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Dosimetric study of CO-60 source step size in uterine cervix intracavitary HDR brachytherapy
Brachytherapy
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(2019)
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Dosimetric study of CO-60 source step size in uterine cervix intracavitary HDR brachytherapy Arvind Kumar Shukla1,2, Bhupendra Singh Rana1,3,*, Narinder Paul Singh1, Sanjeev Kumar4 1
Department of Applied Sciences, IK Gujral Punjab Technical University (PTU), Jalandhar, India 2 Department of Radiotherapy, RNT Medical College, Udaipur, India 3 Department of Radiodiagnosis, PGIMER, Chandigarh, India 4 Department of Physics, G.G.D.S.D. College, Chandigarh, India
ABSTRACT
PURPOSE: The present work reports effects of source step sizes on dose distribution in patients treated with cobalt-60 (Co-60) high-dose-rate afterloading brachytherapy in carcinoma cervix (Ca-cx). METHODS AND MATERIALS: The retrospective study is based on data of 15 patients of Ca-cx treated with Co-60 high-dose-rate intracavitary brachytherapy with dose of 21 Gy in three fractions with source step size of 2.5 mm after external beam radiotherapy of 46 Gy. The effect of source step size on overall treatment procedure was evaluated from prescribed dose volume, dose to organ at risks, and treatment time for source step sizes of 1 mm, 2.5 mm, 5 mm, and 10 mm for each patient. RESULTS: The mean dose to bladder point for 1 mm, 2.5 mm, 5 mm, and 10 mm source step sizes was found to be 3.37 Gy (SD: 1.36), 3.44 Gy (SD: 1.38), 3.54 Gy (SD: 1.41), and 3.74 Gy (SD: 1.46), respectively. Similarly, the mean dose received by rectum point for these source step sizes were 2.86 Gy (SD: 0.64), 3.02 Gy (SD: 0.67), 3.25 Gy (SD: 0.71), and 3.63 Gy (SD: 0.73), respectively. The treatment time and prescribed dose coverage volume were both found to be gradually increasing with increase in step size. CONCLUSIONS: Our results on Ca-cx brachytherapy using Co-60 source indicate that the prescribed dose volume gradually increases from smaller source step to larger source step size. This results in increase of dose to the bladder and rectum and may lead to increase in toxicity and reduces quality of life. The study recommends that step size more than 5 mm should not be used for uterine cervix intracavitary application using Co-60 source. Ó 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.
Keywords:
Step size; Intracavitary brachytherapy; High dose rate; Computed tomography
Introduction Carcinoma of uterine cervix (Ca-cx) is the second most common cancer among women in countries with low and medium human development index [1], which can be treated with external beam radiotherapy and intracavitary brachytherapy. Brachytherapy can be used for successful treatment of cancer of the prostate, esophagus, head and neck, and cervix. High-dose-rate (HDR) brachytherapy is widely used in
Received 19 July 2018; received in revised form 22 November 2018; accepted 10 December 2018. Conflict of interest: No potential conflict of interest relevant to this article was reported. * Corresponding author. Department of Radiodiagnosis, PGIMER, Sector 12, Chandigarh, India. Tel.: þ919855336326; fax: þ91722745768. E-mail address: [email protected] (B.S. Rana).
treatment of Ca-cx and often results in high dose especially to the bladder and rectum [2, 3]. In clinical practice, Ir-192 source is most commonly used, but use of Co-60 source for HDR brachytherapy applications is gradually picking up part due to its longer half-life and its availability in miniaturized form (with dimensions comparable to those of Ir-192 HDR sources) [4, 5]. The use of Co-60 as source for HDR brachytherapy poses a question, whether the organ at risks (OARs) would receive higher radiation dose, owing to Co-60 radioisotopes relative higher average gamma energy of 1.25 MeV compared to 0.38 MeVof Ir-192 [6].The potential logistic advantage of Co-60 is that, it uses only 33% of the activity of Ir-192 source needed to yield an equivalent dose rate. In typical brachytherapy application, there is no significant difference between Ir-192 and Co-60 with respect to treatment planning, dose prescription, and resultant isodose distributions to target volume. The relative comparison of
1538-4721/$ - see front matter Ó 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved. https://doi.org/10.1016/j.brachy.2018.12.006
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radial dose function, qualitative isodose distributions, and dose anisotropy of Co-60 and Ir-192 sources has been reported in literature [7, 8]. However, there are economic aspects of Co-60 nuclide, which make it an interesting option in brachytherapy. The primary requirements of brachytherapy is accurate and careful treatment planning for prescribed dose delivery to the target and simultaneously minimize risk of radiation toxicity to the surrounding OARs [6]. Sometimes, to spare the normal tissue, optimization has to be performed that compromises the tumor dose. Quality of treatment plan generated for HDR unit equipped with Co-60 or Ir-192 depends on source dwell positions and accuracy of dose delivered to the tumor site depend on source position to obtain desired dose gradient. In brachytherapy treatment plan, the treatment time (TT) is determined by prescribed dose and half-life of the source used [9]. Furthermore, dose distribution depends on by TT, source dwell positions, and number of dwell positions [10]. The source dwell position can be altered by changing the source step size. Therefore, the TT can be varied by changing source step size. Unfortunately, there is no report in literature that reports optimization of source step size for the radioisotopes used in HDR brachytherapy to obtain appropriate treatment plan based on the prescribed dose. In present work, we have studied the role of step size in brachytherapy treatment planning as an important parameter to evaluate optimum source step size for proper treatment using a Co-60. In this study, we have studied parameters affected by varying step size from 1 mm to 10 mm and its influence on overall treatment plan and process.
