Original Article
Comparison of ICRU Point Doses and Volumetric Doses to the Rectum during High Dose Rate Cobalt-60 Brachytherapy of Cervical Cancer
1Yun, S. Y., 1,2Jamalludin, Z., 1Abdul malik, R. and1,*ung, n. m.
ABSTRACT
Brachytherapy is an important component of radiotherapy for cervical cancer treatment. High dose rate (HDR) intracavitary brachytherapy (ICBT) is one of the currently used techniques for cervical cancer treatment. The volume-based brachytherapy planning method recommended in the International Commission on Radiation Units and Measurements (ICRU) report No. 89 is believed to be more useful compared to the conventional point-based method recommended in ICRU report No. 38 in terms of organs at risks (OARs) dosimetry. However, more resources in terms of manpower and time are required to implement volume-based brachytherapy planning. This study aims to evaluate the point-based and volume-based methods for rectal dose estimation in HDR Cobalt-60 (Co-60) brachytherapy of cervical cancer. A total of 150 patient plans receiving full insertion HDR ICBT treatment in University of Malaya Medical Centre (UMMC) from 2017 to 2019 were retrospectively selected. The patients’ data including rectal volume and rectal dose parameter calculated using both point-based and volume-based techniques were extracted from the brachytherapy treatment planning system. Data were compared and analysed for possible correlations between rectal point dose (ICRUr) and rectal volumetric doses. The ICRUr was shown to have no significant difference with the most exposed rectal volumes of 2 cm3 (D2cc) (p=0.960). The results of correlation analyses indicated strongest correlations between ICRUr versus D1cc and ICRUr versus D2cc with the correlation coefficients of 0.674 and 0.669 (p<0.05) respectively.
The findings of the study fit with the existing theory that the volumetric doses can be explained well by the ICRU point dose with considerable significant correlations. The dose to ICRUr can be used to estimate volumetric dose to the rectum, specifically the D2cc for treating institutions that are practicing point-based planning for ICBT of cervical cancer.
Keywords: ICRU point dose, volumetric dose, rectal dose
1Clinical Oncology Unit, Faculty of Medicine, University of Malaya, 50603, Kuala Lumpur, Malaysia
2Medical Physics Unit, University of Malaya Medical Centre, Kuala Lumpur, 59100, Malaysia
*Corresponding Author: Ung, N. M.
Email: [email protected] Tel: +603-79493010 Fax: +603-79563072 Received: 7 July 2021 Revised: 30 August 2021
Accepted for publication: 1 September 2021
Publisher: Malaysian Association of Medical Physics (MAMP) http://www.mamp.org.my/
https://www.facebook.com/MedicalPhysicsMalaysia
INTRODUCTION
Cervical cancer is the world fourth most common cancer in women (WHO 2013). Intracavitary brachytherapy (ICBT) is a technique whereby the applicators are placed into the cavity of the patient’s uterus with radionuclides in a number of planned dwell positions.
The dose to the organs at risks (OARs) in cervical brachytherapy is a hot topic for discussion. In brachytherapy of cervix, the rectum is one of the most concerned OARs due to its close proximity with the target volume.
The conventional method of dose estimation to the rectum and other OARs during cervical cancer brachytherapy is based on point dose as recommended by the International Commission on Radiation Units and Measurement (ICRU) report No. 38 (ICRU 38).
This method determines rectal doses on a specific representative orthogonal radiograph or on computed tomography (CT) images. The more and current and advanced method for rectal dose determination is the volume-based method recommended by ICRU report No. 89 (ICRU 89). The volumetric doses are estimated from the dose-volume histogram (DVH) through the advanced uses of three-dimensional planning for ICBT.
Although the new method described in ICRU report No.
89 provides a better dose estimation (Wibowo and Haris
2017), the rectal point dose as specified by ICRU report No. 38 is still important and used these days especially for reporting purposes. The dose determination for rectum in volumetric based planning is performed on CT images (sometimes aided by MR images) and the process is more resource-intensive and complicated compared to the process in which rectal point dose is determined.
This study aims to determine and compare rectal doses based on the point-based and volumetric methods.
It evaluates the possible association between the point-based versus volume-based technique in the brachytherapy of cervical cancer. By performing the correlation between rectal point and volumetric doses, the question of how well the rectal volumetric dose can be predicted or explained by rectal point dose can be answered. This is important to investigate the relevance of point-based method in the era of CT imaging for brachytherapy. In addition, this study evaluates the relationship between rectal volume and rectal dose parameters to investigate if the rectal dose parameters can be explained by rectal volume. The results of this part of the study may contribute to the bowel preparation protocol during ICBT of cervical cancer.
EXPERIMENTAL METHODS
This study was approved by the Medical Ethics Committee (MEC) of University of Malaya Medical Centre (UMMC) (MREC ID no: 2017123-4840). A total of 150 patient’s planning data with cervical carcinomas receiving full insertion HDR ICBT treatment in UMMC from 2017 to 2019 were selected in this study.
