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ORIGINAL ARTICLE
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Evaluating different radiotherapy treatment plans, in terms of critical organ scoring index, conformity index, tumor control probability, and normal tissue complication probability calculations in early glottic larynx carcinoma


1 Department of Radiation Oncology, Medical Physics Division, Antalya Research and Treatment Hospital, Medical Sciences University, Antalya, Turkey
2 Department of Radiation Oncology, Antalya Research and Treatment Hospital, Medical Sciences University, Antalya, Turkey
3 Department of Radiation Oncology, Suleyman Demirel University, Isparta, Turkey

Correspondence Address:
Aysun Inal,
Department of Radiation Oncology, Medical Physics Division, Antalya Research and Treatment Hospital, Medical Sciences University, Antalya
Turkey
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.JCRT_888_18

 > Abstract 


Purpose: In this study, it is aimed to compare three different radiotherapy treatment planning techniques in terms of critical organ scoring index (COSI), two different conformity index (CI), tumor control probability (TCP), and normal tissue complication probability (NTCP) calculations in early (T1) glottic larynx carcinoma (T1GL). Furthermore, it is aimed to investigate these parameters compliance with dose-volume histograms (DVH) parameters.
Materials and Methods: Ten T1GL patients were immobilized in a supine position with a head and neck thermoplastic mask. Treatment plans were created with opposed lateral fields (OLAFs) and intensity-modulated radiation therapy (IMRT) techniques with a total dose of 66 Gy in 33 fraction with 2 Gy/day. IMRT fields were selected as five fields (5IMRT) and seven fields (7IMRT). Dosimetric evaluation of three different treatment plans for T1GL carcinoma was performed in two consequential steps. First step was the assessment of planning target volume (PTV), all organs at risks (OARs), and normal tissue (NT) dose calculations according to given dose constraint directions and comparing the plans via DVH. In the second step, for PTV, the compatibility of DVH data with CIs-TCP was investigated where COSI-NTCP was compared with DVH for OARs. The DVH data were considered as reference in all evaluations.
Results: The CIRTOG mean values were significantly closer to 1 with IMRT plans when compared to OLAF plans (P = 0.005). The CIPADDICK mean values revealed that OLAF plans were significantly worse than IMRT plans (P = 0.005). No statistically significant difference was found between all three plans in terms of homogeneity index mean values (P = 0.076). The calculated mean TCP values were significantly better for 7IMRT plans when compared to OLAF and 5IMRT plans (P = 0.007 and P = 0.017, respectively). Both NTCP and COSI evaluations, which is compatible with DVH, significantly favored OLAF plan for spinal cord and 7IMRT for thyroid gland. The COSI evaluations, which are compatible with DVH, significantly favored 7IMRT plan for carotid arteries and 5IMRT plan for NT.
Conclusion: Our results demonstrated that CIPADDICK-TCP calculations for PTV and COSI-NTCP calculations for OARs were compatible with DVH in T1 GL plans. Therefore, we suggest such parameters as valuable tools for choosing the feasible one among multiple plans and even with different treatment machines.

Keywords: Conformity index, critical organ scoring index, normal tissue complication probability, tumor control probability



How to cite this URL:
Inal A, Duman E, Ozkan EE. Evaluating different radiotherapy treatment plans, in terms of critical organ scoring index, conformity index, tumor control probability, and normal tissue complication probability calculations in early glottic larynx carcinoma. J Can Res Ther [Epub ahead of print] [cited 2019 Oct 16]. Available from: http://www.cancerjournal.net/preprintarticle.asp?id=268777