Methods and materials The present retrospective study has been conducted on 15 patients of Ca-cx treated with Co-60 HDR intracavitary brachytherapy using standard source step size of 2.5 mm. All patients were treated with dose of 46 Gy in 23 fractions of external beam radiotherapy, followed by HDR brachytherapy with a dose of 21 Gy in three fractions of 7 Gy each, after 1 week interval between each fraction as per institution protocol. We have used Fletcher suite applicator for brachytherapy treatment in all patients with a suitable tandem and ovoid. The brachytherapy applicator was inserted in lithotomic position under general anesthesia after getting preanesthetic clearance. A Foley’s catheter was also inserted into the urinary bladder and its balloon inflated with 7 cc of diluted urografin dye to identify bladder reference point. The vagina was packed with gauze to immobilize the applicator, displace rectum posteriorly, and bladder anteriorly to minimize dose to these organs. Patients were treated on BEBIG Multisource HDR brachytherapy unit equipped with Co-60 miniature source having active core diameter of 0.5 mm and active core length of 3.5 mm (Eckert and Ziegler, BEBIG, Germany). Treatment planning was performed on BEBIG HDR 2.5 plus
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(Eckert and Ziegler, BEBIG, Germany) treatment planning system (TPS) with option of selecting source step size from 1 mm to 10 mm. CT scan of patients were performed using 3 mm slice thickness as a standard protocol for patient for intracavity brachytherapy procedure. Contouring of the bladder and rectum was done in axial planes. Volumetric parameters, that is, D0.1cc (dose to 0.1 cubic centimeter volume) and D2cc (dose to 2 cubic centimeter volume) were calculated for both the bladder and rectum as per Gynaecological, The Groupe Europeen de Curietherapie and the European Society for Radiotherapy & Oncology (GYN GEC-ESTRO) recommendations [11, 12]. Applicator reconstruction, International Commission on Radiation Units & Measurements (ICRU) bladder and rectum point were also defined using reconstructed planes from CT scan. A typical plan of intracavitary brachytherapy in axial, sagittal, and coronal planes with isodoses and OAR structures is shown in Fig. 1. For the study, retrospective treatment planning of each patient was performed for source step size of 1 mm, 2.5 mm, 5 mm, and 10 mm on the same CT images acquired during treatment of the patient. Dose of 7 Gy was prescribed to point A as per institute protocol, which is 2 cm superior along the tandem from surface of the ovoid and 2 cm lateral to the central tandem [13]. Contouring of gross tumor volume and clinical target volume were not performed in these plans as the intuitional protocol of prescription dose at point A was followed. Inclusion of MRI images for point A dose prescription was not attempted in the present study. Previous study by one of the coauthor reported that dose delivered to OAR (bladder and rectum) in CT and MRI imageebased planning for point A prescription did not differ significantly [14]. The rectum ICRU reference point was marked 5 mm behind the posterior vaginal wall from center of vaginal sources in the sagittal plane with the help of axial and coronal planes in the TPS. The bladder ICRU reference point was marked as posterior most point on the surface of the balloon filled with 7 cc diluted radioopaque fluid in sagittal plane with the help of axial and coronal planes in the TPS [15, 16]. Dose calculation was done using American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report (TG-43) recommendations [17]. Overall TT, ICRU bladder and rectum point dose and volume of prescribed dose were compared. A paired two-tailed student t-test with threshold for statistical significance of p # 0.05 was used for data analysis.
Results and Discussion Dose to bladder and rectum points, TT and volume covered by the prescribed dose in 15 patients of Ca-cx for different source step size used in the study is listed in Table 1. The mean dose to bladder points for 1 mm, 2.5 mm, 5 mm, and 10 mm source step sizes are 3.37 Gy, 3.44 Gy, 3.54 Gy, and 3.74 Gy, respectively. Likewise, the mean dose to rectum point for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm are 2.86 Gy, 3.02 Gy, 3.25 Gy, and
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Fig. 1. Typical plan of intracavitary brachytherapy in axial, sagittal, and coronal planes with isodoses and OAR structures.
3.63 Gy, respectively. The prescribed dose coverage volume range from 78.66 to 92.88 cc with average dose coverage of 84.64 cc for step size of 1 mm, 82.31e97.13 cc with average of 88.19 cc for step size of 2.5 mm, 86.72e104.38 cc with average of 93.94 cc for step size of 5 mm, and 93.19e 114.75 cc with average 103.77 cc for step size of 10 mm. The mean TT for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm is 31.46 min, 32.36 min, 33.55 min, and 36.15 min, respectively. The variation of dose to OARs, TT, and prescribed dose coverage volume for selected step sizes is shown in Figs. 2ae2c. Figure 2a shows that bladder and rectum dose gradually increase with increase in step size. This is because increase in source step size leads to increase in dose nonuniformity around the source with hotspot near the source. Dose to the bladder point for every step size is reported to be more than dose to the rectum point. The variation of 15% in bladder to rectum dose at 1 mm step size is reduced to 3% for step size of 10 mm. This reflects that, as source step size increases, dose to rectum and bladder becomes nearly equal. Figure 2b shows that TT increases linearly with increase in step size. TT of 31.67 min for step size of 1 mm increased to 36.04 min for source step size of 10 mm. The mean of prescribed dose coverage volume (Fig. 2c) also increases from 84.64 cc for 1 mm step size to 103.77 cc for 10 mm step size. Figure 2c infers that as source step size increases from 1 mm to 10 mm, tissue beyond target volume may also get significant dose. Variation of dose to D0.1cc, D2cc, and ICRU reference point with source step sizes for bladder and rectum OARs is shown in Figs. 3a and 3b. Figure 3a shows that with increase in source step size, bladder’s D0.1cc, D2cc, and bladder reference point dose also increases. The average percentage difference of bladder (D0.1cc) for step size of 1 mme 2.5 mm is 1.3%, for step size of 2.5 mme5 mm is 1.58%, and