Before the brachytherapy treatment, each patient was first given external beam radiotherapy (EBRT) with a standard dose of 48.6 Gy in 27 fractions for 6.5 weeks to the pelvic based on the protocol used in Clinical Oncology Unit, UMMC. Four-field box technique with 10 MV photon energy to the pelvic region was used to allow the dose distribution to cover the whole treatment region.
Following the EBRT, full insertion HDR ICBT was given at week 4, 5 and 6 with the prescribed dose of 7 Gy for each fraction using Cobalt-60 (Co- 60) as the HDR source. Brachytherapy was performed using an Eckert & Ziegler BEBIG MultiSource® remote after- loader unit model 1322-0012 (Eckert & Ziegler, Berlin, Germany), which housed a single Co-60 source (model Co0.A86) emitting average gamma energy 1.25 MeV.
During brachytherapy treatment planning, the rectal volumes were delineated based on the GEC-ESTRO recommendations, which began at anorectal junction and ended at the rectosigmoid flexure (Haie-Meder et al. 2005). Treatment planning was performed in
both 2D and 3D and the related data including OARs volume and doses were recorded and analysed using the IBM SPSS (IBM SPSS Statistics for Windows, Version 23.0; IBM Corp, Armonk, NY, USA). More detailed descriptions on planning for brachytherapy in our institution can be found in our previous published study (Jamalludin et al. 2021).
The treatment data including the rectal volumes, ICRU point and volumetric doses were then retrospectively collected to be used for analysis. All data were extracted from the HDRplus TPS version 2.6 (Eckert & Ziegler BEBIG GmbH, Germany). For volume-based treatment planning, the doses to most exposed volumes of 0.1 cm3, 1 cm3, 2 cm3 and 5 cm3 (denoted as D0.1cc, D1cc, D2cc, and D5cc) for the rectum were recorded. In addition, the volume of the rectum was also recorded. For point-based treatment planning, only the ICRU point dose (denoted as ICRUr) to the rectum was extracted for analysis.
RESULTS AND DISCUSSION
The collected data from 150 cervical cancer brachytherapy planning including rectal volumes and rectal point and volumetric doses are shown in Fig. 1.
The maximum volume recorded was 85 cm3 and the minimum volume was 25 cm3. The range of the rectal volumes among the 150 patient’s treatment planning was 60 cm3 without considering any outlier. The median volume recorded was 50 cm3, with the first and third quartiles of 20 cm3 and 40 cm3 respectively.
The D0.1cc had the largest range of about 7 Gy followed by ICRUr, D1cc, D2cc, and D5cc with the ranges of 5.5 Gy, 4.5 Gy, 3.5 Gy, and 3 Gy respectively. The D0.1cc recorded the largest value of 60 Gy. The D1cc, ICRUr and D2cc recorded approximately the same median of 4.5 Gy. The D5cc yielded the smallest median dose with the value of 4 Gy. The D0.1cc and D5cc resulted in the largest and smallest interquartile range, with values of 2.0 Gy and 1.0 Gy respectively.
COMPARISON OF ICRU POINT DOSE AND VOLUMETRIC DOSES
The normalities of all groups of data collected were first investigated using Kolmogrov-Smirnov (K-S) test the normality of data. The results revealed that data for all groups of point and volumetric doses were not normally distributed (p<0.05). The Wilcoxon-signed rank test was then used to test for differences between point doses versus volumetric rectal dose groups.
Table 1 shows the results of the comparison with the respective p-values.
TABLE 1 Summary of the p-values of the Wilcoxon-signed rank test on rectal dose comparison between ICRUr and volumetric dose parameters
Compared groups Sig (p-value)
ICRUr vs. D0.1cc <0.05
ICRUr vs. D1cc <0.05
ICRUr vs. D2cc 0.960
ICRUr vs. D5cc <0.05
The Wilcoxon-signed rank test showed that all rectal volumetric dose groups were statistically different from the ICRUr except D2cc (p=0.960). This result confirmed that D2cc was the best volumetric dose to use to study their relationship with or predicted by the ICRUr. This result was in agreement with the results of several other studies on such comparison. Srivastava et al. performed a comparison of ICRUr and volumetric doses to OARs in cervical cancer brachytherapy of 20 patients. The ICRUr was not significantly different from the D2cc (p=0.964) (Srivastava et al. 2019). Another study compared ICRUr and volumetric doses based on the comparisons of means using the Kruskal-Wallis Test (non-parametric ANOVA). The rectum was the only OAR which was resulted in no significant difference between mean ICRUr and volumetric doses with values of 4.01 and 4.28 Gy respectively (Vinod et al. 2011).