 > Introduction Top


Emphasis on larynx cancer among all head and neck malignancies is not only because of its prevalence but also because of its high ratio of early stage at diagnosis (T1-T2N0M0).[1] Neither surgery nor radiation therapy (RT) is proven to be superior to one another as single modality.[2] When voice preservation is taken into account as the main issue, RT becomes the mainstay of the treatment.[3],[4],[5],[6] The goal is to provide cure with least morbidity. To achieve this, different treatment planning techniques can be used in RT.[2] Classical techniques such as lateral opposed fields are known to be effective [3],[4],[5],[6],[7] but unfortunately insufficient in terms of sparing normal tissues (NTs) of concern such as the skin, bilateral carotid arteries, and thyroid gland.[8],[9],[10] This encouraged novel approaches and techniques in RT which can reduce the dose to uninvolved adjacent tissues and avoid long-term morbidity.[1],[2],[11],[12]

The International Commission on Radiation Units and Measures (ICRU) report 83 has defined evaluation (prescribing, reporting, and comparison) of different treatment plans in three levels.[13] Level 1 recommends assessment of absorbed dose in the central axis and two-dimensional dose distribution (2D). Level 2 recommends volumetric dose evaluation for target volume (TV), organs at risks (OARs), and NT. Hence, parameters that determine plan quality such as 3D isodose distribution, dose-volume histograms (DVHs), homogeneity index (HI), and conformity index (CI) acquired currency in this level.[13],[14] The most important disadvantage for CI and HI is that these models do not take the overall dosimetric information provided by DVHs into consideration. Consequently, DVHs are still accepted as key indicators of the compliance with clinical requirements. Another crucial drawback is lack of estimations in terms of OAR and healthy tissue sparing while evaluating target coverage and conformity which is a dose-limiting issue in radiotherapy. Therefore, some volume-based indices have been proposed such as healthy tissue overdosage factor,[15] healthy tissue CI,[16] and critical organ scoring index (COSI).[17] Level 3 includes radiobiological evaluations such as tumor control probability (TCP) and NT complication probability (NTCP) which are not used in the clinical standard.[14]

It is obvious that decision-making process between different treatment plans is highly subjective. A skillful, experienced, and careful eye view on DVHs and isodose curves is needed to pick up the best plan.[18] Although several approaches and parameters have been advocated to ease and ameliorate this process, none of these decision support tools are widely accepted in clinical use alone yet.[19]

In this study, it is aimed to compare three different radiotherapy treatment planning techniques in terms of COSI, two different CI, TCP, and NTCP calculations in early (T1) glottic larynx carcinoma (T1GL). Furthermore, it is aimed to investigate these parameters compliance with DVH parameters.


 > Materials and Methods Top


Patient selection

Ten patients with larynx cancer who were treated with definitive radiotherapy from March 2014 to August 2016 were enrolled in this study. Inclusion criteria were tumors located in the glottic larynx with T1a-bN0 disease. Larger tumors invading supraglottic and/or subglottic tissues were excluded. Planning tomography images of the patients were reevaluated retrospectively.

Ethical considerations

This study was approved by the Institutional Scientific Research Ethics Committee. All procedures performed were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent was waived owing to the retrospective chart review nature of the study.

Simulation and volume definition

Ten T1 GL patients were immobilized in a supine position with a head and neck thermoplastic mask. Axial slices were obtained from the planning computed tomography (CT) scans (GE-Light Speed 64, GE, USA) with a 1.25 mm slice thickness. Intravenous contrast media was used in all patients. The CT data were then transferred to XiO Focal system (CMS Co., Ltd, St Louis, MO, USA). The bilateral true vocal cords are outlined as gross tumor volume (GTV). The clinic TV (CTV) included true vocal cords, arytenoids, false vocal cords, anterior, and posterior commissure, from the top of the thyroid cartilage to the bottom of the cricoid cartilage. The planning TV (PTV) encompassed the CTV with 5 mm margin in all directions. The OARs outlined were the thyroid gland, the medulla spinalis superiorly from foramen magnum to 5 cm below PTV inferiorly. The bilateral carotid arteries which were labeled as left and right are contured from the aortic arch on the left and brachiocephalic trunk on the right and extended superiorly to at least 2 cm superior to PTV. Furthermore, the entire area except the PTV in the transverse slices where the PTV was located is defined as NT.

