|Ahead of print publication
Effect of different definitions of prescription point “A” in high dose rate brachytherapy for cervical cancer
Shraddha Srivastava1, Nirmal Kumar Painuly1, Surendra Prasad Mishra2, Kirti Srivastava1, Navin Singh1, Madan Lal Brahma Bhatt1
1 Department of Radiotherapy, King George's Medical University, Lucknow, Uttar Pradesh, India
2 Department of Radiation Oncology, Dr. Ram Manohar Lohia Institute of Medical Sciences, Lucknow, Uttar Pradesh, India
Asst. Professor, Department of Radiotherapy, King George's Medical University, Lucknow - 226 003, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Aim: This study intended to compare the dosimetric parameters using different definitions of prescription point A in high dose rate (HDR) brachytherapy of cervical cancer patients.
Background: Manchester point A has been widely used for prescribing dose in brachytherapy. However, due to certain limitations of this point, a new definition of point A has been recommended by the American Brachytherapy Society (ABS).
Materials and Methods: We retrospectively investigated 55 computed tomography-based plans of 20 cervical cancer patients treated with Ir-192-based intracavitary HDR brachytherapy. The dose of 7 Gy in 3 fractions each was prescribed to point A using revised Manchester definition of point A (AMAN) and ABS guideline definition (AABS). The effect of both definitions on various parameters including dose to point A and 90% of tumor volume (D90), dose received by 2cc volume of bladder, rectum and small bowel and treatment volume receiving 100% of prescription dose (V100) was analyzed.
Results: Mean percentage difference of point AMAN dose and AABS dose with respect to prescription dose was 1.25% ± 1.43% and 1.21% ± 1.01%, respectively. Mean V100 was 80.4 ± 20.45cc and 88.47 ± 16.78cc for AMAN and AABS plans, respectively, while mean percentage difference between prescribed dose and D90 was found to be –37.90% ± 25.06% and –30.47% ± 25.50% respectively for both the definitions.
Conclusion: Doses to both Manchester point A and ABS point A may be recorded during the transition period. However, ABS point A can be preferred over the Manchester point A as it conforms better with the desired dosimetric outcome and is found to be more static.
Keywords: Brachytherapy, cervical cancer, high dose rate, point A
|How to cite this URL:|
Srivastava S, Painuly NK, Mishra SP, Srivastava K, Singh N, Bhatt ML. Effect of different definitions of prescription point “A” in high dose rate brachytherapy for cervical cancer. J Can Res Ther [Epub ahead of print] [cited 2019 Aug 18]. Available from: http://www.cancerjournal.net/preprintarticle.asp?id=260132
| > Introduction|| |
The role of intracavitary brachytherapy (ICBT) in the treatment of cervical cancer is well established. Conventional treatment planning based on two-dimensional (2D) orthogonal radiographs where dose is prescribed to point A has been widely used across the globe. However, with recent advancement in three-dimensional (3D) imaging, computed tomography (CT), and magnetic resonance imaging (MRI)-based treatment planning has gained popularity over 2D-planning technique as they can help in volumetric dose assessment of target and organs at risk (OARs)., Target-based prescription can be used if 3D imaging is available. Better soft-tissue visualization on MRI image makes it a modality of choice. However, its limited availability makes CT, a less sensitive tool though, an alternate choice for target assessment., Dose-volume parameters such as D90 and D100 (minimum dose delivered to 90% and 100% of the target volume) that are used in 3D-image-based brachytherapy although give volumetric dose assessment of target, GEC-ESTRO still recommends to report dose to point A. Therefore, Manchester point A still remains a popular choice for dose recording.
Since the geometry of application can get disturbed with patient movement, the position of point A can change relative to change in tandem position. As point A lies in high-dose gradient region, any positional change leads to variation in dose received by point A. Therefore, it becomes necessary to acknowledge and minimize these variations. One such initiative was taken by the American Brachytherapy Society (ABS) which gave its recommendation in 2012 on new definition of point A., This new point A was defined to remove the practice of defining point A relative to flange as the dose values observed with this concept were not found consistent owing to applicator displacement or anatomical reasons.
