|Year : 2017 | Volume
| Issue : 1 | Page : 379-384
Evaluation of dose conformity and coverage of target volume for intensity-modulated radiotherapy of pelvic cancer treatment
M Atiq1, A Atiq1, K Iqbal2, Q Shamsi1, F Andleeb1, SA Buzdar1
1 Department of Physics, The Islamia University of Bahawalpur, Bahawalpur, Pakistan
2 Shaukat Khanum Cancer Hospital & Research Center, Lahore, Pakistan
|Date of Web Publication||1-Dec-2017|
Miss. M Atiq
Department of Physics, The Islamia University of Bahawalpur, Bahawalpur
Source of Support: None, Conflict of Interest: None
BACKGROUND: Better conformity may help in delivering minimum dose to organs at risk (OARs) and maximum dose to planning target volume (PTV). As per the requirements of modern radiotherapy, 95% isodose should cover the PTV, so conformity indices (CIs) are used for evaluating quality of conformation of treatment plans. AIM: This study aimed to investigate degree of conformity for pelvic patients using intensity-modulated radiotherapy (IMRT) technique. Three formulas of CIs described in literature were analyzed in this study. SETTINGS AND DESIGN: This study was performed to evaluate degree of conformity of 18 patients treated with radiotherapy treatment plan using cumulative dose volume histogram. Effectiveness of different CIs was explored for IMRT plans using 15 MV photon beam. Doses delivered to OAR were also studied. STATISTICAL ANALYSIS USED: CI suggested by the International Commission on Radiation Units and Measurements, radiation CI and CI prescription isodose to target volume (PITV) had mean ± standard deviation values of 1.02 ± 0.018, 0.98 ± 0.017, and 1.63 ± 0.333, respectively. RESULTS AND CONCLUSION: Dose distribution for all patients was highly conformal and clinically acceptable. Values of CI PITV exceeded acceptable value for 27% patients with minor deviation. No statistically significant differences were observed for three CIs reported. Target volume lies between 95% and 107% of prescribed dose which shows ideal target coverage. This simple parameter is advantageous since it is easy to interpret and helped determine quality of treatment plan. This study clearly demonstrated that favorable dose distribution in PTV and OARs is achieved using IMRT technique, and hence, the risk of damage to normal tissues is reduced.
Keywords: Conformity index, coverage, intensity-modulated radiotherapy, pelvic cancer, planning target volume, treatment plan evaluation
|How to cite this article:|
Atiq M, Atiq A, Iqbal K, Shamsi Q, Andleeb F, Buzdar S A. Evaluation of dose conformity and coverage of target volume for intensity-modulated radiotherapy of pelvic cancer treatment. Indian J Cancer 2017;54:379-84
|How to cite this URL:|
Atiq M, Atiq A, Iqbal K, Shamsi Q, Andleeb F, Buzdar S A. Evaluation of dose conformity and coverage of target volume for intensity-modulated radiotherapy of pelvic cancer treatment. Indian J Cancer [serial online] 2017 [cited 2020 Apr 5];54:379-84. Available from: http://www.indianjcancer.com/text.asp?2017/54/1/379/219602
| » Introduction|| |
Intensity-modulated radiation therapy (IMRT) is advance radiation therapy technique which provides high coverage to target by precise delivery of radiation to the tumor site. IMRT can generate dose distributions using computer optimization to minimize dose to organs at risk (OARs) and to achieve high dose to the cancerous part. IMRT plans were found to be dosimetrically and clinically advantageous for gynecological tumors, prostate cancer, head and neck cancer, and brain tumors., Dose-volume histogram (DVH) is the representation of quantified three-dimensional dose distribution. It quantifies minimum, maximum, modal, and mean dose values delivered to volume of interest and critical organs. Dose distribution in DVH is easy to elucidate for the volume of tumor cells because it marks isodose that covers given percentage of target volume.,, In clinical situations, it becomes inevitable to provide full coverage to the target. Hence, adjacent organs to tumor, which should be spared, are sacrificed and receive radiations which result in discomfort to patients. To minimize damage to OAR, they are outlined in computed tomography (CT) images. Therefore, when producing optimum treatment plan, fulfillment of dose requirement is assessed by various indices, one of which is conformity index (CI). This simple parameter is a good quality indicator of how well the dose distribution conforms to the shape of tumor. Deviation of values from ideal CI proposes over treatment or under treatment. The idea of CI was put forward to analyze dose distribution section by section. The Radiation Therapy Oncology Group (RTOG) proposed definition of CI in 1993 which was presented in International Commission on Radiation Units and Measurements (ICRU) Report 62., Its limitations were pointed out by Knöös et al. because the definition of isodose volume may differ from center to center. This volume may either be 95% isodose volume as per guidelines of ICRU Report 50, or it may correspond to least isodose volume encompassing the target volume. Therefore, CI may vary in accordance with isodose selected., As per the requirements of modern radiotherapy, 95% isodose should cover the planning target volume (PTV); hence, CI is simple and useful tool for quantitative analysis of this criterion.