Planning

Treatment plans were created with opposed lateral fields (OLAFs) and intensity-modulated RT (IMRT) techniques with a total dose of 66 Gy in 33 fraction with 2 Gy/day using Elekta XiO treatment planning system (TPS) (CMS Co., Ltd, St Louis, MO, USA). IMRT fields were selected as five fields (5IMRT) and seven fields (7IMRT). In all plans, superposition–convolution calculation algorithm was used, and IMRT plans were optimized with step and shoot technique. The planning techniques and gantry angles used for T1 GL are shown in [Table 1].
Table 1: Planning techniques and gantry angles

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Both OLAF and IMRT plans are designed to give 98% of the prescribed dose to 95% of the PTV with 6 MV photon energy. The median dose of PTV was also provided to be as close as possible (±1 Gy) to the prescribed dose. The dose constrains for OARs were as follows: maximum medulla spinalis dose <40 Gy, mean carotid dose <30 Gy, and mean thyroid dose <40 Gy. For the NT, plans were adjusted to minimize V50% which defined as the volume (cc) receiving half of the prescribed dose. In all plans, primary concern was the PTV dose requirements meanwhile power for OARs was evaluated equally.

Plan evaluation

Convenience of dose distributions in transvers, sagittal, and coronal slices for all plans were visually checked, and DVH was evaluated in terms of PTV, OARs, and NT. It is confirmed via DVH data that 95% of PTV volume was given the 98% of prescribed dose in all plans.

For OARs, the volumes V20Gy, V26Gy, V30Gy, and V40Gy of the thyroid gland, V20Gy and V30Gy of medulla spinalis, and V30Gy, V40Gy, V45Gy, and V50Gy values for carotid arteries were calculated from DVH data. Here, VxGy defines the percent volume of the concerned organ receiving X Gy dose.

The mean values of left and right carotid arteries were taken as a single value for carotid artery volume. All of these volumes plotted as VxGy in graphics. In addition, mean doses (Dmean) of thyroid gland and carotid arteries and maximum doses (Dmax) of medulla spinalis were calculated. The volumes (cc) receiving 90% and 50% (V90% and V50%) of prescribed doses were calculated for NT.

The HI and two different CI parameters (CIRTOG and CIPADDICK) were calculated. where PIV was reference isodose volume and TV was Tumor Volüme. The TV was equal to PTV.

Equation 1:



The CIPADDICK calculation was described below in Equation 2 which was defined to use in evaluating stereotactic radiosurgery plans.[20] Here, TVPIV was TV covered by the reference isodose.

Equation 2:



In addition, the HI was calculated according to the Equation 3 which was described in ICRU 83. The doses received by 2%, 50%, and 98% volume of TV were defined as D2%, D50%, and D98%, respectively.

Equation 3:



In addition to the Level 2 evaluation methods, the COSI values were calculated (Equation 4).[17] In this equation, V(OAR)>tol is volume of organ at risk receiving more than tolerance dose, TCV is TV covered by the reference isodose (PIV).

Equation 4:



TCP (%) were evaluated for all plans. The linear-quadratic cell survival model (LQ) was used for TCP calculation on the XiO TPS.[21],[22],[23] On the basis of fundamental formulation (Equation 5), other current TCP model formulations (Equations 6-10) and explanations of parameters were described below.

Equation 5:



  • Ns: Number of cells surviving after irradiation
  • N0: initial number of tumor cells.


Equation 6:



No, i= Initial number of tumor cells in voxel “i”

Equation 7:



  • ρ (cells/cc) = The clonogenic cell density of the tumor (ρ = 8 × 105 cells/cc)
  • αmean(1/Gy) = Linear radiosensitivity term from LQ model (αmean= 0.35 1/Gy).


αα= The standard deviation or level of interpatient variability of radiosensitivity (αα= 0.05)

Di = Dose to voxel “i” (uniform within voxel).

Equation 8:



Teff(days) = The effective doubling time of tumor clonogens (Teff= 7 days).