for step size of 5 mme10 mm is 3.96%. Likewise, Fig. 3b shows the variation of rectum’s D0.1cc, D2cc, and rectum reference point dose for different step sizes used in the study. The rectum’s D0.1cc, D2cc, and ICRU rectum reference point dose also increases with increase in step size. The average percentage difference of rectum (D0.1cc) for step size of 1 mme 2.5 mm is 4.72%, for 2.5 mme5 mm is 6.95%, and for 5 mme10 mm is 10.7%. The percentage difference of bladder’s D0.1cc for 1 mm, 5 mm, and 10 mm source step size relative to standard 2.5 mm step size is found to be 1.29%, 1.58%, and 5.60%, respectively. For bladder’s D2cc, the percentage difference for step size mentioned previously is found to be 2.0%, 2.0%, and 6.74%. The percentage differences of dose parameters for ICRU bladder points are 2.04%, 3.06%, and 8.79%. Likewise, the percentage difference of rectum’s D0.1cc for 1 mm, 5 mm, and 10 mm step size relative to standard 2.5 mm source step size is found to be 4.73%, 7.47%, and 20.43%, respectively. For rectum’s D2cc, the percentage difference for step size mentioned previously is found to be 4.96%, 7.33%, and 20.26%. The percentage differences of dose parameters for ICRU rectum point are 5.3%, 7.62%, and 20.2%. TT and prescribed dose volume for 1 mm, 5 mm, 10 mm source step size relative to standard 2.5 mm source step size are found to be 2.4%, 4.04%, 11.07%, and 4.03%, 6.52%, and 17.66%, respectively. The p-values (#0.05) of dosimetric parameters are found to be statistically significant for all reported parameters. The abovementioned observation indicates that use of step size greater than 5 mm may increase dose to OARs and overall TT. For an ideal treatment plan, dose to OAR should be minimum and TT should be optimum and prescribed dose
Step size 5 1 mm
Step size 5 2.5 mm
D2cc bladder (Gy)
ICRU Bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Mean SD
5.1 4.3 9.2 4.7 6.4 3.5 4.1 4.1 5.3 4.3 3.6 4.9 2.5 5 1.7 4.58 1.71
4.3 3.8 6.3 3.8 4.8 2.4 3.7 3.6 4.4 3.5 2.9 3.4 2 3.6 1.3 3.59 1.19
3.74 4.3 4.91 4.99 6.18 3.36 1.85 3.22 3.19 1.71 2.24 3.79 1.92 3.57 1.54 3.37 1.36
2.7 5.1 5.8 5.3 8 3.5 4.6 4.1 3.4 2.9 4.8 3.5 2.6 4.8 1.4 4.17 1.60
2.1 3.1 4.7 3.5 4.4 2.9 3.3 3.1 2.6 2.4 3.6 2.6 1.8 3.2 0.8 2.94 0.98
1.91 3.44 2.65 1.8 2.2 2.38 3.58 3.13 4.11 2.99 3.05 3.08 2.55 3.4 2.65 2.86 0.64
31.54 31.4 29.11 36.07 31.45 32.04 30.05 32.26 31.4 32.08 31.46 30.5 31.54 30.59 33.53 31.67 1.59
Patient
Step size 5 5 mm
Prescribed dose volume (cc) 87.02 83.13 84.38 92.88 82.75 82.88 78.66 86.63 84.38 84.38 84.94 83.06 85.13 84.47 84.94 84.64 2.99
D0.1cc bladder (Gy)
D2cc bladder (Gy)
ICRU Bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
5.1 4.3 9.3 4.7 6.4 3.6 4.1 4.2 4.4 5.3 3.7 5 2.6 5.1 1.8 4.64 1.70
4.4 3.8 6.4 4.1 4.8 2.5 3.7 3.6 3.6 4.4 2.9 3.5 2.1 3.7 1.4 3.66 1.18
3.84 4.35 5.07 5.12 6.22 3.49 1.88 3.28 1.76 3.25 2.28 3.85 1.99 3.59 1.59 3.44 1.38
2.8 5.3 6.2 5.7 8.2 3.7 4.8 4.3 3 3.6 5 3.7 2.8 5 1.5 4.37 1.65
2.2 3.2 4.9 3.7 4.6 3.1 3.5 3.3 2.5 2.8 3.8 2.7 1.9 3.3 0.9 3.09 1.01
2.03 3.57 2.82 1.91 2.32 2.47 3.76 3.27 3.16 4.31 3.23 3.25 2.7 3.67 2.81 3.02 0.67
32.47 32.25 29.59 36.52 32.28 32.49 30.42 33.25 33 32.31 32.36 31.35 32.45 31.46 34.5 32.45 1.61
Prescribed dose volume (cc) 91.64 86.75 87.38 97.13 85.88 85.03 82.31 90.94 87.75 87.66 88.13 87 87.84 88.31 89.16 88.19 3.32
Step size 5 10 mm D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU rectum point
TT (min.)
Prescribed dose volume (cc)
D0.1cc bladder (Gy)
D2cc bladder (Gy)
ICRU bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
Prescribed dose volume (cc)
4.4 5.1 9.5 4.7 6.4 4.2 3.8 4.2 4.5 5.3 1.9 2.7 5 5.2 3.8 4.71 1.71
3.9 4.4 6.6 4.2 4.9 3.8 2.6 3.7 3.6 4.4 1.4 2.2 3.5 3.8 3 3.73 1.21
4.42 4 5.32 5.32 6.29 1.93 3.69 3.36 1.84 3.3 1.67 2.09 3.95 3.62 2.34 3.54 1.41
5.4 3.1 6.9 6.3 8.5 5.2 4.1 4.7 3.3 3.9 1.6 3 4 5.2 5.3 4.7 1.72
3.4 2.4 5.3 4.1 5 3.8 3.3 3.5 2.7 3 0.9 2 2.9 3.5 4 3.32 1.11
3.79 2.21 3.08 2.07 2.52 4.04 2.6 3.48 3.43 4.64 3.06 2.92 3.5 3.89 3.52 3.25 0.71
33.36 34.08 31.14 38.03 33.36 31.39 33.58 34.42 34.21 33.51 36.2 34.05 32.46 33 33.55 33.76 1.69
92.13 95.91 93.75 104.38 91.63 86.72 90.56 96.19 94.03 93.38 95.53 94.59 91.31 93.94 95.06 93.94 3.79
9.8 5.2 5.1 6.4 4.1 4.4 4.3 4.7 5.3 2.1 4.1 5 2.9 5.5 4.6 4.9 1.71
6.9 4.5 4.4 5 2.8 3.9 3.9 3.8 4.6 1.6 3.2 3.6 2.4 3.9 4.1 3.91 1.21
5.76 4.28 5.69 6.41 4.05 2.03 3.52 1.99 3.42 1.82 2.45 4.13 2.3 3.68 4.56 3.74 1.46
7.9 3.6 7.3 9.1 4.6 5.9 5.4 3.8 4.5 1.8 5.9 4.5 3.4 5.6 5.7 5.27 1.87
5.9 2.7 4.7 5.5 3.7 4.2 3.9 3.1 3.4 1.1 4.4 3.3 2.3 3.8 3.8 3.72 1.20
3.52 2.53 2.35 2.85 2.84 4.55 3.87 3.92 4.88 3.52 4.04 3.96 3.32 4.12 4.18 3.63 0.73
33.27 36.36 40.1 35.37 36.04 33.22 37 36.48 36.15 39.02 36.17 34.53 36.3 35.14 35.42 36.04 1.82
104.75 106.68 114.75 100.63 99.56 93.19 104.94 105.75 105.38 109.78 104.06 99.75 105.19 101.91 100.25 103.77 4.98
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ICRU bladder point
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D2cc bladder (Gy)
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D0.1cc bladder (Gy)
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Patient
D0.1cc bladder (Gy)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Mean SD
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Table 1 Dosimetric parameters of treatment plans for Co-60 brachytherapy source for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm
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a
b b
c
Fig. 3. Variation of dose to D0.1cc, D2cc, and ICRU reference point with source step sizes for (a) bladder and (b) rectum.