This work aims to investigate dose conformity of IMRT plans for pelvic cancer. Acceptable treatment plans for gynecological cancers have been created by this technique.
| » Subjects and Methods|| |
Clinac Varian DHX was used to deliver 15 MV photon beams to all the plans under study. Plans were created, for which no hotspot lies outside PTV. Eighteen patients were selected at random, enrolled at Shaukat Khanum Memorial Cancer Hospital and Research Centre for the treatment of pelvic cancer. Planning process includes acquiring CT images using CT simulator. After determination of coordinates of organs to be treated by virtual simulation, laser system was used to tattoo the patients. Seven-field multileaf collimator technique was used for IMRT photon beam delivery. Increased number of beams is associated with better CI., Gantry angles were 30°, 60°, 105°, 180°, 255°, 300°, and 330°. Large number of fields is an indication of better conformity of dose distribution to the tumor site. Static treatment planning was conducted with Eclipse treatment planning system. Collimator and couch angle were 0° for all cases. Dose of 5000 cGy was delivered in 25 equal fractions. According to RTOG Report 0724, PTV for pelvic cancer patients should either receive 5040 cGy in 28 fractions  and same criterion was followed in our study.
In this study, we analyzed CIs in three-ways for pelvic cancer to indicate its clinical and technical aspects. CI was calculated for 18 patients; first, using formula as suggested by ICRU Report 62 which was originally reported in RTOG 90-05 protocol.
ICRU Report 50 and 62 included the concepts of target volumes and PTV. TV is treated volume which is defined as the volume for target enclosed by 95% of isodose lines, i.e., V95. PTV is geometric volume calculated from DVH which includes gross target volume and tumor site. While calculating PTV, setup uncertainties, patient movement, and internal motion are also taken into account., If value of CI is >1, it suggest less conformal plan as treated volume is larger than PTV. If value is <1, target volume is not completely covered, and hence, treatment does not compile with protocol of ICRU Report 50. CI has been criticized for its inability to consider spatial overlap of treated and target volume. To account for spatial overlap, modified CI was proposed by Paddick. However, review of dose distribution proved that both indices effectively provide the same information.
Knöös et al. suggested radiation CI (RCI) referred to as RCI:
RCI is inverse of CI previously described.
CI prescription isodose to target volume (PITV) is defined by simple ratio:,
Prescription isodose volume (PIV) that completely envelops the tumor volume. Quality of conformation is precisely defined by PITV values. Observing RTOG guidelines, if values of PITV lie between 1 and 2, treatment plan is acceptable. There will be minor violation in treatment plan if index lies between 0.9 and 1 or 2 and 2.5. When CI values exceed 2.5 or fall behind 0.9, plan is considered to have major violation, but plan may be considered acceptable., Various formulas are available in literature to calculate CI. Nonetheless, factors that influence CI are unknown yet. However, CI might be influenced by factor such as PTV volume.
Coverage index is defined as ratio of minimum dose within target volume to prescribed dose.
Prescribed dose PD is minimum dose to tumor volume. If target volume is completely covered by 90% of prescription isodose, then plan is considered per protocol. There will be minor deviation if target is covered by 80% of prescribed dose. If 80% of PD does not completely encompass the target, it is regarded as major deviation. For patients suffering from gynecological cancer, ICRU Report 50 suggests a homogeneous dose of −5% to +7% of PD to the target volume. However, conventionally in most clinical practices, ±10% is considered as an acceptable variation. Commonly used indices in radiotherapy with their ideal value and acceptable variation is given in [Table 1].,,,,,
| » Results|| |
Minimum, mean, and maximum doses were given for PTV and OARs in [Table 2] for 18 patients. In all the treatment plans, doses to rectum and bladder were well below the tolerances as recommended by RTOG guideline P0126. Bladder volume was 220.85 cm 3 and rectum volume was 89.3 cm 3 on average. Average values of Different indices are graphically represented in [Figure 1].
|Figure 1: Summary of average values of conformity indices and coverage for 18 clinical cases|
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|Table 2: Minimum, mean, and maximum doses of intensity-modulated radiation therapy treatment plans|
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Eighteen cases were evaluated using three CIs. Significance or nonsignificance of treatment plan is described by t value by taking significance level ≤0.05. Dose coverage for PTVs and OARs was observed in our treatment plans from comparison of average values of CIs. Frequently used CI definitions from literature with three parameters (TV, PIV, and PTV) were investigated.
