physician to minimize potential bias associated with multiple planners when comparing treatment plans. The CTV consisted of the prostate gland plus the seminal vesicles. The planning target volume (PTV) included the CTV plus a 7-mm safety margin, except at the prostate gland–rectum interface, where a 6-mm margin was used to decrease the risk of rectal toxicity. The craniocaudal rectal extension was defined as ± 10 mm from the range where the PTV was delineated.

Treatment planning

The PT plans were generated by the XiO-M using the beam data from Hitachi's proton-type particle therapy system (Hitachi, Kashiwa, Japan). Two treatment plans (supine position and prone position) were generated for each patient. The irradiation method was the wobbler method, which is a passive scattering method.^[10] The PT plans were designed using our standard lateral opposed fields with 210 MeV proton beams. The key parameters for passive scattering PT plans are distal, proximal, lateral, and smearing margins. Most of the planning parameters are selected using the methods described by Moyers et al.^[11] The compensator was designed for the CTV using a custom distal margin that included a 3.5% depth to account for uncertainty for CT number accuracy and conversion to proton relative linear stopping power and a 3-mm range of uncertainty to take into account uncertainties in the accelerator energy, variable scattering system thickness, and compensator density. The radiation field was formed using a multi-leaf collimator built into the snout. The prescribed dose for each plan was set to 78 Gy (Relative biological effectiveness)/39 fractions to 50% of the PTV. Given that the purpose of this study was to compare the relative risk organ doses in the supine and prone positions, we did not make individual fine adjustments as performed in actual clinical practice. Therefore, it should be noted that the OAR doses could have been higher than usual in some cases.

Analysis

The analysis was performed using a dose–volume histogram (DVH), and a comparative study was performed using rectal D₀₅, D₁₀, D₂₀, D₃₀, D_mean, and V₉₀ as indexes. D_n represents the dose at which n% of the volume of each OAR is irradiated. Furthermore, V₉₀ means the percent of the OAR volume receiving 90% of the prescribed irradiation dose. For V₉₀, when the relative quantity is displayed (% display), the error due to contour delineation and the volume change due to the change of posture could affect the analysis result, so the absolute value was displayed. The same analysis was performed not only for the rectum but also for the bladder, which is one of the OARs. A Wilcoxon test was used to analyze the difference between the supine and prone positions in each dose parameter. P < 0.05 was considered statistically significant. All statistical analyses were performed using R version 4.1.2 (the R Foundation for Statistical Computing, Vienna, Austria).

Results

The accuracy of structural delineation directly influenced the results of this comparative study; thus, confirmation is important. No significant differences in volumes were observed except for the bladder [Table 1]. [Figure 1] shows the increase in distance between the posterior edge of the CTV and the anterior rectal wall with a change in the treatment position from supine to prone measured at seminal vesicle and prostate levels. It was confirmed that the increases in distance were 5.7 ± 8.4 (−7.2–23.0) mm at the seminal vesicle level and 2.3 ± 3.9 (−2.1–13.1) mm at the prostate level. As for the overall tendency, the distance between the prostate and the rectum tended to be separated by the prone position; however, the distance was <5 mm. In some cases, the prone position increased the distance at the seminal vesicle level. There were some cases with a separation of 10 mm or more; however, there was a large variation between the cases. [Table 2] shows the results of the summarized rectal and bladder parameters. The rectal dose index showed a significant difference in all measurements, whereas the bladder did not show a significant difference except for D_mean. [Figure 2] and [Figure 3] show the dose distribution and DVHs of a typical case, respectively. In this case, rectal V₉₀ was found to have a 60% reduction effect in the prone position compared with the supine position.

Figure 1: Increase in the distance between the posterior edge of the CTV and the anterior rectal wall in 15 patients due to changes in the treatment position from supine to prone measured at seminal vesicle and prostate levels. Positive values indicate displacement of the CTV away from the rectum when the patient is in the prone position compared to the supine position. CTV: Clinical target volume

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Figure 2: Isodose distribution of central axis for supine (left) and prone (right) PT plans in a typical case. The dose in each figure is relative to the prescribed dose of 78 Gy (RBE) (i.e., 100% = 78 Gy[RBE]). The isodose levels shown in each figure are red, 100%; green, 90%; blue, 80%; yellow, 50%; light blue, 20%, RBE: Relative biological effectiveness

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Figure 3: Representative normalized, cumulative DVH comparison in a typical case shown in Figure 1. DVH for PTV (orange), rectum (green), and bladder (yellow) of the supine (solid line) and prone (dashed line) treatment positions. DVH: Dose–volume histogram, PTV: Planning target volume, RBE: Relative biological effectiveness

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Table 1: Structure volume information for the two treatment positions (n=15)

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Table 2: Dose-volume metrics of supine and prone proton therapy plans for the rectum and bladder

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Discussion

Several reports have found that the rectal dose can be reduced by setting the irradiation position to prone in external irradiation of prostate cancer;^[6],[7],[8] there have been no reports analyzing the effect of prone position irradiation in PT. To the best of our knowledge, this study is the first to quantitatively evaluate how much the PT dose to the OARs is affected in the prone position compared with the supine position in PT for prostate cancer.

