Essay: Radiation therapy

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CHAPTER 1
INTRODUCTION AND AIM OF THE WORK
1.1. Introduction:
Radiation Therapy:
The main goal of radiation therapy is to control the primary tumor either alone (prostate, head and neck, anus) or association with surgery (breast, rectum, uterus) or chemotherapy (Hodgkin’s disease). The dose delivered to the tumor (and the pathological type of the tumor) is the main point for local control. The history of one century of radiotherapy can be schematically summarized as a continuous escalation of dose to the tumor enabled by improved radiation devices. [1]
Radiation therapy techniques:
Two-dimensional (2D) radiotherapy takes the form of a single beam from one to four directions. Plans frequently consist of opposed lateral fields or four-field ‘boxes’ (1). Three-dimensional (3D) was a major advance because it took into account axial anatomy and complex tissue contours such as the hourglass shape of the neck and shoulders. While 3D planning became available for accurate dose calculations to such irregular shapes, we were still limited in the corrections we could make.
As its name implies, intensity-modulated radiotherapy (IMRT) enables us to modulate the intensity of each radiation beam, so each beam may have one or many areas of high intensity radiation and any number of lower intensity areas within the same field, thus allowing for a better control of the dose distribution with the target. By modulating both the number of fields and the intensity of radiation within each one, we have limitless possibilities to sculpt radiation dose (see 2). Advanced treatment planning software has enhanced our ability to modulate radiation dose. Instead of the manual choosing every beam angle and weighting, treatment planning optimization techniques can now help determine the distribution of beam intensities across a treatment target volume, which often include a non-intuitive distribution of ‘beamlets,’ or 1-cm2 areas of iso-intensity.
In the traditional external beam photon radiation therapy, most treatments are delivered with radiation beams that are of uniform intensity across the field (within the flatness specification limits). Occasionally, wedges or compensators are used to modify the intensity profile to offset contour irregularities and/or produce more uniform composite dose distributions. This process of changing beam intensity profiles to meet the goals of a composite plan is called intensity modulation. Thus, the compensators and wedges may be called intensity modulators, albeit much simpler than the modern computer-controlled intensity modulation systems such as dynamic multileaf collimators. 3 , 4
Intensity-modulated radiation therapy (IMRT) is an advanced mode of radiation therapy which allows a more precise delivery of radiation to the targeted volume. The radiation dose is designed to conform to the three-dimensional shape of the treatment target by modulating the intensity of the radiation beam to deliver a higher radiation dose to the tumor while limiting radiation exposure to surrounding normal tissues. Typically, a combination of several intensity-modulated fields with different gantry angle directions produces a customized radiation dose distribution that increases the dose to tumor while at the same time minimizing the dose to the nearby normal tissues.
IMRT is one of the most widely used delivery modalities for radiotherapy for cancer patients. It is a radiation therapy technique in which non-uniform fluence is delivered to the patient from any given position of the treatment beam to enhance the composite dose distribution. The treatment criteria for plan optimization are determined by the planner and the optimal fluence profiles for a given set of beam directions are specified through ‘inverse planning.’ The fluence files thus generated are electronically delivered to the linear accelerator (Linac), which is computer controlled, equipped with the required hardware and software to deliver the calculated intensity-modulated beams (IMBs). 3 , 4
The clinical application of IMRT requires at least two systems; (a) a treatment-planning computer system for calculating non-uniform intensity maps for multiple beams coming from different directions to deliver higher dose to the target volume while protecting normal structures, and (b) a system of transferring the non-uniform planned fluences. Each of these systems must be adequately tested and commissioned before actual clinical use. 5 , 6 , 7
The principle of IMRT is to treat a patient from a number of different directions (or continuous arcs) with beams of non-uniform radiation dose intensity, which have been optimized to deliver a high dose to the tumor and a very low dose to the adjacent normal tissues/structures. The treatment-planning system divides each beam into a large number of smaller beams (beamlets) and determines optimum setting of their fluences or weights. The optimization process involves inverse planning in which weights or intensities of the beamlet are automatically adjusted to achieve predefined dose distribution criteria for the composite plan. 3