Fig. 2. Variation of (a) dose to bladder and rectum points, (b) overall treatment time (TT), and (c) prescribed dose volume with source step size.
volume should be near to the target volume. In the present study, plans were evaluated for a 2 Ci Co-60 source. Therefore, even a difference in TT of 1 s may result in over dose to the surrounding structures. Manikandan et al. [18] have shown that plan optimization strongly depends on initial input parameters such as step size and maximum dwell time. Their study on prostate implants identified the step size of 5 mm and maximum dwell time of 40 s as optimum values. With the increase in step size, target coverage gradually increases and then starts decreasing after reaching saturation. The reported target coverage was nearly the same for 5 mm and 7 mm step size. The plans with 5 mm step size were seen to have clinically acceptable tumor coverage, minimal normal structure doses, and less TT as compared with the other step sizes. They recommended that step size more than 5 mm should not be used for HDR brachytherapy treatment. Park et al. [19] studied the effect of source step size in HDR brachytherapy in esophagus cancer and found that step size of 4e6 mm provides the optimal and most homogenous dose distributions. The study by Odgers et al. [20] shows that increasing the source step size in the treatment plan increases the magnitude of dose to hot spots. Bhadur et al. [21] presented similar results with Ir-192 HDR
brachytherapy Ca-cx with target coverage of 100% to D90 for clinical target volume and rectum’s and bladder’s D2cc were 82.85% and 105.71%, respectively. Palmer et al. [22] compared the use of Co-60 source in HDR brachytherapy to Ir-192ebased HDR brachytherapy. TT with Co-60 source is about 1.7 times more than TT with Ir-192 source of 10 Ci activity and listed logistical benefits of Co-60 as compared to Ir-192 source for Ca-cx HDR brachytherapy. In the present study, mean dose to bladder and rectum for 10 mm step size increases by 8.72% and 20.2%, respectively with increase in TT of ~4 min as compared to treatment with standard 2.5 mm step size.
Conclusion Our results on Ca-cx brachytherapy using Co-60 radioactive source show that all source step size provides clinically acceptable prescribed dose distribution. As step size increases, dose to bladder and rectum also increases, which may lead to increase in radiation-induced toxicity and reduces the quality of life [23]. With increase in source, step size TT gradually increases. TT difference of around 4 min between minimum selected step size (1 mm) and maximum selected step size (10 mm) is observed that is not found to be significant in terms of patient discomfort as compared to overall duration of treatment procedure. However, optimum doses to OARs and acceptable prescribed dose volume coverage relative to step size of 2.5 mm were achieved at source step size of 5 mm or less. This study recommends step size more than 5 mm should not be used for HDR
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brachytherapy treatment. The incorporation of Co-60 sourceebased brachytherapy due to its long half-life is economical and attractive for low-resource settings especially in developing countries. References [1] Available at: http://gco.iarc.fr/today/home. Accessed September 18, 2018. [2] Kirisits C, Rivard MJ, Baltas D, et al. Review of clinical brachytherapy uncertainties: Analysis guidelines of GEC-ESTRO and the AAPM. Radiother Oncol 2014;110:199e212. [3] Bravo-Miranda C, Burg Rech A, Francisco Oliveira H, et al. Measurement of rectum dose by in vivo alanine/ESR dosimetry in gynecological 192Ir HDR brachytherapy. Radiat Meas 2015;75:45e52. [4] Ntekim A, Adenipekun A, Akinlade B, et al. High dose rate brachytherapy in the treatment of cervical cancer: Preliminary experience with cobalt 60 radionuclide sourced -A prospective study. Clin Med Insights Oncol 2010;4:89e94. [5] Granero D, Perez-Calatayud J, Ballester F. Technical note: Dosimetric study of a new Co-60 source used in brachytherapy. Med Phys 2007;34:3485e3488. [6] Zaman ZK, Ung NM, Malik RA, et al. Comparison of planned and measured rectal dose in-vivo during high dose rate Cobalt-60 brachytherapy of cervical cancer. Phys Med 2014;30:980e984. [7] Strohmaier S, Zwierzchowski G. Comparison of 60Co and 192Ir sources in HDR brachytherapy. J Contemp Brachytherapy 2011;3: 199e208. [8] Richter J, Baier K, Flentje M. Comparison of 60Co and 192 Ir sources in high dose rate afterloading brachytherapy. Strahlenther Onkol 2008;184:187e192. [9] Rogus RD, Smith MJ, Kubo HD. An equation to QA checks the total treatment time for single-catheter HDR brachytherapy. Int J Radiat Oncol Biol Phys 1998;40:245e248. [10] Wong T, Wallace S, Fernando W, et al. Dose errors in the near field of an HDR brachytherapy stepping source. Phys Med Biol 1999;44:357. [11] Potter R, Haie-Meder C, Van-Limbergen E, et al. Recommendations from gynaecological (GYN) GEC ESTRO working group (II): Concepts and terms in 3D image-based treatment planning in cervix cancer brachytherapy-3D dose volume parameters and aspects of 3D
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image-based anatomy, radiation physics, radiobiology. Radiother Oncol 2006;78:67e77. Potter R, Van Limbergen E, Wambersie A. Reporting in brachytherapy: Dose and volume specification. In: Gerbaulet A, Potter R, Mazeron JJ, et al, editors. The GEC ESTRO handbook of brachytherapy. Brussels: ESTRO; 2002. p. 155e215. Nag S, Erickson B, Thomadsen B, et al. The American brachytherapy society recommendations for high-dose-rate brachytherapy for carcinoma of the cervix. Int J Radiat Oncol Biol Phys 2000;48: 201e211. Krishnatry R, Patel FD, Singh P, et al. CT or MRI for image-based brachytherapy in cervical cancer. Jpn J Clin Oncol 2012;42:309e 313. ICRU report 38. Dose and volume specification for reporting intracavitary therapy in gynaecology. Bethesda, MD: International Commission on Radiation Units and Measurements; 1985. p. 1e20. ICRU report 89. Prescribing, recording, and reporting brachytherapy for cancer of the cervix. J ICRU 2013;13:138e139. Nath R, Anderson L, Luxton G, et al. Dosimetry of interstitial brachytherapy source: Recommendation of the AAPM radiation therapy committee task group No. 43. Med Phys 1995;22:209e234. Manikandan A, Sarkar B, Rajendran VT, et al. Role of step size and max dwell time in anatomy based inverse optimization for prostate implants. J Med Phys 2013;38:148e154. Park SJ, Kamrava M, Banerjee R, et al. Effects of source step size on dose distribution in high-dose-rate brachytherapy for esophageal cancer. Brachytherapy 2013;12:S11eS77. Odgers D, Haque M. Near-catheter dosimetry of a HDR brachytherapy source using Gafchromic film. Australas Phys Eng Sci Med 2013; 36:159e166. Bahadur YA, Constantinescu CT, Hassouna AH, et al. Treatment planning for high dose rate brachytherapy ofcervical cancer based on total dose constraints. Saudi Med J 2011;32:495e503. Palmer A, Hayman O, Muscat S. Treatment planning study of the 3D dosimetric differences between Co-60 and Ir-192 sources in high dose rate (HDR) brachytherapy for cervix cancer. J Contemp Brachytherapy 2012;4:52e59. Adams E, Boulton MG, Horne A, et al. The effects of pelvic radiotherapy on cancer survivors: Symptom profile, psychological morbidity and quality of life. Clin Oncol (R Coll Radiol) 2014;26: 10e17.