Tk(days) = The lag time between the first treatment and beginning of tumor proliferation.

(Tk= 5 days).

T (days) = The overall time of radiotherapy treatment (T = 43 days).

Equation 9:



Equation 10:



According to Lyman's model, the NTCP of a fractional organ volume was calculated by Equations 11–14 on XiO TPS.[24]

Equation 11:



Equation 12:



Equation 13:



  • m = Parameter describing the slope of NTCP vs. dose
  • V = Fraction of reference volume irradiated and
  • Vref= Fraction of reference volume irradiated
  • TD50(1) = The dose to reference volume leading to 50% complication probability
  • TD50(sυ) = The dose to partial volume leading to 50% complication probability.


Equation 14:

TD (υ) = TD (1) *υ−n

n = Parameter describing the volume dependence.

The calculated n, m, and TD50 values for NTs and clinical endpoints of toxicity in case of exceeding tolerance doses are shown on [Table 2].[25]
Table 2: Fitting of normal tissue tolerance data on analytic function

Click here to view


If the CIRTOG value is “1,” the plan is considered as ideally conformal. In a similar manner, CIPADDICK value of “1” indicates an ideal plan while “0” value necessitates replanning. On the contrary, HI value of “0” indicates an ideal plan. COSI value closer to 1 means better OARs protection. TCP and NTCP values of a best plan should be “100%” and “0%,” respectively. In this study, the COSI, DVH, and NTCP calculation results revealed perfect plans which are described above.

Herein, we performed the dosimetric evaluation of three different treatment plans for T1GL carcinoma in two consequential steps. First step was the assessment of PTV, all OARs, and NT dose calculations according to given dose constraint directions and comparing the plans via DVH. In the second step, for PTV, the compatibility of DVH data with CIs-TCP was investigated where COSI and NTCP were compared with DVH for OARs. The DVH data were considered as reference in all evaluations. While the NTCP was evaluated only for medulla spinalis and thyroid gland in the absence of NT tolerance data for carotid arteries, the COSI was calculated for all selected OARs and NT. All volumes covered by the selected tolerance doses such as V30Gy for carotid arteries and medulla spinalis, V40Gy for thyroid gland and V50% for NT were converted to geometrical shapes in the TPS, and these created volumes were used in COSI calculations for OARs evaluations.

Statistical analysis

The Statistical Package for Social Sciences version 15.0 (SPSS Inc., LEAD Technologies, 1991/Charlotte, North Carolina/USA) was used for statistical analysis. The Friedman test and Wilcoxon signed-rank test were used for comparisons. P < 0.05 was considered to be statistically significant.


 > Results Top


Three different treatment plans for each patient (30 plans in total) were evaluated. Ninety-five percent of the PTV was enclosed by at least 98% of the prescribed dose (64.68 Gy) for all treatment planning techniques.

The CIRTOG mean values were found 1.66 (±0.16), 1.17 (±0.05), and 1.18 (±0.06) for OLAF, 5IMRT, and 7IMRT treatment plans, respectively. The CIRTOG mean values were significantly closer to 1 with IMRT plans when compared to OLAF plans (P = 0.005). No significant difference was found between 5IMRT and 7IMRT plans. The CIPADDICK mean values were found 0.52 (±0.08), 0.83 (±0.03), and 0.85 (±0.05) for OLAF, 5IMRT, and 7IMRT treatment plans, respectively. Moreover, the difference revealed that OLAF plans were significantly worse than IMRT plans (P = 0.005). Results between 5IMRT and 7IMRT slightly favored 7IMRT, but the difference was not statistically significant (P = 0.09). The HI mean values were found 0.13 (±0.02), 0.16 (±0.01), and 0.15 (±0.02) for OLAF, 5IMRT, and 7IMRT treatment plans, respectively, where the difference was not statistically significant (P = 0.076).

