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 Table of Contents  
Year : 2013  |  Volume : 9  |  Issue : 3  |  Page : 422-429

Forward versus inverse planning in oropharyngeal cancer: A comparative study using physical and biological indices

1 Department of Radiation Oncology, Valavadi Narayanaswamy Cancer Centre, G. Kuppuswamy Naidu Memorial Hospital, Coimbatore, India
2 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bangalore, India
3 Department of Physics, PSG College of Technology, Coimbatore, India
4 Department of Physics, Bharathiar University, Coimbatore, India

Date of Web Publication8-Oct-2013

Correspondence Address:
T Sundaram
V. N. Cancer Centre, G. Kuppuswamy Naidu Memorial Hospital, Coimbatore - 641 037
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Source of Support: None, Conflict of Interest: None

PMID: 24125977

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 > Abstract 

Context: Possible benefits of inverse planning.
Aims: To analyze possible benefits of inverse planning intensity modulated radiation therapy (IMRT) over field-in-field 3D conformal radiation therapy (FIF-3DCRT) and to evaluate the differences if any, between low (6 Million Volts) and high energy (15 Million Volts) IMRT plans.
Materials and Methods: Ten patients with squamous cell carcinoma of oropharynx, previously treated with 6 MV step and shoot IMRT were studied. V 100 , V 33 , V 66 , mean dose and normal tissue complication probabilities (NTCP) were evaluated for parotid glands. Maximum dose and NTCP were the parameters for spinal cord.
Statistical Analysis Used: A two-tailed t-test was applied to analyze statistical significance between the different techniques.
Results: For combined parotid gland, a reduction of 4.374 Gy, 9.343 Gy and 7.883 Gy were achieved for D 100 , D 66 and D 33 , respectively in 6 MV-IMRT when compared with FIF-3DCRT. Spinal cord sparing was better in 6 MV-IMRT (40.963 ± 2.650), with an average reduction of maximum spinal cord dose by 7.355 Gy from that using the FIF-3DCRT technique. The uncomplicated tumor control probabilities values were higher in IMRT plans thus leading to a possibility of dose escalation.
Conclusions: Though low-energy IMRT is the preferred choice for treatment of oropharyngeal cancers, FIF-3DCRT must be given due consideration as a second choice for its well established advantages over traditional conventioan technique.

Keywords: 3D conformal radiation therapy, conformity index, intensity modulated radiation therapy, normal tissue complication probabilities, oropharynx, sigma index, tumor control probabilities, uncomplicated tumor control probabilities

How to cite this article:
Sundaram T, Nagarajan M, Nagarajan V, Supe SS, Mohanraj R, Balaji T, Jayakumar S, Balasubramaniam M, Govindarajan K N. Forward versus inverse planning in oropharyngeal cancer: A comparative study using physical and biological indices. J Can Res Ther 2013;9:422-9

How to cite this URL:
Sundaram T, Nagarajan M, Nagarajan V, Supe SS, Mohanraj R, Balaji T, Jayakumar S, Balasubramaniam M, Govindarajan K N. Forward versus inverse planning in oropharyngeal cancer: A comparative study using physical and biological indices. J Can Res Ther [serial online] 2013 [cited 2022 Sep 30];9:422-9. Available from: https://www.cancerjournal.net/text.asp?2013/9/3/422/119326

 > Introduction Top

Intensity modulated radiation therapy (IMRT) is a new radiation delivery technique that allows more precise delivery of radiation and optimization of the dose intensity to specific volumes while sparing the dose to critical normal structures. IMRT has been credited with the ability to generate highly conformal including concave dose distributions, with particular relevance in head and neck squamous cell carcinomas (SCC) due to their complex target volumes, proximity to critical structures, and a well-defined dose response relationship.