In the present study, we have compared the effect of using two different point A definitions, Manchester point A (AMAN) and ABS point A (AABS), on volumetric and dosimetric parameters of target and critical structures. We have also tried to observe the relation between point A and 3D-dose-volume parameters.
| > Materials and Methods|| |
Fifty-five CT-based high dose rate-ICRT applications of 20 retrospectively treated cervical carcinoma patients between 2016 and 2017 with FIGO staging from IIB to IIIB using Fletcher Williamson stainless steel applicators consisting of two ovoids (diameter 20 mm, 25 mm, and 30 mm) and a uterine tandem (angles 15°, 30°, and 45°) were studied. High-risk clinical target volume (HR-CTV) and OARs such as bladder and rectum were contoured using GYN GEC-GESTRO guidelines,, in Oncentra Brachy Treatment planning system (version 4.5.2), Elekta, The Netherlands. In these previously treated plans, point A (AMAN) was defined using revised Manchester definition, i.e., 2 cm superior to the flange of the tandem and 2 cm lateral to uterine tandem. The dose of 7 Gy per fraction delivered in three fractions was prescribed to point AMAN. Plans were optimized in patients where rectum and bladder doses exceeded their tolerance values in accordance to institutional protocol.
New point A (AABS) was introduced in same CT-based plans in accordance with 2012 ABS recommendations. To determine AABS in treatment-planning computer a line passing through center of two ovoids was drawn. At the intersection point of this line with tandem, a distance equal to radius plus 2 cm superiorly was marked along the tandem and from this point 2 cm laterally to tandem, point AABS was marked. [Figure 1] represents the different isodose levels when the two definitions, Manchester and ABS, are used for prescription point.
|Figure 1: Manchester and American Brachytherapy Society definition of point A. (a) Manchester point A and (b) ABS point A|
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Source positions in new point AABS-based plans were kept same as in AMAN-based plans however normalization was done at this new point AABS. Dose received by both point AMAN and AABS, 2cc volume of OARs (D2cc) and minimum dose received by 90% and 100% of HR-CTV, i.e., D90 and D100 were recorded in both plans. Treatment volumes enclosed by 100% isodose line (V100), total reference air kerma (TRAK), and geometric shifts between point AMAN and AABS were also reported.
Statistical analysis using Paired t-test was performed to compare mean point A dose, D90, D2cc, and V100 values between Manchester and ABS plans.
| > Results|| |
On comparing ABS and Manchester point A-based plans, it was found that mean percentage dose difference between higher single point AMAN values (of the two-point As, left and right to the tandem) with respect to prescription dose was 1.25% ± 1.43% while for AABS values with respect to prescription dose was 1.21% ±1.01%. On statistical analysis, no significant difference (P = 0.42) was found between the mean point A doses of AMAN and AABS. [Figure 2] compares the point A dose in AMAN and AABS plans with respect to number of applications.
|Figure 2: Point A dose for Manchester and American Brachytherapy Society plans versus number of applications|
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Dose to 2cc, 1cc, and 0.1 cc volumes of bladder, rectum, and small bowel were obtained and listed in [Table 1]. The ratio of D2cc dose and prescription dose was found to be 86.76% ± 30.90%, 62% ± 18%, and 54% ± 23% for bladder, rectum, and bowel for AMAN based plans while 101% ± 32%, 71% ± 16%, and 62 ± 26% for AABS based plans [Table 2]. These values were significantly different (P < 0.05) in both plans.
|Table 1: Comparison of D2cc, D1cc and D0.1cc volume of bladder, rectum and bowel between Manchester and American Brachytherapy Society plans|
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|Table 2: Comparison of different dosimetric parameters between Manchester and American Brachytherapy Society plans|
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Mean percentage difference between prescribed dose and D90 for AMAN plans was found to be −37.90% ± 25.06% and −30.47% ± 25.50% for AABS plans. Mean percentage difference between prescribed dose and D100 was −62.85% ± 13.12% and −58.31% ± 15.65% for and AMAN and AABS plans, respectively [Table 2]. A significant difference (P = 0.002) was observed between ABS and Manchester plans in terms of mean percentage change between D90 and prescribed dose and D100 and prescribed dose, respectively.
Plans based on point AABS, normalization exhibited 8.73% ± 18.10% higher TRAK values. Average volumes of HR-CTV receiving the 100% of prescription dose in AMAN and AABS plans were 44.09 ± 17.29cc and 50.65 ± 16.98cc, respectively. Mean treatment volumes enclosed by 100% isodose line (V100) were 80.4 ± 20.45cc and 88.47 ± 16.78cc for AMAN and AABS plans, respectively. V100 differed by a mean percentage difference of 15.18% ± 29.34% and significant difference (P = 0.003) was observed between the two definitions. The comparison of these dosimetric parameters has been shown in [Table 2]. The average shift between AABS and AMAN location was found to be 9.6 ± 1.3 mm. ABS point A was thus found to be situated superiorly to Manchester point A. [Figure 3] demonstrates the relation between the ratio V100ABS/V100MAN and ratio DABS/DMAN with shift in point A where V100ABS/V100MAN represents the ratio of treatment volume enclosed by 100% isodose line in AABS and AMAN plans respectively and DABS/DMAN represents the ratio of mean doses received by point AABS and AMAN, respectively. It is visible from the figure that there is no correlation between the shift and the ratios V100ABS/V100MAN and DABS/DMAN.