The calculated mean TCP values were 65.21% (±1.10), 64.59% (±1.43), and 66.8% (±1.06) for OLAF, 5IMRT, and 7IMRT treatment plans, respectively. Results were significantly better for 7IMRT plans when compared to OLAF and 5IMRT plans (P = 0.007 and P = 0.017, respectively).

The DVH evaluation of medulla spinalis, thyroid gland, carotid arteries, and NT volume values were summarized in [Table 3]. The V30Gy of medulla spinalis, which was used for COSI evaluation at the same time, was significantly lower in the OLAF treatment plans compared to IMRT treatment plans (P < 0.001). In the comparison of 5IMRT and 7IMRT treatment plans, significant differences were found in favor of 7IMRT for medulla spinalis (P = 0.007).
Table 3: Dose volume histograms evaluation of medulla spinalis, thyroid gland, carotid arteries, and normal tissue

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The Dmean, V26Gy, V30Gy, V40Gy, and V50Gy values of thyroid glands were lower in the IMRT treatment plans compared to OLAF. Comparison of the V40Gy values of DVH parameters for thyroid gland which was used for COSI evaluation revealed a decrease with IMRT planning techniques (P < 0.001). Between 7IMRT and 5IMRT, significant differences were found in favor of 7IMRT (P = 0.005).

The Dmean, V30Gy, V40Gy, V45Gy, and V50Gy values of carotid arteries were lower in the IMRT treatment plans compared to OLAF. When we compared 7IMRT and 5IMRT, results were significantly superior in 7IMRT for V30Gy values (P = 0.005).

The V50% of DVH evaluation revealed that 5IMRT treatment plans provided best protection for NT compared to other two planning techniques which were OLAF and 7IMRT (P = 0.005 and P = 0.005, respectively). No statistically significant difference was found between OLAF and 7IMRT treatment plans (P = 0.169).

The calculated NTCP of medulla spinalis was found higher with 5IMRT treatment plans compared to 7IMRT plans (P = 0.005), and the NTCP with OLAF treatment plans was lower than 7IMRT treatment plans (P = 0.005). The calculated COSI results were correlated with NTCP results that is to say 5IMRT had the lowest value according to the remaining two other plans which were OLAF and 7IMRT (P = 0.005 and P = 0.004, respectively). For spinal cord, (a) NTCP-V30Gy, (b) COSI-V30Gy, and (c) VxGy graphics are shown in [Figure 1].
Figure 1: Medulla spinal is normal tissue complication probability - V30 (cc) (a), critical organ scoring index - V30 (cc) (b) and Vx (cc) (c) graphics

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The NTCP of thyroid gland was higher with OLAF treatment plans compared to both 5IMRT and 7IMRT plans (P = 0.005 and P = 0.005, respectively). Best thyroid gland protection was achieved with 7IMRT treatment plans compared to 5IMRT (P = 0.028). When we compared 7IMRT versus 5IMRT and 5IMRT versus OLAF in terms of the COSI, the results were correlated with NTCP results (P = 0.01 and P = 0.005, respectively). For thyroid gland, (a) NTCP-V40 Gy, (b) COSI-V40 Gy, and (c) VxGy graphics are shown in [Figure 2].
Figure 2: Thyroid gland normal tissue complication probability - V40 Gy (cc) (a), critical organ scoring index - V40 Gy (cc) (b) and VXGy (cc) graphics

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While tolerance data of carotid arteries and NT were not defined in the literature, the NTCP of those organs was not calculated. According to COSI values, the best protection for carotid arteries was achieved with 7IMRT treatment plan compared with 5IMRT and OLAF plans (P = 0.003 and P = 0.004, respectively). For carotid arteries, (a) COSI-V30 Gy and (c) VxGy graphics are shown in [Figure 3].
Figure 3: Carotid arteries critical organ scoring index - V30 Gy (cc) (a) and VxGy (cc) (b) graphics