In the planning of conformal techniques like IMRT, target delineation is usually carried out slice by slice on computed tomography (CT) images or magnetic resonance (MR) fused image sets. This has tremendously increased the geometric accuracy of target volumes when compared with conventional 2D planning. [1] Thus, target shapes are usually irregular and hence dose distributions must conform to these irregular 3D shapes. Intensity modulation is an attractive tool to achieve such irregular conformal dose distributions.

Conformal planning in head and neck is challenging because of the complex anatomy of that region. Functional organs for breathing, visualizing (vision), eating, speech and facial expression are usually present in close proximity to target volumes. [2],[3]

The present study was undertaken to compare the forward-planning field-in-field 3D conformal radiation therapy (FIF-3DCRT) and inverse-planning IMRT (6 MV-IMRT) techniques regarding target-volume coverage and critical organ protection. Low energy photons (6 MV) were employed in these techniques. In addition, inverse-planning IMRT plans were also created with 15 MV photons (15 MV-IMRT) and compared with 6 MV-IMRT plans to investigate the differences that arise due to energy, if any. Many physical and biological indices were evaluated and compared.

Several trials have demonstrated organ preserving chemo-radiation as the key management strategy for oropharyngeal tumors. [4],[5] But, conventional treatments (two lateral primary fields and an anterior supraclavicular field) often cause salivary gland dysfunction, causing permanent xerostomia. After conventional RT, 60-75% of the patients are expected to have grade 2 or higher xerostomia. [6] Permanent loss of saliva can affect the quality of life to a large extent. The post treatment complications due to loss of saliva include nutritional, dental and communication problems apart from oral cavity infections. 3D-CRT may offer some benefits over conventional techniques but this is usually not enough to reduce complications to an acceptable level. Thus, oropharyngeal cancers are ideally suited for intensity modulated treatments which are characterized by high-dose gradients near the tumor (thus good sparing of nearby critical organs especially parotid glands) and high dose conformity. [1],[2] Eisbruch et al. have demonstrated a strong correlation between mean dose of salivary glands and degree of salivary flow following radiotherapy. [7] They also showed that it is feasible to achieve good parotid sparing without compromising local control.

 > Materials and Methods Top

In this retrospective study, 10 patients with oropharyngeal cancer were selected. The distribution of patients in T and N stages are in given in [Table 1]. All the patients were previously treated with 6 MV static IMRT between March 2011 and July 2011. The datasets were chosen such that the gross tumor bed lies in the medial part of the body. Initially, all the patients were simulated in supine position and with customized thermoplastic masks and with lead markers for defining laser center. The lead markers are used to evaluate translational distances between laser center and isocenter during treatment execution. 3 mm CT axial slices were acquired during intravenous contrast administration.
Table 1: Distribution of ten patients in T and N staging (M=0)

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Contouring was done on Philips ADAC Pinnacle treatment planning system version 7.4 f. 66 Gy Planning Target Volume (PTV 66), 60 Gy (PTV 60) and 50 Gy (PTV 50) target volumes were delineated on the CT images. This corresponds to gross tumor with margins, elective high risk volumes and low risk volumes respectively. Combined parotid gland (right and left), spinal cord, oral cavity and mandible were also delineated as organs-at-risk (OAR's) on CT Images. Unspecified tissue (body minus delineated volumes) was also derived. However, this study focused on combined parotid and spinal cord in view of their clinical relevance. The CT images of all the patients along with their structure sets (delineated volumes) were transferred to PrecisePlan planning system version 2.03 (Elekta Medical systems, Crawley, UK) for planning and dose calculation.