|Figure 3: Ratio of V100ABS/V100MAN and DABS/DMAN versus shift in point A|
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| > Discussion|| |
Manchester point A has been used for decades as prescription point in ICBT of cervix cancer. However, since the location of this point A varies with the geometry of application, there arises a need for a more stable prescription point. In this study, we have compared plans based on Manchester point A and on point A introduced by ABS 2012 guidelines. Our findings suggest that the mean point A doses of Manchester and ABS plans did not differ significantly. The mean difference of point A with respect to prescription dose was 1.25% ± 1.43% for Manchester and 1.21% ± 1.01% for ABS plans which was similar to findings of Anderson et al. whose values were 1.7% and 1.5% for Manchester and ABS definitions, respectively. Kim et al. also reported similar results (mean 2.2% ± 3.0% for Manchester point and mean 1.8% ± 1.3% for ABS point). Howell et al. also concluded that the average percentage dose to Manchester point was greater than the average percentage dose to ABS point as Manchester point lies more close to ovoids thus receiving more dose. These results were in congruence with our findings.
As shown in [Table 2], the average ratio of D2cc dose to prescription dose was found to be higher for ABS plans which resembled results of Zhang et al. who found average D2cc doses for rectum and bladder to be 51% and 89% of prescribed dose respectively for Manchester point A and 60% and 106% of prescribed dose respectively for ABS point A. Similar results were derived from the study done by Howell et al. who showed that prescribing dose to ABS point A leads to higher bladder and rectum dose over prescribing dose to Manchester point A. The reason for higher OAR doses in ABS plans could be due to the fact that ABS point A lies superiorly above the Manchester point A. Significant difference in OAR doses observed in our study was similar to the results of Chennupati et al.
V100 was higher for ABS plans which matches with the observation of Anderson et al. and Zhang et al. who concluded that treatment volume enclosed by 100% isodose level was higher for plans normalized to ABS point A at this point was situated superiorly to Manchester point A. Kim et al. also demonstrated percentage difference of −3% between V100 values of ABS and Manchester plans and established that using ABS point A as prescription point increased the total delivered dose. However, the mean percentage difference in our study was 15.18% which was higher than the findings of Anderson et al. who calculated mean percentage change of 3%. The most probable reason for the large variation may be due to anatomical constraints, small pelvis and smaller separation between rectovaginal septum.
For HR-CTV, we found that mean percentage difference between D90 and prescribed dose was lower in ABS plans, and thus dose received by 90% volume of tumor volume was higher in plans using ABS point A definition for prescribing dose. There was a significant difference in D90 values of AABS and AMAN plans. This differed from Anderson et al. findings who found mean percentage difference between D90 parameter of two definitions to be not more than 2%. This variation is again due to anatomical constraints and small pelvic diameters. In another study done by Chennupati et al. it was reported that placement errors in point A with respect to various point A definitions have different effects on tumor doses. This could be a possible reason for the variation in D90 values of our results.
AABS plans in our study had on average 8.73% higher TRAK as compared to AMAN plans. This was somewhere close to the results of Anderson et al. who had found this value to be around 2%–3% and Kim et al. who showed that plans normalized to ABS point A had higher TRAK and thus prescribing to this point increased the total dose up to 2%. Zhang et al. had reported TRAK to be 14% higher for ABS point A-based plans which is different from our result. This difference could be due to the use of LDR brachytherapy machine in their methodology.
The mean distance of AABS point from AMAN point was found to be 9.6 ± 1.3 mm in our study. Zhang et al. and Anderson et al. calculated this shift to be 8.9 ± 5.4 mm and 6 mm, respectively. Chennupati et al. results were also close to our value.
| > Conclusion|| |
Point A has remained a consistent choice for dose prescription so far. However, due to variation in point A location, several definitions of point A have been explored. In this study, we compared the traditionally used Manchester point A definition and recently introduced point A definition by ABS and found that each of them had an impact on the dosimetric parameters. The study revealed that doses to both the points may be recorded during the transition period. However, ABS point A can be preferred over the Manchester point A as it conforms better with the desired outcome, is less susceptible to dose variations with geometrical changes and found to be more static.
Financial support and sponsorship
This work was funded by Intramural faculty research seed grant from King George's Medical University.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]