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The COSI evaluation of NT revealed a statistical significance in favor of 5IMRT compared to OLAF and 7IMRT treatment plans (P = 0.011 and P = 0.005). NT protection was worse with OLAF plans compared to 7IMRT plans (P = 0.063). For NT, (a) COSI-V50% and (b) Vx% graphics are shown in [Figure 4].
Figure 4: Normal tissue critical organ scoring index - V30 Gy (a), and Vx% (cc) (b) graphics showed in Figure 4

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 > Discussion Top


Planning target volume evaluation

In the DVH evaluations, enclosure of 95% of PTV in 98% isodose line and all other parameters were found in accordance with the required dose directives. Furthermore, when treatment plans were compared via CI and TCP calculations, it was found that PTV coverage was best in 7IMRT plans.

CIRTOG is a useful adjunct for the evaluation done via DVH from CT slices.[15] Even PTV and PIV encompasses a similar mathematical volume, CI does not consider the spatial intersection ratio and cannot reflect dose homogeneity in PTV, so examining isodose distribution is obligatory.[26] CIPADDICK takes both PTV and organs nearby PTV into consideration. Although it has the widest range of utilization, currently, as a comparison index, it is inadequate due to lack of radiosensitivity differentiation between organs.[15] Besides, the data we have to date are insufficient to ensure the correlation between clinical results and CI evaluation models.[27] All the above-mentioned studies suggest the need for a new comparison index for different treatment plans in terms of PTV. In addition, treatment plans are evaluated via CI values in all slices, and it is confirmed that all the plans ensure the safety and conformity limits. When difference between IMRT plans in terms of DVH and CI evaluations is not significant for PTV, the choice may be compelling. It is known that even very small cold points can cause a great decrease in TCP, likewise hot points (if hot point has a measurable volume) also change TCP significantly.[28] Therefore, utilization of this TCP equation sensitive for “cold points” in the PTV – which could be overlooked – is critical in multiple plan evaluations.

Organs at risks and normal tissue evaluations

When spinal cord Dmax was evaluated according to DVH data, OLAF was the best planning technique. In the average dose calculations, the best plan for thyroid gland and carotid arteries was 7IMRT, while 5IMRT was optimal for NT. Menhel et al. speculated that graphical analysis of COSI-CIRTOG values are useful while it provides a quick, simple, and accurate result.[17] We used the same graphic in our study to detect the consistence among the parameters of critical organ evaluation (COSI-DVH and NTCP-DVH). Volume effect concept is fundamental for modeling dose–response relations of NTs in NTCP calculation. In parallel organs, maintaining the organ function in a considerable extend is provided via decreasing the irradiated volume and is strongly correlated with a determined dose to a certain organ volume. However, in serial, it is correlated with maximum organ dose and “hot point.”[29]

In our study, both NTCP and COSI evaluations, which is compatible with DVH, significantly favored OLAF plan in consistence with spinal cord. The spinal cord dose was increased in IMRT plans, although it remained within the recommended tolerance dose range.

The OLAF plan primarily avoid the dose to spinal cord; however, a certain proportion of the thyroid is exposed to higher dose. Between two IMRT plans, mean thyroid gland dose was higher in 5IMRT. Kim et al. reported that thyroid gland Dmean and V35–V50 volumes were significantly associated with hypothyroidism.[30] In the study of El-Shebiney et al.,[31] V30 was defined as dose-volumetric threshold for low- and high-risk radiation-induced hypothyroidism in head and neck radiation treatment. Radiation-induced hypothyroidism was found 29.4% and 71.4%, in the group with V30Gy <42.1% and ≥42.1%, respectively (P = 0.002). In 7IMRT plans, V30Gy and V40Gy for thyroid glands were found <42.3% and <32%, respectively. These results were compatible with NTCP and COSI evaluations which may allow the radiation oncologist to select the feasible plan in terms of late toxicity.