Many classes of intensity-modulated systems have been developed. These include transmission blocks, compensators, wedges, dynamic jaws, multileaf collimators, tomotherapy collimators, moving bar, and scanned elementary beams of variable intensity. Of these, only the last five allow dynamic intensity modulation. Wedges, transmission blocks and compensators are manual techniques that are inefficient, time consuming, and do not belong to the modern class of IMRT systems. Dynamic jaws are suited for applying wedge-shaped distributions but are not significantly superior to conventional metal wedges. Although a scanning beam accelerator can deliver intensity-modulated primary beams, the Gaussian half-width of the photon ‘pencil’ at the isocenter can be as large as 4 cm and therefore does not have the required resolution by itself for full-intensity modulation. However, scanning beam may be used in combination with a dynamic multileaf collimator (MLC) to get rid of this problem and provide an additional degree of freedom for full dynamic intensity modulation. The case of dynamic MLC with upstream intensity modulation is a powerful but complex technique that is currently available only with scanning beam accelerators such as Microtrons. 9
The patient input data required for the inverse planning algorithm, is the same as that required for forward planning. Also three-dimensional image data, image registration, and segmentation are all needed when planning for IMRT. For each target (planning target volume PTV), the user enters the plan criteria: minimum dose, maximum dose, and a dose volume histogram (DVH). For the organs at risk (normal tissues/structures), the treatment planning program requires the desired limiting dose and a dose volume histogram. Depending on the IMRT planning system, the user may be required to enter other data such as beam directions, beam energy, number of iterations, etc., before proceeding to optimizing intensity profiles and calculating the resulting dose distribution. The evaluation process of an IMRT treatment plan also requires the same considerations as the ‘conventional’ three-dimensional conventional radiotherapy (3-D CRT) plans, namely displaying isodose curves in orthogonal planes, individual slices, or 3-D volume surfaces. The isodose distributions are usually evaluated by dose volume histograms.
Field-In-Field technique:
Three Dimensional Conformal Radiotherapy (3D-CRT) planning software allows displaying the 3D radiation dose distribution at different levels in the planning target volume (PTV). Physical or dynamic wedges are commonly used to achieve homogeneous dose distribution in the tumor or PTV. Despite all these planning efforts, there are about 10% increased dose hot spots encountered in final plans. To overcome the effect of formation of over dose regions, a manual forward planning method has been used. In this method, two more beams with multi-leaf collimator (MLC) of different dose weights are added in addition to the main used beams in the major plan and sometimes; when we use the FIF technique, we can dispense the physical and dynamic wedges.
FIF technique allows us to use additional MLC fields to achieve better homogeneity in dose distributions. So, in this study we used the FIF technique in the all three sites under study (brain, prostate and left sided breast tumors) because it can get us a homogeneous dose distribution in the PTV and decrease the dose hot spots in the treatment planning.
Field-In-Field process:
Using the Three Dimensional Conformal Radiotherapy (3D-CRT) planning, a forward treatment planning can be performed as follows;
– In the FIF technique, a forward TPS is used to eneble the medical physicist (and/or the dosimetrist) to try to reach the expected results by the try and error method.
– Using a multi-leaf collimator (MLC) facility, we can add the main required radiation treatment beams with their parameters such as; Gantry angle, Collimator angle, Field size, ‘. Etc.,
– The dose distribution from this simple configuration will be computed. But naturally, there will be some dose hot spots in the plan.
– So, two or more beams with multi-leaf collimator (MLC) of different weights are added in addition to the main used beams in the major plan. The regions of hot spots will be covered with the new segmented beams by editing in the MLC. A small radiation dose weight was given to these smaller fields, and the plan was recalculated.
– Again the hot spots formed will be projected in the BEV, and the region will be covered with the segments of MLC from another segmented field.
– Again a small weight was given to this field and the plan was recalculated for the distribution of the dose.
– Finally the dose distribution will be optimized with the all fields by adjusting the leaf positions and weights of MLC fields. And the plan is saved to be used in the verification and treatment.
Aim of the work:
The aim of this study is to:
1- Evaluate FIF technique for different tumor citations.
2- Improve the performance using the FIF technique compared with the conventional and IMRT planning techniques.
3- State the required conditions and establish fixed steps for applying FIF technique as an alternative radiotherapy treatment plan.
4- Apply and verify our final recommendations on different patients.