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Dosimetric study of CO-60 source step size in uterine cervix intracavitary HDR brachytherapy Arvind Kumar Shukla1,2, Bhupendra Singh Rana1,3,*, Narinder Paul Singh1, Sanjeev Kumar4 1
Department of Applied Sciences, IK Gujral Punjab Technical University (PTU), Jalandhar, India 2 Department of Radiotherapy, RNT Medical College, Udaipur, India 3 Department of Radiodiagnosis, PGIMER, Chandigarh, India 4 Department of Physics, G.G.D.S.D. College, Chandigarh, India
ABSTRACT
PURPOSE: The present work reports effects of source step sizes on dose distribution in patients treated with cobalt-60 (Co-60) high-dose-rate afterloading brachytherapy in carcinoma cervix (Ca-cx). METHODS AND MATERIALS: The retrospective study is based on data of 15 patients of Ca-cx treated with Co-60 high-dose-rate intracavitary brachytherapy with dose of 21 Gy in three fractions with source step size of 2.5 mm after external beam radiotherapy of 46 Gy. The effect of source step size on overall treatment procedure was evaluated from prescribed dose volume, dose to organ at risks, and treatment time for source step sizes of 1 mm, 2.5 mm, 5 mm, and 10 mm for each patient. RESULTS: The mean dose to bladder point for 1 mm, 2.5 mm, 5 mm, and 10 mm source step sizes was found to be 3.37 Gy (SD: 1.36), 3.44 Gy (SD: 1.38), 3.54 Gy (SD: 1.41), and 3.74 Gy (SD: 1.46), respectively. Similarly, the mean dose received by rectum point for these source step sizes were 2.86 Gy (SD: 0.64), 3.02 Gy (SD: 0.67), 3.25 Gy (SD: 0.71), and 3.63 Gy (SD: 0.73), respectively. The treatment time and prescribed dose coverage volume were both found to be gradually increasing with increase in step size. CONCLUSIONS: Our results on Ca-cx brachytherapy using Co-60 source indicate that the prescribed dose volume gradually increases from smaller source step to larger source step size. This results in increase of dose to the bladder and rectum and may lead to increase in toxicity and reduces quality of life. The study recommends that step size more than 5 mm should not be used for uterine cervix intracavitary application using Co-60 source. Ó 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.
Keywords:
Step size; Intracavitary brachytherapy; High dose rate; Computed tomography
Introduction Carcinoma of uterine cervix (Ca-cx) is the second most common cancer among women in countries with low and medium human development index [1], which can be treated with external beam radiotherapy and intracavitary brachytherapy. Brachytherapy can be used for successful treatment of cancer of the prostate, esophagus, head and neck, and cervix. High-dose-rate (HDR) brachytherapy is widely used in
Received 19 July 2018; received in revised form 22 November 2018; accepted 10 December 2018. Conflict of interest: No potential conflict of interest relevant to this article was reported. * Corresponding author. Department of Radiodiagnosis, PGIMER, Sector 12, Chandigarh, India. Tel.: þ919855336326; fax: þ91722745768. E-mail address: [email protected] (B.S. Rana).
treatment of Ca-cx and often results in high dose especially to the bladder and rectum [2, 3]. In clinical practice, Ir-192 source is most commonly used, but use of Co-60 source for HDR brachytherapy applications is gradually picking up part due to its longer half-life and its availability in miniaturized form (with dimensions comparable to those of Ir-192 HDR sources) [4, 5]. The use of Co-60 as source for HDR brachytherapy poses a question, whether the organ at risks (OARs) would receive higher radiation dose, owing to Co-60 radioisotopes relative higher average gamma energy of 1.25 MeV compared to 0.38 MeVof Ir-192 [6].The potential logistic advantage of Co-60 is that, it uses only 33% of the activity of Ir-192 source needed to yield an equivalent dose rate. In typical brachytherapy application, there is no significant difference between Ir-192 and Co-60 with respect to treatment planning, dose prescription, and resultant isodose distributions to target volume. The relative comparison of
1538-4721/$ - see front matter Ó 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved. https://doi.org/10.1016/j.brachy.2018.12.006
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radial dose function, qualitative isodose distributions, and dose anisotropy of Co-60 and Ir-192 sources has been reported in literature [7, 8]. However, there are economic aspects of Co-60 nuclide, which make it an interesting option in brachytherapy. The primary requirements of brachytherapy is accurate and careful treatment planning for prescribed dose delivery to the target and simultaneously minimize risk of radiation toxicity to the surrounding OARs [6]. Sometimes, to spare the normal tissue, optimization has to be performed that compromises the tumor dose. Quality of treatment plan generated for HDR unit equipped with Co-60 or Ir-192 depends on source dwell positions and accuracy of dose delivered to the tumor site depend on source position to obtain desired dose gradient. In brachytherapy treatment plan, the treatment time (TT) is determined by prescribed dose and half-life of the source used [9]. Furthermore, dose distribution depends on by TT, source dwell positions, and number of dwell positions [10]. The source dwell position can be altered by changing the source step size. Therefore, the TT can be varied by changing source step size. Unfortunately, there is no report in literature that reports optimization of source step size for the radioisotopes used in HDR brachytherapy to obtain appropriate treatment plan based on the prescribed dose. In present work, we have studied the role of step size in brachytherapy treatment planning as an important parameter to evaluate optimum source step size for proper treatment using a Co-60. In this study, we have studied parameters affected by varying step size from 1 mm to 10 mm and its influence on overall treatment plan and process.