Evidence associating late cerebrovascular events such as transient ischemic attack and stroke with carotid doses during radiotherapy is gradually increasing. The vascular events can be seen even years after radiotherapy.[32] A significant increase in intima–media thickness after 35 Gy is shown by Martin et al.[33] A total of 35 Gy has been indicated as the threshold for intima–media thickness and wall abnormalities of the carotid artery.[33] A recent retrospective cohort study evaluated carotid artery stenosis in 366 head and neck patients after definitive or adjuvant radiotherapy. Actuarial risk for stenosis was 29% at 8years. In multivariate analysis, diabetes was found to be predictive; however, carotid dose parameters were not significantly associated with time to stenosis.[34]

An additional radiation-related carotid toxicity is carotid blowout syndrome, a rare but devastating complication following reirradiation of the head and neck region for recurrence or a second malignancy.[35] Therefore, it is important to minimize the unnecessary dose to the carotid artery during initial treatment. In an attempt to estimate a model to present the dose–response relations of carotid blowout syndrome (CBOS) after stereotactic body radiotherapy (SBRT), Mavroidis et al. reported a retrospective study on 61 inoperable locally recurrent head and neck cancer patients treated with SBRT. Lyman-Kutcher-Burman, relative seriality, and Logit NTCP models were used to fit the clinical data. Conclusively, Dmax to the internal carotid <34 Gy significantly reduced the risk for CBOS.[36] In our study, V30Gy is decreased by 86.8% with 7IMRT compared to OLAF. COSI evaluation was in consistence with DVH calculations which also revealed best results via 7IMRT.

The IMRT more generous volumes will be exposed to low doses compared to OLAF which unfortunately will lead to an increase in secondary malignancy risk.[37],[38] Secondary malignancy probability in a 10 years' survival is predicted to be 1% in OLAF where it can rise to 1.75% with IMRT.[39] In this study, 5IMRT plans were found to provide the best protection for NT via V50% and V90% values. These results were also in consistency with COSI calculations. The V50% of NT could inadequate for comparing the risk of secondary radiation-induced malignancies; these results allowed us to compare the volume of tissue which was irradiated unnecessary.

There are two major limitations of this study to our concern. First is the absence of NT low dose evaluation which is an important factor for late toxicity and secondary malignancy issues. Second one is the small number of patients included. Especially when the fact of several P values being slightly near significance is taken into consideration, our results could be more accurate with more patients.

In radiation treatment, DVHs provide a graphical demonstration of dosimetric data about PTV and OARs, and it is currently the most common tool to compare different treatment plans. However, it is all but impossible for the radiation oncologist to evaluate all the relative data obtained from alternative plans sufficiently and accurately. As there is no way to see all the DVH data overlapped to provide comparison at a glance, we need another constant strongly in convenience DVH.[17]

All dosimetric evaluations are definitely related to accurate organ delineations done by clinicians. It should also be considered that variables such as tissue structure, fractionation, serial, or parallel designation of organ are independent from geometric definitions.[15] Therefore, radiobiological evaluation became more critical with developing technology.[29] NTCP and TCP calculation models are not widely accepted for common utilization due to suspicions in robustness of predictions and accuracy of parameter values.[29] Another insufficiency for NTCP is that only severe complications are accepted as end points of toxicity. However, it has been widely known that DVH evaluations should be replaced by biological indices to obtain better reflection of clinical goals of treatment.[40] Many researchers included TCP and NTCP models in their computer programs to evaluate multiple treatment plans.[41]


 > Conclusion Top


Our results demonstrated that CIPADDICK-TCP calculations for PTV and COSI-NTCP calculations for OARs were compatible with DVH for PTV in T1 GL plans. Therefore, we suggest such parameters as valuable tools in choosing the feasible one among multiple plans and even with different treatment machines. In the future, it is thought that taking factors such as patient age, smoking history, pretreatment organ function, chemotherapy scheme, surgery, beam energy, dose scheme, and dose rate into consideration will make NTCP a more accurate, widely used, and important tool for plan evaluation. Moreover, future studies with higher number of patients may support our results.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
 > References Top

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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3]



 

 
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