CHAPTER2
Literature Review and
Theoretical Consideration
2.1. Literature Review of IMRT Applications in radiotherapy
‘ showed that despite of using 3D-CRT planning software that helps the planner in displaying the 3D dose distribution at different levels in the PTV in addition to applying physical or dynamic wedges to achieve homogeneous dose distribution in the PTV, there are about 10% increased dose hot spots encountered in final plans. They investigated that to overcome the effect of formation of over dose (hot spots), a manual trial and error forward planning method has been used. In this method, two more beams with multi-leaf collimator (MLC) of different weights of radiation doses are added in addition to both medial and lateral wedged tangent beams. Fifteen patient treatment plans were taken up to check and compare the validity of using additional MLC fields to obtain better homogeneity in dose distributions. The resultant dose distributions with and without presence of MLC were compared objectively. The dose volume histogram (DVH) of each plan for the PTV was evaluated. The 3D dose maximum values were reduced by 4% to 7%, and hot spots assumed point size. Optimizations of 3D-CRT plans with MLC fields achieved good homogeneity and conformability of dose distribution in the PTV.
‘ investigated that using three-dimensional conformal radiation therapy (3D-CRT) and multi-segmented conformal radiation therapy (MS-CRT) for breast cancer treatment, both of the dose coverage of the planning target volume (PTV) and the radiation burden on the organs at risk (OARs) were evaluated.
The multi-segmented plans furnished significantly (p < 0.0001) better target coverage (PTVD95'107% 82.8% vs. 90.9%, PTVD107% 5.9% vs. 0.3% and PTV Dmax. 56.6 vs. 54.3 Gy). The mean OAR doses remained almost unchanged: ipsilateral lung 10.5 versus 10.4 Gy, contralateral lung 0.4 versus 0.4 Gy, contralateral breast 0.8 versus 0.8 Gy, and whole heart (for left-sided cancers) 4.8 versus 4.8 Gy. The smaller segments required a mean of 9.8 MU (monitor units), i.e., a mean total 7.6 MU increment. The planning took from 10 min. to 20 min, and the delivery from 5 to 10 min. Thus, they investigated that MS-CRT is a good alternative to breast intensity-modulated radiation therapy (IMRT) and seems adequate for right-sided cancers, whereas left-sided cancers necessitate a longer follow-up of heart-related side effects before a final assessment.
… investigated that the FIF technique can be used for upper abdominal malignancies (UAM) in place of wedge-based conformal treatment plans. They studied feasibility of using a field-in-field (FIF) in different UAM and its efficacy in reducing the over dose regions. Twelve patients of UAM (which included malignancies of the gastroesophageal junction, pancreas, gall bladder, and stomach) were selected for this study. CT scans were performed and three-dimensional conformal wedge plans were generated for all the cases under this study. They copied the same plan with the wedges removed and a FIF plan was designed. They compared the two plans for mean, maximum, and median doses; dose received by 2% (D2) and 98% (D98) of the target volume; volume receiving >107% (V > 107%) and <95% (V < 95%) of the prescribed dose; conformity index (CI); and total monitor units. The doses to organs at risk such as kidneys, liver and spinal cord were also compared. For all the cases, the FIF technique was better than wedge-based planning in terms of maximum dose, D2, V > 107%, and CI; there was a statistically significant reduction in monitor units. With regard to doses to organs at risk, there was a relatively dose reduction for the kidneys and spinal cord with FIF when compared to wedge-based planning.