We confirmed that the rectal dose can be significantly reduced in the prone position compared with the supine position as reported in photon radiation therapy. However, we also clarified that the effect varies greatly among individuals, which could occur because the distance between the prostate and the rectum differs from case to case when the patient is in the prone position. One report found that the hydrogel spacer insertion technique achieves a distance of 7–10 mm between the prostate and the rectum.^[4] Hydrogel spacer insertion is currently performed in the supine position at our institution, but it has been confirmed that a distance of approximately 10 mm can almost certainly be secured if properly inserted. By contrast, in the prone position, it was 5 mm or less in most cases. From the point of view of certainty that the distance between the prostate and the rectum can be secured, the spacer insertion technique was considered to be superior to the prone position irradiation. It would be of great interest to examine how the rectal dose differs between the hydrogel spacer insertion and the prone position. However, since the number of data that can be analyzed is not sufficient at present, we would like to make it an issue for future study.

In 1 of 15 cases, the rectal dose increased in the prone position compared with the supine position. For cases in which the prostate is protruding into the bladder in the supine position, the distance between the CTV and the rectum becomes shorter as the bladder shifts to the ventral side in the prone position. It was considered that the prone position should not be adopted in such a case. However, in 1 case, we confirmed that the distance between the CTV and rectum was as large as 20 mm, especially at the seminal vesicle level. Maintaining the distance between the seminal vesicles and the rectum by hydrogel spacer insertion is difficult.^[3] In contrast, in the prone position, the distance can be maintained at the seminal vesicle level. Therefore, the prone position has still been worth considering in cases where the seminal vesicles were present around the rectum and in the high-risk group, where the CTV had to contain the seminal vesicles. At present, it is technically difficult to separate the seminal vesicles from the rectum with a hydrogel spacer, so combining the prone position and the hydrogel spacer insertion is worth considering for the high-risk group.

The bladder was also analyzed, but we found there was no clear benefit because the prone position did not separate the CTV from the bladder as it did in the rectum. This result was consistent with previous reports of photon radiation therapy.^[6],[7],[8] The D_mean of the bladder showed a significant difference between the two positions, which could be due to the order of CT imaging. Given that the CT was taken later in the prone position, the volume of the bladder was significantly greater, which appeared to indicate that the D_mean was lower in the prone position. Although there was a significant difference in bladder volume between the supine and prone positions, the absolute difference was sufficiently small. Therefore, the effect of the difference in bladder volume itself on the shape and position of the rectum was considered to be quite small. To ensure a comfortably full bladder, the patient was instructed to drink 200 ml of water 30–60 min prior to the treatment planning CT scan performed in this study. However, the bladder volume tended to be low generally; this may be because the waiting time between the patient drinking water and undergoing CT scan may be insufficient. In such a situation, instructing the patient to be in the prone position on a flat table top is unlikely to be a serious problem, however, in case of a completely full bladder, it is necessary to consider actively using a belly board.^[12],[13] In addition, although it was not used in our patients, it will be necessary to consider the use of a belly board to place a patient with a large belly in the prone position. Results may vary depending on whether or not a belly board is used, but even previous reports of photon radiation therapy have not been consistent,^[6],[7],[8] so caution is required in interpreting.

This study has several limitations. First, only 15 patients were evaluated. However, even with this small number of cases, we were able to determine significant differences in dose-volume indexes between supine and prone positions in PT. Second, dose–volume indexes were only evaluated by the passive scattering method of the standard lateral opposed fields and not evaluated by the latest pencil beam scanning (PBS) method. The principal limitation here is that it is difficult to form a concave dose distribution with the passive scattering method of the standard lateral opposed fields. However, the passive scattering method still exists in a certain proportion, and if the results of this study are interpreted appropriately, it is thought that there are many points that can be applied to the PBS method. Therefore, the significance of this research is considered to be not small. There is a possibility that the rectal dose at the seminal vesicle level could be further reduced by using intensity-modulated PT,^[14] which will be a topic for future study. Third, intrafractional and interfractional variations were not fully considered in both treatment positions. The internal organ motion in this study was assumed to be the same between both treatment positions, as reported by Stroom et al.;^[15] however, there have also been reports that the positional accuracy of both is actually different.^[16],[17] In addition, PT is sensitive to changes in the beam path, and Trofimov et al. reported the effect of reproducibility of the femoral head.^[18] There is a lack of information on the extent to which the reproducibility of the femoral head differs between the two positions, however, this point is also considered to be an ongoing issue in the future. The positional accuracy depends on the patient fixation method and the image-guided method, so it might not be uniquely determined. Although treatment in the prone position has been used for a relatively long time in photon radiation therapy, it might be worthwhile to continue employing it given that the prone position could have new usefulness in PT in combination with the latest technology.

Conclusions

We quantitatively evaluated the rectal dose reduction effect of the prone position in PT for prostate cancer. It was confirmed that all the parameters of D₀₅, D₁₀, D₂₀, D₃₀, D_mean, and V₉₀ examined in the rectum were significantly reduced in the prone position. Focusing on rectal V₉₀, there was a case in which a maximum reduction effect of 60% was observed by placing the patient in the prone position, while there was also a case in which it increased by 7%, indicating a large variation between cases. We conclude that the prone position might be a method to reduce the rectal dose in PT; however, further research is needed to confirm this hypothesis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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