In addition to 6 MV-IMRT plans which were used for treating patients with precise digital linear accelerator (Elekta Medical Systems, Crawley, UK), 6 MV FIF-3DCRT and 15 MV-IMRT plans were also generated for each patient (three plans for each patient). Thus, altogether 30 plans were generated for a cohort of 10 patients. Seven equally spaced coplanar beams (0, 51,103, 154, 206, 257, 309 degrees) were used for all plans. Inverse segmental optimizer of PrecisePlan was used for static IMRT optimization process which is based on Cimmino projection technique. Final dose calculation was carried out by an area integration algorithm based on Clarkson's technique. 40 pair multileaf collimator having a width of 1 cm at isocentre distance (Elekta Medical Systems, Crawley, UK) was used for field shaping. The field shaping margin was 0.8 cm in all techniques. The 3D view of the delineated organs with the beam orientations are shown in [Figure 1].
Figure 1: 3D view with beam orientations and delineated organs

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In 6 MV and 15 MV IMRT plans, to find the appropriate energy and to ensure that the differences are due to energy alone, all other beam parameters like beam angles, number of beams, number of segments, shapes of the segments, dose constraints etc. were kept constant. The same segments can be used for both energies in segmental inverse optimizer of PrecisePlan planning system.

For FIF-3DCRT and IMRT plans, International Commission on Radiation Units and Measurements (ICRU) 50 guidelines for conformal radiotherapy were used to define our plan acceptance criteria for target coverage. That is, at least 95% of the PTV must be covered by prescription dose of 66 Gy. This ensures good target coverage. However, it does not guarantee the homogeneity. This study requires both to be met to bring the PTV dose distributions similar in all plans. This can be achieved by keeping the mean doses and homogeneity indices as close as possible. Dose homogeneity was evaluated by calculating sigma index (SI) which was recently proposed by Yoon et al. [8] and used by Sundaram et al. [9] For each patient, for this fixed PTV dose distributions in all 3 plans (and fixed [TCP] values), critical organ doses and normal tissue complication probabilities's (NTCPs) can be evaluated.

For PTV 66, in addition to above, the target coverage was assessed by comparing the volume that received 95% and 105% of the prescribed dose (V 95% and V 105% ). Cold and hot spots were quantified by evaluating dose to 99% and 1% volume of PTV 66 (D99 and D1 ).

The dose prescription was 66 Gy to tumor bed with margins (PTV 66), 60 Gy to elective high risk PTV (PTV 60) and 50 Gy to low risk PTV (PTV 50) all treated in 33 fractions by simultaneous integrated boost technique.

Concerning normal tissue, this study analyses the following questions. Do IMRT plans result in better sparing of OAR's than FIF-3DCRT plans? If so, how must it manifest as improvement in NTCP? Do high energy IMRT (15 MV-IMRT) plans have benefits in head and neck region?

In case of critical organs, a maximum dose of 45 Gy for spinal cord was the constraint in inverse planning optimizer. For combined parotid gland, a mean dose of 26 Gy was the objective constraint. The goal for FIF-3DCRT was not different from that of IMRT. Dose to 33%, 66% and 100% volume of combined parotids were compared in view of their documented clinical relevance. [10],[11] This study reports dosimetric and biological data for parotid glands and spinal cord since the early and late complications, respectively are well established.

The other parameters that were compared include conformity index (CI), mean dose volume histograms, TCP, NTCP and uncomplicated tumor control probabilities (UCP). The ability to cover target volumes can be assessed by the Conformity index (CI). In this work, CI is defined as the ratio of the volume of the body receiving the prescription dose to the volume of the PTV receiving the same dose.

BIOPLAN software was used for evaluating TCP, NTCP, UCP etc. [12] To compute the TCP and NTCP, BIOPLAN uses the Poisson model and Lyman-Kutcher-Burman (LKB) model respectively. [13],[14],[15],[16],[17] These models and their corresponding equations are given below.

The brief description of the Poisson model with its equations is as follows:

The calculated TCP corresponds to the TCP averaged over a population with variability in radiosensitivity (simulated as a Gaussian distribution of αi values with mean and σa standard deviation) where a fraction g i of patients have αi radiosensitivity and . TCP (αi , βi ) represents the TCP of a patient with radiosensitivity αi and with a non-uniform tumor dose distribution given by {Dj,vj}. The subscript j refers to the j th -volume (vj ) that receives a dose dj in each of the n fractions (Dj =dj x n). The β parameter is allowed to vary over the population of patients such that α/β is always constant.