Methods and materials The present retrospective study has been conducted on 15 patients of Ca-cx treated with Co-60 HDR intracavitary brachytherapy using standard source step size of 2.5 mm. All patients were treated with dose of 46 Gy in 23 fractions of external beam radiotherapy, followed by HDR brachytherapy with a dose of 21 Gy in three fractions of 7 Gy each, after 1 week interval between each fraction as per institution protocol. We have used Fletcher suite applicator for brachytherapy treatment in all patients with a suitable tandem and ovoid. The brachytherapy applicator was inserted in lithotomic position under general anesthesia after getting preanesthetic clearance. A Foley’s catheter was also inserted into the urinary bladder and its balloon inflated with 7 cc of diluted urografin dye to identify bladder reference point. The vagina was packed with gauze to immobilize the applicator, displace rectum posteriorly, and bladder anteriorly to minimize dose to these organs. Patients were treated on BEBIG Multisource HDR brachytherapy unit equipped with Co-60 miniature source having active core diameter of 0.5 mm and active core length of 3.5 mm (Eckert and Ziegler, BEBIG, Germany). Treatment planning was performed on BEBIG HDR 2.5 plus
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(Eckert and Ziegler, BEBIG, Germany) treatment planning system (TPS) with option of selecting source step size from 1 mm to 10 mm. CT scan of patients were performed using 3 mm slice thickness as a standard protocol for patient for intracavity brachytherapy procedure. Contouring of the bladder and rectum was done in axial planes. Volumetric parameters, that is, D0.1cc (dose to 0.1 cubic centimeter volume) and D2cc (dose to 2 cubic centimeter volume) were calculated for both the bladder and rectum as per Gynaecological, The Groupe Europeen de Curietherapie and the European Society for Radiotherapy & Oncology (GYN GEC-ESTRO) recommendations [11, 12]. Applicator reconstruction, International Commission on Radiation Units & Measurements (ICRU) bladder and rectum point were also defined using reconstructed planes from CT scan. A typical plan of intracavitary brachytherapy in axial, sagittal, and coronal planes with isodoses and OAR structures is shown in Fig. 1. For the study, retrospective treatment planning of each patient was performed for source step size of 1 mm, 2.5 mm, 5 mm, and 10 mm on the same CT images acquired during treatment of the patient. Dose of 7 Gy was prescribed to point A as per institute protocol, which is 2 cm superior along the tandem from surface of the ovoid and 2 cm lateral to the central tandem [13]. Contouring of gross tumor volume and clinical target volume were not performed in these plans as the intuitional protocol of prescription dose at point A was followed. Inclusion of MRI images for point A dose prescription was not attempted in the present study. Previous study by one of the coauthor reported that dose delivered to OAR (bladder and rectum) in CT and MRI imageebased planning for point A prescription did not differ significantly [14]. The rectum ICRU reference point was marked 5 mm behind the posterior vaginal wall from center of vaginal sources in the sagittal plane with the help of axial and coronal planes in the TPS. The bladder ICRU reference point was marked as posterior most point on the surface of the balloon filled with 7 cc diluted radioopaque fluid in sagittal plane with the help of axial and coronal planes in the TPS [15, 16]. Dose calculation was done using American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report (TG-43) recommendations [17]. Overall TT, ICRU bladder and rectum point dose and volume of prescribed dose were compared. A paired two-tailed student t-test with threshold for statistical significance of p # 0.05 was used for data analysis.
Results and Discussion Dose to bladder and rectum points, TT and volume covered by the prescribed dose in 15 patients of Ca-cx for different source step size used in the study is listed in Table 1. The mean dose to bladder points for 1 mm, 2.5 mm, 5 mm, and 10 mm source step sizes are 3.37 Gy, 3.44 Gy, 3.54 Gy, and 3.74 Gy, respectively. Likewise, the mean dose to rectum point for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm are 2.86 Gy, 3.02 Gy, 3.25 Gy, and
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Fig. 1. Typical plan of intracavitary brachytherapy in axial, sagittal, and coronal planes with isodoses and OAR structures.
3.63 Gy, respectively. The prescribed dose coverage volume range from 78.66 to 92.88 cc with average dose coverage of 84.64 cc for step size of 1 mm, 82.31e97.13 cc with average of 88.19 cc for step size of 2.5 mm, 86.72e104.38 cc with average of 93.94 cc for step size of 5 mm, and 93.19e 114.75 cc with average 103.77 cc for step size of 10 mm. The mean TT for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm is 31.46 min, 32.36 min, 33.55 min, and 36.15 min, respectively. The variation of dose to OARs, TT, and prescribed dose coverage volume for selected step sizes is shown in Figs. 2ae2c. Figure 2a shows that bladder and rectum dose gradually increase with increase in step size. This is because increase in source step size leads to increase in dose nonuniformity around the source with hotspot near the source. Dose to the bladder point for every step size is reported to be more than dose to the rectum point. The variation of 15% in bladder to rectum dose at 1 mm step size is reduced to 3% for step size of 10 mm. This reflects that, as source step size increases, dose to rectum and bladder becomes nearly equal. Figure 2b shows that TT increases linearly with increase in step size. TT of 31.67 min for step size of 1 mm increased to 36.04 min for source step size of 10 mm. The mean of prescribed dose coverage volume (Fig. 2c) also increases from 84.64 cc for 1 mm step size to 103.77 cc for 10 mm step size. Figure 2c infers that as source step size increases from 1 mm to 10 mm, tissue beyond target volume may also get significant dose. Variation of dose to D0.1cc, D2cc, and ICRU reference point with source step sizes for bladder and rectum OARs is shown in Figs. 3a and 3b. Figure 3a shows that with increase in source step size, bladder’s D0.1cc, D2cc, and bladder reference point dose also increases. The average percentage difference of bladder (D0.1cc) for step size of 1 mme 2.5 mm is 1.3%, for step size of 2.5 mme5 mm is 1.58%, and