CORRELATION ANALYSIS
The correlations analysis between rectal volume and rectal dose parameters were performed to investigate if the sizes or volume of rectum could have a significant impact on rectal doses during cervical brachytherapy. Fig. 2 shows the scatter plot of rectal volume versus ICRUr as well as volumetric dose parameters of D0.1cc, D1cc, D2cc, and D5cc. The plots are shown with the aid of regression lines to help in identifying their relationships.
The correlations between rectal volumes versus rectal doses resulted in significant correlations to all rectal dose groups (p < 0.05). However, the correlation coefficients for all groups were weak. The strongest correlation occurred in the correlation of rectal volume with D5cc with the correlation coefficient of 0.397 followed by D2cc, D1cc, D0.1cc and ICRUr with correlation coefficients of 0.384, 0.371, 0.351 and 0.184 respectively.
Fig. 3 show the scatter plots of correlations between ICRUr with the volumetric dose parameters of D0.1cc, D1cc, D2cc, and D5cc. The strongest correlation occurred between ICRUr and D1cc with the correlation coefficient of 0.674 (r=0.674) followed by D2cc, D5cc and D0.1cc with the correlation coefficients of 0.669, 0.650 and 0.617 respectively. The strongest correlation obtained in this study was D1cc followed by D2cc. Further analysis revealed that these two parameters were highly correlated with a correlation coefficient of 0.988 (p<0.05). Therefore, both D1cc and D2cc can be good indicators of volumetric doses which had a good relationship with ICRUr. However, D2cc is a better choice of metric to represent rectal dose as the volume of D1cc is extremely small and thus, poses some uncertainties in its determination. In addition, it was shown on the results from the previous section that the dose to ICRUr is significantly different from D1cc.
The results of comparison of ICRUr and volumetric doses that were significantly correlated and the common FIGURE 1 (a) The boxplot shows the rectal volumes of 150 cervical HDR ICBT
treatments in UMMC, (b) the boxplot of ICRUr and volumetric doses
FIGURE 2 Scatter plots of rectal volumes versus (a) ICIUr (b) D0.1cc (c) D1cc (d) D2cc (e) D5cc
FIGURE 3 Scatter plots of ICRUr versus (a). D0.1cc. (b). D1cc. (c). D2cc. (d). D5cc
best-correlated was between ICRUr and D2cc has also been concluded in several studies. Yaparpalvi et al.
study was a retrospective study of 10 patients involving 30 individual plans combining ICBT with EBRT.
The best correlation occurring between ICRUr and volumetric dose of D2cc with correlation coefficient r=0.91 (p=0.0003) (Yaparpalvi et al. 2008). Patil et al. studied the prediction of volumetric dose by ICRUr and ICRUb defined by point-based on the analysis of 150 CT based scanning of cervix treatment among 80 patients underwent two sessions of HDR ICBT treatment. The results of this study suggested that the strongest correlation was with the D2cc (r=0.77, p<0.001) (Patil et al. 2011). Vinod et al. compared point and volumetric doses to OARs in cervical cancer brachytherapy treatment of 20 patients receiving brachytherapy treatment based on CT-MRI images planning. There was a significant correlation between point and volumetric dose to the rectum and bladder with correlation coefficients of 0.76 (p=0.0001) and 0.78 (p=0.01) respectively for D2cc (Vinod et al. 2011).
Our results showed that doses to ICRUr were significantly correlated to doses to D1cc and D2cc of the rectum. This implies that volumetric dose to the rectum during ICBT of cervical cancer can be reasonably estimated from the ICRUr. This is a favorable result to treating institutions that still practice point-based method in planning or have challenges moving on to volume-based planning which is more resource intensive in terms of time, facilities and manpower. However, 3D volumetric planning is the preferred method for brachytherapy as it permits full appreciation and utilisation of the intrinsically 3D nature of irradiation. Training to personnel involved in brachytherapy, specifically the radiation oncologist, medical physicist as well as dosimetrist is warranted for transitioning from point-based planning to volume- based planning for ICBT of cervical cancer.
CONCLUSION
There was no statistical difference between the mean ICRUr and the most exposed D2cc (p=0.960). The correlations between ICRUr and volumetric doses were all statistically significant. The highest correlation was between the ICRUr versus D1cc (r=0.674) and ICRUr versus D2cc (r=0.669). The findings of the study fit with the existing theory that the volumetric doses can be explained well by the ICRU point dose with considerable significant correlations. The dose to ICRUr can be used to estimate volumetric dose to the rectum, specifically the D2cc for treating institutions that are practicing point- based planning for ICBT of cervical cancer. The results
also build on the existing evidence that rectal volume has very little impact on rectal dose parameters. Based on this, it is envisaged that the bowel preparation protocol minimally affects the dose received by the rectum during brachytherapy.
ACKNOWLEDGEMENTS
We acknowledge the Clinical Oncology Unit, UMMC for the facilities, assistance and support provided to conduct the research. The first author would like to thank IAEA for the support in terms of an academic scholarship.
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