In order to allow cell proliferation during the course of the treatment, a final term working in the opposite direction to cell-killing has been added; where g = ln2/Td , Td is the average doubling time, T is the overall treatment time and Tk the time at which proliferation begins after the start of the treatment. Thus, the parameters required by this model are: (initial clonogenic cell density), Td and Tk .

An estimate of clonogenic cell density is required to calculate TCP (= e-sfm ) where sf is the survival fraction and m is the clonogenic number. Thames and Bentzen estimated the number of clonogens for a large series of patients with SCC of the oropharynx as being between 1.8 × 10 3 and 6.6 × 10 5 per cc. [18] Others have reported similar values. [19],[20] Chatterjee et al. have used a more realistic value in this range by using the BIOPLAN software. [20] The mean value in this range was used to evaluate TCP. α/β value of 10 was assumed for SCC of oropharynx.

The brief description of the LKB model with its equations is as follows:

This model, also called normal or probit model, calculates the probability of complication of a partially uniformly irradiated critical organ. The equations for the model are:

TD 50 (v) = TD 50 (1) · v -n

Where, TD 50 (1) is the dose which uniformly delivered over the whole organ will produce a 50% chance of complications and v is the fraction of the organ irradiated uniformly. The three parameters in the model TD 50 (1), m and n have been tabulated by Burman et al. [21] for different organs and specified end-points based on the clinical tolerance data by published by Emami et al. [22] BIOPLAN contains a library with some of them, otherwise they can be entered by the user. The radiobiological parameters are shown in [Table 2].
Table 2: Radiobiological parameters used for calculating normal tissue complication probabilities

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BIOPLAN can generate TCP, NTCP tot and UCP values for different doses. It requires the dose volume histograms as inputs. From the plotted data of TCP and NTCP tot , BIOPLAN can calculate the UCP as a function of prescribed dose where NTCP tot is the total or combined normal tissue complication probability of parotids (NTCP P ) and spinal cord (NTCP SC ). The expression for UCP is given below:

UCP values provide an objective function that denotes the biological benefit that could be provided by that particular treatment plan depending on the TCP and NTCP values. Thus this tool has helped to predict the outcomes of dose escalation.

There is no interface compatibility between BIOPLAN and the planning system used for dose calculations. Hence dose-volume histograms of all plans were extracted from the treatment planning system and manually inputted into BIOPLAN software in a compatible ASCII format.

The mean of all the ten patients were reported in this study along with the standard deviation. A two tailed t-test was applied to analyze statistical significance between the different techniques. P values less than 0.05 was considered statistically significant.

 > Results Top

IMRT plans were more conformal than FIF plans. The evaluation of the dose volume histogram based and radiobiological parameters of delineated organs is summarized in [Table 3]. The mean ± SD values are reported.
Table 3: Descriptive statistics of dose volume histogram (DVH) parameters for different structures. The values are reported as mean±SD

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Not much difference was observed between 6 MV and 15 MV-IMRT plans for all the parameters and hence all the P values show poor statistical significance (P > 0.05). Thus 6 MV-IMRT plans being in widespread clinical use were given priority and compared with FIF-3DCRT technique (rather than 15 MV-IMRT plans).

Quantitative analysis of target dosimetric parameters shows that dose coverage is very similar among the different plans. The mean DVH's of different techniques are shown in [Figure 2]. Though plans were generated with the objective to create similar dose distributions, there were some differences observed due to inherent characteristics of different techniques.
Figure 2: Mean DVH of PTV 66 for field-in-field 3D conformal radiation therapy (solid purple), 6 MV (solid blue) and 15 MV- intensity modulated radiation therapy (solid red)

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The average SI (homogeneity) in 6 MV-IMRT (2.251 ± 0.443) and 15 MV-IMRT (2.211 ± 0.434) plans are better than of FIF-3DCRT technique (3.710 ± 0.765). Thus, though the 6 MV and 15 MV-IMRT plans exhibit similar homogeneity (P = 0.843), statistically significant differences were observed when compared with FIF-3DCRT technique (P = 0.000). This was further ascertained by high V 95 (99.330 ± 1.487, 99.228 ± 1.169) and low V 105 (1.970 ± 0.276, 1.849 ± 0.384) in IMRT groups. The D1 (near maximum) and D99 (near minimum) values are 71.560 ± 1.146 and 60.700 ± 1.849 in 6 MV-IMRT, an improvement of 2.739 Gy and 4.870 Gy from that using the FIF-3DCRT technique.