for step size of 5 mme10 mm is 3.96%. Likewise, Fig. 3b shows the variation of rectum’s D0.1cc, D2cc, and rectum reference point dose for different step sizes used in the study. The rectum’s D0.1cc, D2cc, and ICRU rectum reference point dose also increases with increase in step size. The average percentage difference of rectum (D0.1cc) for step size of 1 mme 2.5 mm is 4.72%, for 2.5 mme5 mm is 6.95%, and for 5 mme10 mm is 10.7%. The percentage difference of bladder’s D0.1cc for 1 mm, 5 mm, and 10 mm source step size relative to standard 2.5 mm step size is found to be 1.29%, 1.58%, and 5.60%, respectively. For bladder’s D2cc, the percentage difference for step size mentioned previously is found to be 2.0%, 2.0%, and 6.74%. The percentage differences of dose parameters for ICRU bladder points are 2.04%, 3.06%, and 8.79%. Likewise, the percentage difference of rectum’s D0.1cc for 1 mm, 5 mm, and 10 mm step size relative to standard 2.5 mm source step size is found to be 4.73%, 7.47%, and 20.43%, respectively. For rectum’s D2cc, the percentage difference for step size mentioned previously is found to be 4.96%, 7.33%, and 20.26%. The percentage differences of dose parameters for ICRU rectum point are 5.3%, 7.62%, and 20.2%. TT and prescribed dose volume for 1 mm, 5 mm, 10 mm source step size relative to standard 2.5 mm source step size are found to be 2.4%, 4.04%, 11.07%, and 4.03%, 6.52%, and 17.66%, respectively. The p-values (#0.05) of dosimetric parameters are found to be statistically significant for all reported parameters. The abovementioned observation indicates that use of step size greater than 5 mm may increase dose to OARs and overall TT. For an ideal treatment plan, dose to OAR should be minimum and TT should be optimum and prescribed dose
Step size 5 1 mm
Step size 5 2.5 mm
D2cc bladder (Gy)
ICRU Bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Mean SD
5.1 4.3 9.2 4.7 6.4 3.5 4.1 4.1 5.3 4.3 3.6 4.9 2.5 5 1.7 4.58 1.71
4.3 3.8 6.3 3.8 4.8 2.4 3.7 3.6 4.4 3.5 2.9 3.4 2 3.6 1.3 3.59 1.19
3.74 4.3 4.91 4.99 6.18 3.36 1.85 3.22 3.19 1.71 2.24 3.79 1.92 3.57 1.54 3.37 1.36
2.7 5.1 5.8 5.3 8 3.5 4.6 4.1 3.4 2.9 4.8 3.5 2.6 4.8 1.4 4.17 1.60
2.1 3.1 4.7 3.5 4.4 2.9 3.3 3.1 2.6 2.4 3.6 2.6 1.8 3.2 0.8 2.94 0.98
1.91 3.44 2.65 1.8 2.2 2.38 3.58 3.13 4.11 2.99 3.05 3.08 2.55 3.4 2.65 2.86 0.64
31.54 31.4 29.11 36.07 31.45 32.04 30.05 32.26 31.4 32.08 31.46 30.5 31.54 30.59 33.53 31.67 1.59
Patient
Step size 5 5 mm
Prescribed dose volume (cc) 87.02 83.13 84.38 92.88 82.75 82.88 78.66 86.63 84.38 84.38 84.94 83.06 85.13 84.47 84.94 84.64 2.99
D0.1cc bladder (Gy)
D2cc bladder (Gy)
ICRU Bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
5.1 4.3 9.3 4.7 6.4 3.6 4.1 4.2 4.4 5.3 3.7 5 2.6 5.1 1.8 4.64 1.70
4.4 3.8 6.4 4.1 4.8 2.5 3.7 3.6 3.6 4.4 2.9 3.5 2.1 3.7 1.4 3.66 1.18
3.84 4.35 5.07 5.12 6.22 3.49 1.88 3.28 1.76 3.25 2.28 3.85 1.99 3.59 1.59 3.44 1.38
2.8 5.3 6.2 5.7 8.2 3.7 4.8 4.3 3 3.6 5 3.7 2.8 5 1.5 4.37 1.65
2.2 3.2 4.9 3.7 4.6 3.1 3.5 3.3 2.5 2.8 3.8 2.7 1.9 3.3 0.9 3.09 1.01
2.03 3.57 2.82 1.91 2.32 2.47 3.76 3.27 3.16 4.31 3.23 3.25 2.7 3.67 2.81 3.02 0.67
32.47 32.25 29.59 36.52 32.28 32.49 30.42 33.25 33 32.31 32.36 31.35 32.45 31.46 34.5 32.45 1.61
Prescribed dose volume (cc) 91.64 86.75 87.38 97.13 85.88 85.03 82.31 90.94 87.75 87.66 88.13 87 87.84 88.31 89.16 88.19 3.32
Step size 5 10 mm D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU rectum point
TT (min.)
Prescribed dose volume (cc)
D0.1cc bladder (Gy)
D2cc bladder (Gy)
ICRU bladder point
D0.1cc rectum (Gy)
D2cc rectum (Gy)
ICRU Rectum point
TT (min.)
Prescribed dose volume (cc)
4.4 5.1 9.5 4.7 6.4 4.2 3.8 4.2 4.5 5.3 1.9 2.7 5 5.2 3.8 4.71 1.71
3.9 4.4 6.6 4.2 4.9 3.8 2.6 3.7 3.6 4.4 1.4 2.2 3.5 3.8 3 3.73 1.21
4.42 4 5.32 5.32 6.29 1.93 3.69 3.36 1.84 3.3 1.67 2.09 3.95 3.62 2.34 3.54 1.41
5.4 3.1 6.9 6.3 8.5 5.2 4.1 4.7 3.3 3.9 1.6 3 4 5.2 5.3 4.7 1.72
3.4 2.4 5.3 4.1 5 3.8 3.3 3.5 2.7 3 0.9 2 2.9 3.5 4 3.32 1.11
3.79 2.21 3.08 2.07 2.52 4.04 2.6 3.48 3.43 4.64 3.06 2.92 3.5 3.89 3.52 3.25 0.71
33.36 34.08 31.14 38.03 33.36 31.39 33.58 34.42 34.21 33.51 36.2 34.05 32.46 33 33.55 33.76 1.69
92.13 95.91 93.75 104.38 91.63 86.72 90.56 96.19 94.03 93.38 95.53 94.59 91.31 93.94 95.06 93.94 3.79
9.8 5.2 5.1 6.4 4.1 4.4 4.3 4.7 5.3 2.1 4.1 5 2.9 5.5 4.6 4.9 1.71
6.9 4.5 4.4 5 2.8 3.9 3.9 3.8 4.6 1.6 3.2 3.6 2.4 3.9 4.1 3.91 1.21
5.76 4.28 5.69 6.41 4.05 2.03 3.52 1.99 3.42 1.82 2.45 4.13 2.3 3.68 4.56 3.74 1.46
7.9 3.6 7.3 9.1 4.6 5.9 5.4 3.8 4.5 1.8 5.9 4.5 3.4 5.6 5.7 5.27 1.87
5.9 2.7 4.7 5.5 3.7 4.2 3.9 3.1 3.4 1.1 4.4 3.3 2.3 3.8 3.8 3.72 1.20
3.52 2.53 2.35 2.85 2.84 4.55 3.87 3.92 4.88 3.52 4.04 3.96 3.32 4.12 4.18 3.63 0.73
33.27 36.36 40.1 35.37 36.04 33.22 37 36.48 36.15 39.02 36.17 34.53 36.3 35.14 35.42 36.04 1.82
104.75 106.68 114.75 100.63 99.56 93.19 104.94 105.75 105.38 109.78 104.06 99.75 105.19 101.91 100.25 103.77 4.98
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D0.1cc bladder (Gy)
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D0.1cc bladder (Gy)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Mean SD
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Table 1 Dosimetric parameters of treatment plans for Co-60 brachytherapy source for step size of 1 mm, 2.5 mm, 5 mm, and 10 mm
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a
b b
c
Fig. 3. Variation of dose to D0.1cc, D2cc, and ICRU reference point with source step sizes for (a) bladder and (b) rectum.