‘ made Investigation deals with the implementation of the field-in-field intensity-modulated radiotherapy (FiF) technique in our daily practice for breast radiotherapy. FiF plans and conformal radiotherapy (CRT) plans were compared for doses in the PTV, the dose homogeneity index (DHI), doses in irradiated soft tissue outside the target volume (SST), ipsilateral lung and heart doses for left breast irradiation, and the monitor unit counts (MU) required for treatment. Averaged values were compared. With FiF, the DHI is improved 7.0% and 5.7%, respectively (P < 0.0001) over the bilateral and lateral wedge CRT techniques. When the targeted volumes received 105% and 110% of the prescribed dose in the PTV were compared, significant decreases are found with the FiF technique. With the 105% dose, the SST, heart, and ipsilateral lung doses and the MU counts were also significantly lower with the FiF technique. Thus, the FiF technique, compared to CRT, for breast radiotherapy achieves significantly better dose distribution in the PTV. Significant differences are also found for soft tissue volume, the ipsilateral lung dose, and the heart dose. Considering the decreased MUs acquired for treatment, the FiF technique is preferred over tangential CRT. ' made comparison between wedged beams and field-in-field technique in left breast irradiation for evaluating the probability of late cardiac mortality caused by left breast irradiation planned with tangential fields and for comparing this probability between the wedged beam and field-in-field (FIF) techniques and to investigate whether some geometric/dosimetric indicators can be determined to predict with the cardiac mortality probability before treatment begins. Evaluating the differential DVHs showed that the gain in uniformity between the two compared techniques was about 1.5. With the FIF, the mean dose sparing for the heart, the left anterior descending coronary artery, and the lung was 15% (2.5 Gy vs. 2.2 Gy), 21% (11.3 Gy vs. 9.0 Gy), and 42% (8.0 Gy vs. 4.6 Gy) respectively, compared with the wedged beam technique. Also, the cardiac mortality probability decreased by 40% (from 0.9% to 0.5%). Three geometric parameters, the maximal length, angle subtended at the center of the CT slice by the PTV contour, and thorax width/thickness ratio, were the determining factors (p = .06 for FIF, and p = .10 for wedged beam) for evaluating the cardiac mortality probability. The FIF technique seemed to cause a lower cardiac mortality probability than the conventional wedged beam technique. However, although this study proved that FIF technique improved the dose coverage of the PTV, the breast cancer survival rates of the patients didn't be allowed because of the restricted number of patients enrolled and the short follow-up. ' quantified the low dose regions under geometrical uncertainties in field-in-field technique for whole breast irradiation. Ten consecutive patients from both the right- and left-sided treatment site groups who treated with the field-in-field technique for whole breast radiotherapy were included. Treatment plans were designed with moving isocenters to the posterior direction having two amplitudes (5 and 10 mm) and prescribing the same monitor unit as the original plan (FIF_5 and FIF_10). The PTV for evaluation was defined by excluding the areas within 5 mm from the skin and within 5 mm from the lung from the whole breast. The differences in V90, V95 and D98 of PTV for evaluation were measured between the original and virtual plans. As a reference, the same measurements were taken for the wedge techniques (Wedge 5 and Wedge 10). The differences in V95 were -0.2% on FIF 5, -1.7% on FIF 10, -0.5% on Wedge 5 and -1.5% on Wedge 10. The differences in V90 were -0.02% on FIF 5, -0.3% on FIF 10, -0.05% on Wedge 5 and -0.1% on Wedge 10. The differences in D98 were 0 Gy on FIF 5, -0.1 Gy on FIF 10, -0.2 Gy on Wedge 5 and -0.4 Gy on Wedge 10. The differences in D98% between the original plans and virtual scenarios for field-in-field techniques were significantly smaller than those for wedge techniques, but there were no statically significant differences in V90 and V95. Finally, they investigated that the quantity of the low dose regions caused by the geometrical uncertainties in field-in-field techniques was almost the similar to that for the wedge techniques and was acceptable. ' demonstrated the feasibility of clinical use of a field-in-field (FIF) technique for total body photon irradiation (TBI) using a TPS and to verify the TPS results with in vivo dose measurements using metal-oxide-semiconductor field-effect transistor (MOSFET) detectors. They showed that all patients treated with planned TBI using the FIF technique with 18-MV photon energies and 2 Gy b.i.d. on 3 consecutive days. The difference in tissue maximum ratios between the conventional and extended distances was < 2%. The mean deviation of the manual calculations compared with TPS data was ?? 