As expected, IMRT group was more conformal (CI values of 1.114 and 1.174) when compared with FIF-3DCRT (1.329) technique. TCP values were marginally better in IMRT group (80.368 ± 2.031, 78.713 ± 2.359) than in FIF-3DCRT (74.813 ± 1.324) technique.

Concerning coverage for other target volumes (PTV 60 and PTV 50), as expected, IMRT plans improved the CI, maximum and minimum doses than FIF-3DCRT. However, FIF-3DCRT plans met the practically feasible criteria of 90% volume of PTV 60 and PTV 50 being covered by their prescription doses (60 Gy and 50 Gy) respectively.

The values of D100 , D66 and D33 are 7.486 ± 2.146 Gy, 20.575 ± 2.470 Gy and 39.944 ± 3.631 Gy respectively in 6 MV-IMRT plans, a reduction of 4.374 Gy, 9.343 Gy and 7.883 Gy respectively when compared with FIF-3DCRT. Comparing to FIF-3DCRT technique (37.932 ± 4.458), the 6 MV-IMRT plans reduced the mean dose (25.079 ± 4.432) significantly (P = 0.000). The NTCP values were calculated using BIOPLAN software by inputting the parotid DVH's of individual patients for each technique. [12] Like other parameters, the mean ± SD values were reported which are representative of all patients. The NTCP in 6MV-IMRT (2.666 ± 1.219%) is much better than those in FIF-3DCRT (21.407 ± 5.606%) technique, with the statistical significance (P = 0.000).

Among the OAR's, spinal cord was given top priority in all techniques to keep the maximum doses below 45 Gy. Nevertheless spinal cord sparing was better in 6 MV-IMRT (40.963 ± 2.650), with an average reduction of maximum spinal cord dose by 7.355 Gy from that using the FIF-3DCRT technique. This reduction translated into a better NTCP value of 0.158 ± 0.322% in 6 MV-IMRT, when compared with FIF-3DCRT (1.824 ± 0.675%).

With the possibility of improved therapeutic index with IMRT, this study explored the TCP and UCP values that could be achieved with dose escalation. For FIF-3DCRT and 6 MV-IMRT techniques, mean DVH's of target volumes, combined parotid gland and spinal cord were inputted into BIOPLAN and theoretical consequences of dose escalation was studied. [12]

[Figure 3] shows the TCP (solid blue), UCP (solid white), NTCP tot (solid red), NTCP P (solid yellow) and NTCP SC (solid purple) curves for different prescription doses. The UCP values (solid white) represent the therapeutic index of FIF-3DCRT (left) and 6 MV-IMRT (right) plans. The Gaussian-like bell shaped FIF-3DCRT curve is highly distinguishable from the S-shaped 6 MV-IMRT curve. Poor UCP values (descending part of the curve) characterize the FIF-3DCRT technique whereas it is much better in 6 MV-IMRT techniques. Thus raised UCP values in 6 MV-IMRT provide theoretical confirmation of possible benefits of dose escalation. Moreover the NTCP tot (solid red) and NTCP P (solid yellow) are quite distinguishable with high values in FIF-3DCRT whereas it is not so in case of 6 MV-IMRT (not very distinguishable and with low values). 6 MV-IMRT is characterized by high parotid and spinal cord sparing. NTCP SC (solid purple) is almost zero in 6 MV-IMRT and hence NTCP tot = NTCP P .
Figure 3: Dose versus tumor control probabilities (solid blue), uncomplicated tumor control probabilities (solid white), normal tissue complication probabilities (NTCP)tot (solid red), NTCPP (solid yellow) and NTCPSC (solid purple) for field-in-field 3D conformal radiation therapy (left) and 6 MV-intensity modulated radiation therapy (right) techniques