Fig. 2. Variation of (a) dose to bladder and rectum points, (b) overall treatment time (TT), and (c) prescribed dose volume with source step size.
volume should be near to the target volume. In the present study, plans were evaluated for a 2 Ci Co-60 source. Therefore, even a difference in TT of 1 s may result in over dose to the surrounding structures. Manikandan et al. [18] have shown that plan optimization strongly depends on initial input parameters such as step size and maximum dwell time. Their study on prostate implants identified the step size of 5 mm and maximum dwell time of 40 s as optimum values. With the increase in step size, target coverage gradually increases and then starts decreasing after reaching saturation. The reported target coverage was nearly the same for 5 mm and 7 mm step size. The plans with 5 mm step size were seen to have clinically acceptable tumor coverage, minimal normal structure doses, and less TT as compared with the other step sizes. They recommended that step size more than 5 mm should not be used for HDR brachytherapy treatment. Park et al. [19] studied the effect of source step size in HDR brachytherapy in esophagus cancer and found that step size of 4e6 mm provides the optimal and most homogenous dose distributions. The study by Odgers et al. [20] shows that increasing the source step size in the treatment plan increases the magnitude of dose to hot spots. Bhadur et al. [21] presented similar results with Ir-192 HDR
brachytherapy Ca-cx with target coverage of 100% to D90 for clinical target volume and rectum’s and bladder’s D2cc were 82.85% and 105.71%, respectively. Palmer et al. [22] compared the use of Co-60 source in HDR brachytherapy to Ir-192ebased HDR brachytherapy. TT with Co-60 source is about 1.7 times more than TT with Ir-192 source of 10 Ci activity and listed logistical benefits of Co-60 as compared to Ir-192 source for Ca-cx HDR brachytherapy. In the present study, mean dose to bladder and rectum for 10 mm step size increases by 8.72% and 20.2%, respectively with increase in TT of ~4 min as compared to treatment with standard 2.5 mm step size.
Conclusion Our results on Ca-cx brachytherapy using Co-60 radioactive source show that all source step size provides clinically acceptable prescribed dose distribution. As step size increases, dose to bladder and rectum also increases, which may lead to increase in radiation-induced toxicity and reduces the quality of life [23]. With increase in source, step size TT gradually increases. TT difference of around 4 min between minimum selected step size (1 mm) and maximum selected step size (10 mm) is observed that is not found to be significant in terms of patient discomfort as compared to overall duration of treatment procedure. However, optimum doses to OARs and acceptable prescribed dose volume coverage relative to step size of 2.5 mm were achieved at source step size of 5 mm or less. This study recommends step size more than 5 mm should not be used for HDR
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brachytherapy treatment. The incorporation of Co-60 sourceebased brachytherapy due to its long half-life is economical and attractive for low-resource settings especially in developing countries. References [1] Available at: http://gco.iarc.fr/today/home. Accessed September 18, 2018. [2] Kirisits C, Rivard MJ, Baltas D, et al. Review of clinical brachytherapy uncertainties: Analysis guidelines of GEC-ESTRO and the AAPM. Radiother Oncol 2014;110:199e212. [3] Bravo-Miranda C, Burg Rech A, Francisco Oliveira H, et al. Measurement of rectum dose by in vivo alanine/ESR dosimetry in gynecological 192Ir HDR brachytherapy. Radiat Meas 2015;75:45e52. [4] Ntekim A, Adenipekun A, Akinlade B, et al. High dose rate brachytherapy in the treatment of cervical cancer: Preliminary experience with cobalt 60 radionuclide sourced -A prospective study. Clin Med Insights Oncol 2010;4:89e94. [5] Granero D, Perez-Calatayud J, Ballester F. Technical note: Dosimetric study of a new Co-60 source used in brachytherapy. Med Phys 2007;34:3485e3488. [6] Zaman ZK, Ung NM, Malik RA, et al. Comparison of planned and measured rectal dose in-vivo during high dose rate Cobalt-60 brachytherapy of cervical cancer. Phys Med 2014;30:980e984. [7] Strohmaier S, Zwierzchowski G. Comparison of 60Co and 192Ir sources in HDR brachytherapy. J Contemp Brachytherapy 2011;3: 199e208. [8] Richter J, Baier K, Flentje M. Comparison of 60Co and 192 Ir sources in high dose rate afterloading brachytherapy. Strahlenther Onkol 2008;184:187e192. [9] Rogus RD, Smith MJ, Kubo HD. An equation to QA checks the total treatment time for single-catheter HDR brachytherapy. Int J Radiat Oncol Biol Phys 1998;40:245e248. [10] Wong T, Wallace S, Fernando W, et al. Dose errors in the near field of an HDR brachytherapy stepping source. Phys Med Biol 1999;44:357. [11] Potter R, Haie-Meder C, Van-Limbergen E, et al. Recommendations from gynaecological (GYN) GEC ESTRO working group (II): Concepts and terms in 3D image-based treatment planning in cervix cancer brachytherapy-3D dose volume parameters and aspects of 3D
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image-based anatomy, radiation physics, radiobiology. Radiother Oncol 2006;78:67e77. Potter R, Van Limbergen E, Wambersie A. Reporting in brachytherapy: Dose and volume specification. In: Gerbaulet A, Potter R, Mazeron JJ, et al, editors. The GEC ESTRO handbook of brachytherapy. Brussels: ESTRO; 2002. p. 155e215. Nag S, Erickson B, Thomadsen B, et al. The American brachytherapy society recommendations for high-dose-rate brachytherapy for carcinoma of the cervix. Int J Radiat Oncol Biol Phys 2000;48: 201e211. Krishnatry R, Patel FD, Singh P, et al. CT or MRI for image-based brachytherapy in cervical cancer. Jpn J Clin Oncol 2012;42:309e 313. ICRU report 38. Dose and volume specification for reporting intracavitary therapy in gynaecology. Bethesda, MD: International Commission on Radiation Units and Measurements; 1985. p. 1e20. ICRU report 89. Prescribing, recording, and reporting brachytherapy for cancer of the cervix. J ICRU 2013;13:138e139. Nath R, Anderson L, Luxton G, et al. Dosimetry of interstitial brachytherapy source: Recommendation of the AAPM radiation therapy committee task group No. 43. Med Phys 1995;22:209e234. Manikandan A, Sarkar B, Rajendran VT, et al. Role of step size and max dwell time in anatomy based inverse optimization for prostate implants. J Med Phys 2013;38:148e154. Park SJ, Kamrava M, Banerjee R, et al. Effects of source step size on dose distribution in high-dose-rate brachytherapy for esophageal cancer. Brachytherapy 2013;12:S11eS77. Odgers D, Haque M. Near-catheter dosimetry of a HDR brachytherapy source using Gafchromic film. Australas Phys Eng Sci Med 2013; 36:159e166. Bahadur YA, Constantinescu CT, Hassouna AH, et al. Treatment planning for high dose rate brachytherapy ofcervical cancer based on total dose constraints. Saudi Med J 2011;32:495e503. Palmer A, Hayman O, Muscat S. Treatment planning study of the 3D dosimetric differences between Co-60 and Ir-192 sources in high dose rate (HDR) brachytherapy for cervix cancer. J Contemp Brachytherapy 2012;4:52e59. Adams E, Boulton MG, Horne A, et al. The effects of pelvic radiotherapy on cancer survivors: Symptom profile, psychological morbidity and quality of life. Clin Oncol (R Coll Radiol) 2014;26: 10e17.