1.6% (range, 0.1e2.4%). By using 18-MV photon beams for the FIF technique, a homogenous dose distribution was obtained. The mean lung dose for the FIF technique was 79.2% (9.2 Gy; range, 8.8 e 9.7 Gy) of the prescribed dose. The MOSFET output readings and TPS doses in the body were similar (percentage difference range, 0.5% to 2.5%) and slightly higher in both of the shoulder and lung (percentage difference range, 4.0 e5.5%). Thus, they investigated that the FIF technique used for TBI can achieve a homogenous dose distribution and is simple, feasible and spares time compared with other more complex techniques. The TPS doses were similar to the midline doses showed by MOSFET readings. ' evaluated dose distribution and homogeneity of field-in-field intensity-modulated radiation treatment (FIF-IMRT) compared with standard wedged tangential-beam 3D conformal radiotherapy (CRT) of the left breast in patients who have undergone lumpectomy. They aimed to improve dose distribution homogeneity in the breast and minimize the dose to organs at risk (OAR), i.e., ipsilateral lung, contralateral breast, and heart and vessels. When the aimed volumes receiving 105 % and 110 % of the total prescribed dose in the PTV were compared, significant decreases were found with the FIF-IMRT technique. With the 105 % dose to the OARs, monitor unit (MU) values were significantly lower with the FIF-IMRT technique. V2 of pulmonary artery, aorta, and left atrium and V1 for the contralateral breast were statistically significantly lower with FIF-IMRT plans (p = 0.001). PTV showed a better HI and CI with FIF-IMRT. ' evaluated a simplified 'field-in-field' technique (SFF) that was applied in their department of Radiation Oncology for breast treatment. This study evaluated 15 consecutive patients treated with a simplified field in field technique after breast-conserving surgery for early-stage breast cancer. Radiotherapy consisted of whole-breast irradiation to the total dose of 50 Gy in 25 fractions, and a boost of 16 Gy in 8 fractions localized to the tumor bed. They compared dosimetric results of SFF to state-of-the-art electronic surface compensation (ESC) with dynamic leaves. Then they performed an analysis of early skin toxicity of a population of 15 patients. By determining the median volume receiving at least 95% of the prescribed dose, it was 763 mL (range, 347'1472) for SFFvs. 779 mL (range, 349'1494) for ESC. The median residual 107% isodose was 0.1 mL (range, 0'63) for SFF and 1.9 mL (range, 0'57) for ESC. Monitor units were on average 25% higher in ESC plans compared with SFF. It is noticed that no patient treated with SFF had acute side effects superior to grade 1-NCI scale. SFF created homogenous 3D dose distributions equivalent to electronic surface compensation with dynamic leaves. It allowed the integration of a forward planned accompaniment tumor bed boost as an additional MLC subfield of the tangential fields. Compared with electronic surface compensation with dynamic leaves, shorter treatment times allowed better radiation protection to the patient. Low-grade acute toxicity evaluated weekly during the radiotherapy course and 2 months after treatment completion justified the follow-up of all breast patients, treated with this technique, in their department. ' made comparison of the dosimetry for five different radiotherapy techniques used in radiotherapy treatment planning for the left-sided breast cancer patients. Twenty adequate patients were treated with conservative surgery followed by radiotherapy. They were planned using five different radiotherapy techniques, including: 1) field-in-field (FIF) technique (TW); 2) conventional tangential wedge-based fields; 3) tangential inverse planning intensity-modulated radiotherapy (T-IMRT); 4) multi-field IMRT (M-IMRT); and 5) volumetric modulated arc therapy (VMAT). The CTV, PTV and OARs including the heart, the regions of coronary artery (CA), the left and right lung, the contralateral breast were contoured. The targeted PTV dose was prescribed 50Gy and V47.5'95%. Same radiation dose constraint was used for all five plans. The planned volumetric dose of PTV and PRV-OARs were compared and analyzed. All the other four plans were able to meet the V95% (V47.5) requirement except VMAT (Average V47.5 was 94.72%??1.2%). T-IMRT plan enhanced the PTV dose homogeneity index (HI) by 0.02 and 0.03 when compared to TW plan and VMAT plan, and minimized the V5, V10 and V20 of all PRV-OARs. However, the high dose volume (' 30Gy) of the PRV-OARs in T-IMRT plan had no statistically significant difference when compared with the other two inverse plans. For the all five plans, the dose volume of coronary artery area showed a very strong correlation to the dose volume of the heart (the correlation coefficients were 0.993, 0.996, 1.000, 0.995 and 0.986 respectively). Thus, compared to other techniques, the T-IMRT technology minimized radiation dose exposure to normal structures with remaining reasonable target homogeneity, and VMAT is not recommended for left-sided breast cancer treatment. In five techniques, DVH of the heart can be used to predict the DVH of the coronary artery. ' made comparison dosimeteric benefits of field-in-field radiotherapy (FIF) and conformal radiotherapy (CRT) for early stage endometrial cancer patients. The FIF significantly reduced the maximum dose of the PTV, bladder, rectum, bowel, right femur, left femur and bone marrow ( p values were: < 0.0 