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The different prescription doses and their corresponding TCP, NTCP tot and UCP values are reproduced in [Table 4]. In order to have a better understanding of the variations between these paraameters, pearson correlation coefficients were evaluated between them and shown in [Table 5]. A value close to +1, 1 and 0 indicates strong positive, strong negative and no linear relationships respectively between the studied parameters. The findings are summarized below:
Table 4: Dose versus tumor control probabilities, normal tissue complication probabilitiestot and uncomplicated tumor control probabilities (calculated from mean dose volume histograms (DVH's))

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Table 5: Variation of different radiobiological parameters with dose

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As expected, TCP and NTCP tot showed a strong postive linear association with dose for both FIF-3DCRT and 6MV-IMRT (values close to unity).

Perusing [Table 5] shows a poor (or no) linear association between dose and UCP for FIF-3DCRT technique (0.253). However, a strong linear association existed between them in 6MV-IMRT technique (0.902). This gives us a degree or measure of efficacy of 6MV-IMRT over FIF-3DCRT technique (resulting from higher UCP values because of better sparing of normal tissues).

Strong correlation was observed between TCP and NTCP tot in FIF-3DCRT (0.836) when compared with 6 MV-IMRT (0.645).

An interesting observation was a positive but poor association (0.494) between UCP and NTCP tot in 6 MV-IMRT, whereas a negative but very poor association (−0.145) was observed in FIF-3DCRT. This again emphasized the betterment of efficacy of 6 MV-IMRT over FIF-3DCRT. It is quite obvious that, the higher the conformity of the technique, the larger will be the positive association.

 > Discussion Top

We have compared FIF-3DCRT, 6 MV and 15 MV-IMRT techniques for patients with oropharyngeal cancer. The doses to PTV 66 were compared in terms of D1 (near maximum), D99 (near minimum), V 95 , V 105 and SI, to verify that similar dose coverage was kept in all the techniques. For combined parotid gland, being a parallel architecture organ, D100 , D66 and D33 and mean dose were evaluated and compared. For spinal cord, being a serial organ, the maximum dose was evaluated.

All the physically optimized (or evaluated) plans were also compared by means of TCP (for PTV 66) and NTCP (for OAR's) values. UCP values were also evaluated to study the variation of TCP and NTCP values with different prescribed doses for typical IMRT and FIF dose distributions.

In general, 6 MV-IMRT plans showed superior target coverage, homogeneity, conformity, combined parotid and spinal cord sparing. Particularly, IMRT significantly reduced the mean dose of combined parotid by 12.853 Gy when compared with FIF-3DCRT technique. With regard to D33 , this reduction was 7.883 Gy. This translated into a NTCP value of 2.666% which is significantly lesser than FIF-3DCRT (21.407%) technique.

Concerning spinal cord maximum doses, average reduction of 7.355 Gy was observed in 6 MV-IMRT when compared with FIF-3DCRT technique (P < 0.05).

In particular, for head-and-neck cancers including oropharynx, sufficiently high dose delivery to the PTV is crucial for local control and patient survival. Unfortunately, with conventional technique (two traditional lateral fields and an anterior field for supraclavicular region) and in FIF-3DCRT, even delivering 68 Gy to GTV is practical only at the cost of considerable morbidity. Particularly with the aforementioned conventional technique, radiation-induced myelitis and necrosis have prevented delivering tumorocidal doses and led to the increasing use of FIF-3DCRT and IMRT. Though FIF-3DCRT could comfortably reduce spinal cord dose below 50 Gy, parotid gland sparing could not be effectively achieved which is ascertained in this work. A large tumorocidal doses could be delivered with IMRT with improved conformity and thus with less doses to OAR's. [23] But not all centers have IMRT facility. Nevertheless, such centers could very well adapt to this intermediate technique (between conventional and IMRT) namely FIF, a synonym for forward-planning IMRT. This technique has some potential advantages. Simultaneous integrated boost delivery (delivering different doses to tumor bed,elective high, intermediate and low risk regions simultaneously) is feasible which is impractical or cumbersome with traditional conventional technique. Also certain degree of intensity modulation achieved with this technique has led to aforementioned improvements in dosimetric parameters, particularly in reducing spinal cord doses below 50 Gy.