The value of CI PITV should be between 1 and 2 as per protocol. Seventy-eight percent of plans had CI PITV between 1 and 2 with no major deviation. The formula of RCI used in our study was inverse of CI, which is confirmed in this study. For objective evaluation of plan, values of CIs were presented and graphically analyzed. All the plans of study had satisfactory results in the tested range. CI is used to assess plan quality. Our data suggested that CI, RCI, and CI PITV provided plan quality equally.
Study revealed that no general increasing or decreasing trend is found between CI PITV as illustrated in [Figure 2]. Similar results also hold true for other CIs. Hence, CIs, undertaken in this study, are independent of PTV. Mean values and standard deviation of Conformity indices and coverage are listed in [Table 3].
|Figure 2: Graph of planning target volume versus conformity index prescription isodose to target volume|
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|Table 3: Mean values and standard deviation of conformity indices and coverage of 18 patients|
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Results presented in [Figure 3] show that CIs lie in acceptable range. Thus, requirement of conformal dose to tumor is achieved in our case, with no overdosage or underdosage. Hence, quality of treatment plan is assured.
CI was plotted against target volume for 18 patients as represented in [Figure 4].
Next, CI PITV, CI, and RCI are graphically compared with each other in [Figure 5] and [Figure 6].
|Figure 5: Comparison of conformity indices radiation conformity index and conformity index prescription isodose to target volume|
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|Figure 6: Comparison of conformity indices and radiation conformity index|
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Analysis of results suggests that:
Fit had an R2 value 1 for equation 5 and 0.625 for equation 6.
Graph of Dmin% and Dmax% versus number of patients is illustrated in [Figure 7] and [Figure 8], respectively, 17 out of 18 plans (94%) were created with maximum dose ≤107% of PD and minimum dose ≥95% of PD. Dose distribution and DVH scan of a patient are presented in [Figure 9] and [Figure 10], respectively.
|Figure 7: Graph of minimum dose and number of patients for intensity-modulated radiotherapy treatment plans|
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|Figure 8: Graph of maximum dose and number of patients for intensity-modulated radiotherapy treatment plans|
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|Figure 9: Dose distribution of intensity-modulated radiotherapy plan with 15 MV photon beam for pelvic cancer patient|
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|Figure 10: Cumulative dose volume histogram of planning target volume, rectum, and bladder|
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| » Discussion|| |
This study clearly demonstrated that favorable dose distribution in PTV and OARs is achieved using IMRT technique, and hence, the risk of damage to normal tissues is reduced. Maximum and minimum doses to PTV and tumor volume are significantly associated with tumor control. Review of numerous papers suggested that no secondary cancer is developed in patients treated with 15 MV IMRT as a result of treatment with radiation therapy. Prescription doses to PTV and OAR are according to guidelines: 5000 cGy delivered in 28 fractions. OARs include rectum, bladder, and femurs. Often, rectum is considered as dose limiting organ.,
Quality assurance of treatment plan required that PIV coverage is ≥95%–<98% of PTV. Sixty-five percent of patients followed the RTOG P0126 recommendations with 35% of patients with minor variation in coverage of PTV.,,
No relationship between CI and PTV was found, which was also suggested by Knöös et al. Plans with larger target volumes have smaller values of CI. In this study, as the target volume increased, CI value improved which resulted in improvement of plan. Results of this paper concurred with the result of studies previously published., Definition of target volume is central to CI. Knöös et al. suggested that influence of target volume on CI for small values of PTV is because minute change in volume will result in large relative change. Larger PTVs have better CIs, because for small PTVs, it becomes increasingly difficult to cover isodoses to match the PTVs. CI PITV value <1 means that PTV is not completely covered by reference dose. For values above 2, coverage of PTV is still in acceptable limit; in our results, there was not any CI value <1, which means that reference dose volume is always larger than PTV. Despite customization of optimum parameters, when plan did not meet requirements of OAR constraints, more preference is given to OAR and PTV is sacrificed.
In all the treatment plans, prescription isodose covers 95% of respective PTV. According to ICRU Report 50, minimum dose to tumor should be ≥95% and maximum dose should be ≤107% of PD as is illustrated in [Figure 9] and [Figure 10] of this study. Seventeen out of 18 plans (94%) were created with maximum dose ≤107% of PD and minimum dose ≥95% of PD: hence, if this requirement is not achieved, it may result in overdosage or underdosage to PTV., There is no overdosage or underdosage for patients under study.
| » Conclusion|| |
CI is advantageous since it is easy to interpret and help determine quality of treatment plan, but there is still the need for visual inspection of treatment plans. Good values of CIs are observed in this study, which signify better PTV coverage and superior treatment plan. CI parameter should be set as planning requirement for quality assurance to treatment plan. It is recommended that determination of this parameter has taken into account in clinical practice when investigating a new technique. This study clearly demonstrated that favorable dose distribution in PTV and OARs is achieved using IMRT technique, and hence, the risk of damage to normal tissues is reduced.
Financial support and sponsorship
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
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