01, 0.031, 0.0 03 , <0.0 01, 0.0 01, 0.0 01 and < 0.0 01 respectively). When the OAR volumes received with > 30 and >45Gy were compared, the results were supportive to the FIF technique. The OAR volumes of rectum, bladder, bowel, left femur, right femur and bone marrow receiving more than the prescribed dose of 45Gy were significantly minimized with FIF technique (p values were 0.016, 0.039, 0.01, 0.0 4, 0.037 and 0.01 respectively). The dose homogeneity index (DHI) was significantly better with FIF technique (p < 0.0 01). So they investigated that FIF achieved more homogeneous dose distribution in the PTV and minimized the doses received by OAR. Considering the lower maximum doses in the OAR and PTV, FIF technique seems to be better than CRT during adjuvant radiotherapy for early stage endometrial cancer patients. ' compared between tangential and field-in field IMRT (FIF) techniques to determine the optimal whole breast irradiation technique in small-sized breast cancer patients. Their study included sixteen patients with '3 cm breast height and '350 cc volume were included. Seven patients had 4D CTs performed. The planning target volumes (PTV), editing 5 and 2 mm from the surface on the whole breast, were contoured and called PTV(5) and PTV(2), respectively. DVHs of tangential techniques with open beam (OT) and wedge filter (WT), conventional FIF (cFIF), and modified FIF (mFIF) blocking out the lung were designed. Various dose volume parameters, the dose homogeneity index (DHmI), dose heterogeneity index (DHtrI), and PTV dose enhancement (PDI) were determined. OT compared with WT showed a significantly appropriate V90 of the heart and lung, and PTV(5) dose distribution. Comparing OT and cFIF, OT showed significant enhancement in the V95 of PTV(2), whereas cFIF showed significant enhancement in the V95, DHtrI, DHmI, and PDI of the PTV(5). In comparing cFIF and mFIF, mFIF showed enhanced dose distributions of the heart and lung, while cFIF presented the better V95, DHtrI, DHmI, and PDI of the PTV(5). Respiratory effects on the absolute dose were mostly within 1 %. The ratio of free breathing and each respiratory phase was similar for OT, cFIF, and mFIF. Therefore, cFIF has appropriate dose conformity and is recommended be an optimal method for small-sized breasts. However, OT in order to dose coverage close to the skin and mFIF for normal tissue may also be potential alternatives. Respiratory effects are minimal. ' made comparison of the dosimetry of field-in-field (FIF) and wedged beams (WB) planning techniques for patients with breast cancer receiving adjuvant radiotherapy after conservative surgery. A total of eighty nine patients with breast cancer are selected in this study. Each patient received a CT-based treatment plan with opposed tangential fields. Two FIF and WB planning techniques were designed for each patient by using the Pinnacle TPS. Three indices, uniformity index (UI), conformity index (CI) and the homogeneity index (HI), as well as median dose (D50), maximum dose (Dmax), monitor unit (MU), number of portals, and lung volume at 20Gy (lung20) were used for comparison. The mean values tested using a t-test showed that the WB technique had a significantly higher CI (p < 0.0001), a significantly higher D50 (p = 0.0002) and a significantly lower HI (p < 0.0001), than did the FIF technique. The FIF technique had a significantly greater Dmax compared with the WB technique, but lung20 did not show a significant difference. By contrast, the FIF technique had a significantly greater UI and a significantly lower MU compared with the WB technique, but a significantly greater number of portals were found in the FIF technique. The FIF technique did not proof superior dosimetric results. The WB technique had a significantly higher CI, lower Dmax, lower HI, and lower number of portals; but the FIF technique had a significantly lower MU and higher UI. * From the previous literature survey, we noticed that; ten researches were about the use of the Field-In-Field technique for breast radiation therapy and only three researches talked about using it in different targets; in endometrial cancer radiation therapy, in Total Body Irradiation (TBI) and for upper abdominal malignancies (UAM). In the usual external beam photon radiation therapy, most treatments are applied with radiation fields that are of uniform intensity across the field (within the flatness specification limits). Occasionally, wedges or compensators are used to enhance the intensity profile to offset contour irregularities and/or achieve more uniform composite dose distributions. This process of modifying beam intensity profiles to satisfy the goals of a composite plan is called intensity modulation. Thus, the wedges and compensators may be called intensity modulators, albeit much simpler than the recent computer-controlled intensity modulation tools such as dynamic multileaf collimators. The primary obstacles to getting the maximum possible therapeutic advantage for the patient being treated with conventional radiotherapy are the following: 1- The uncertainties in the true spatial extent of the tumor 2- Insufficient knowledge of the exact shapes and locations of normal tissues/structures. 