As mentioned earlier, statistically insignificant values were observed for all parameters between 6MV and 15 MV-IMRT techniques [P values > 0.05 in last column of [Table 3]. Hence, our focus was to compare the widely practiced 6 MV-IMRT plans with the FIF-3DCRT technique. However, we found that 15 MV-IMRT plans resulted in slightly better values for most of the parameters of PTV and combined parotid (except spinal cord) than in 6 MV-IMRT, however, statistically not significant (P > 0.05). For instance, the mean dose ± SD of 6 MV and 15 MV-IMRT plans are 25.079 ± 4.432 Gy and 24.854 ± 4.109 Gy, respectively. The reasons could be, parotids being at shallow depth, may experience a lesser entry dose (high sparing) in 15 MV plans though the exit dose from other coplanar beams compensates this reduction to some extent, in general. [24]

Comparison of NTCP and UCP values in [Table 4] confirms the superiority of IMRT plans. While TCP and NTCP values in 6 MV-IMRT plans confirm the possibility of better outcomes, the high UCP values widens the scope for dose escalation. For dose escalation in oropharynx, consideration needs to be given to be pharyngeal constrictor muscles, the temporo-mandibular joint and the muscles of the mastication. [20]

Cold spots will rapidly drive down the TCP, which cannot be rescued by hot spots in different locations of the PTV. [20] Nevertheless we could achieve close TCP values between different techniques by trying to keep many parameters similar (D1 , D99 , V 95 , V 105 , SI, mean dose and mean dose-volume histograms). Our radiobiological calculations were based on data of Emami et al. [22] though recent QUANTEC publications have described a predictive model based on pooled data from multiple studies. [25],[26],[27] Though some of their limitations have been highlighted, the new data may well be more accurate. These values if used, could proivde changed NTCP values for the plans in this study. Till the publication of new values for NTCP model parameters, the values of Emami et al. [22] can be used.

The dosimetric and radiobiologcal parameters reported here indicate that similar tumor control can be achieved with FIF-3DCRT technique as well but with higher doses to parotid glands and spinal cord. It should be noted that the sample chosen for this study had gross tumors medially. Had the study involved patients with lateral extension as well, the parotid and spinal cord doses could have been further on the higher side. Many references reported higher parotid doses and spinal cord doses. [20],[23] Our experience also indicates a higher parotid and spinal cord doses for patients with gross tumors having lateral tendency and if both ipilateral and contralateral lymph nodes needs to be irradiated.

This study did not include the traditional conventional technique for comparison. However, much evidance is available regarding the potential disadvantages of this traditional technique against FIF-3DCRT and IMRT techniques and is well-known too. [20],[23] Though this study recommends low energy IMRT as a preferred choice of managing patients with oropharyngeal cancers, FIF-3DCRT should be the next choice if it is deemed practical rather than traditional conventional technique.

 > Conclusion Top

IMRT improves dose homogeneity and target coverage as compared to FIF-3DCRT technique, while significantly improving organ sparing. The differences were very small between 6 MV and 15 MV-IMRT plans and this study prefers using low energy for head-and-neck IMRT since high energy is well-known for neutron contaminations. For centres lacking inverse-IMRT facility, FIF-3DCRT must be the second choice. Further studies are required to assess the possible clinical benefits of 6 MV-IMRT.

 > References Top

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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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