3- The lack of optimal facilities for efficient planning and applying of conformal radiation therapy (CRT). 4- limitations of current methods of producing desirable radiation dose distributions These limitations result in the integration of large safety margins to reduce the risk of local recurrence. However, to ensure that unacceptable normal tissue complications are avoided, the tumor dose often has to be at suboptimal levels, leading to a greater probability of local failures. Therefore, better localization of the extent of the tumor and of organs at risk and the ability to conform the dose distributions accordingly are essential to decrease the margins, allowing increases in tumor doses and minimizing dose to normal tissues. Transition from 2-D Radiotherapy to 3-D Conformal and Intensity Modulated Radiotherapy: 3-D conformal radiotherapy (3-D CRT) is the term used to describe the design and delivery of radiotherapy treatment plans based on three-dimensional image data with treatment fields individually designed to treat only the target volume. The European Dynarad consortium has suggested that the complexity of radiotherapy planning and treatment methodologies can be captured in four levels 30 . Level 0 is a basic radiotherapy where no attempt is made to design the treatment fields and as such cannot be described as conformal. Levels 1 to 3 are illustrated in Table 1. Individually made fields can be designed from planar radiographs or with limited computer tomography (CT) data. This level of conformal radiotherapy (referred to as Level 1 in Table 1) can be applied in any radiotherapy department with the minimal tools described in and is a useful way to begin the move towards full 3-D CRT. Level 2 conformal radiotherapy needs a full 3-D data set, usually of computed tomography images, on which the tumor volume is delineated following the concepts of ICRU 50 and 62 31 , 32 , 33 . This level may include the use of non-coplanar beams. Level 3 represents the most complex radiotherapy treatment plans, including IMRT, many of which are still at the research level in University Hospitals. Table 1 is generated to give a flavor of the progression of techniques that may be available at each level and should not be considered as a prescriptive indication that every treatment should use all the techniques listed. Table 1: Classification of Conformal Therapy According to Methodology and Tools Associated with Each of the Procedure. Conformal radiotherapy allows the delivery of a radical dose of radiotherapy while minimizing the dose to normal tissue structures, thus minimizing the negative effects of treatment. Its principle benefit therefore is to patients who are to be received potentially curative radiotherapy. Where radiotherapy is being delivered with palliative purpose the prescribed total doses are usually lower and the negative effects of palliative radiotherapy are therefore likely to be less. For this reason conformal radiotherapy is not often used when delivering palliative treatment, although it is always required to reduce the volume of non-target tissue that is irradiated. Conformal radiotherapy can be considered as a step towards IMRT. However, the delivery of IMRT, where fields are designed of multiple smaller beams (beamlets), is too more costly than conformal radiotherapy and requires an even higher experience level. There is considerable evidence for the3-D CRT benefits, but the benefits of IMRT are less well established. The gradual benefits in the transition from conventional radiotherapy to 3-D CRT are therefore dramatically greater than those achieved in the transition from 3-D CRT to IMRT. It is therefore recommended that the application of 3-D CRT should be given priority over the application of IMRT. 34 , 36 Types of radiotherapy treatment planning techniques: - Conventional external beam radiation therapy (2DXRT): Conventional external beam radiation therapy (2DXRT) is applied via 2-dimensional beams using Linac machines. 2DXRT consists of a single radiation beam delivered to the patient from several directions: often front or back (anterior to posterior), and both sides. Conventional describes the way the treatment is planned or simulated on a specific calibrated diagnostic x-ray machine known as a simulator because it recreates the linear accelerator actions (or sometimes by eye), and to the usually well-established arrangements of the radiation fields to produce a desired plan. The aim of simulation is to accurately localize the targeted volume which is to be treated. This technique is well established and is generally quick reliable and quick. The worry is that some high-dose treatments may be limited by the radiation toxicity capacity of normal tissues which are closed to the target tumor volume. An example of this problem is seen in radiation of the prostate gland, where the sensitivity of the nearby rectum limited the radiation dose which could be safely prescribed using 2DXRT planning to such an extent that tumor control may not be easily achievable. Prior to the invention of the CT, physicists and physicians had limited knowledge about the true radiation dose delivered to both cancerous and normal tissues. 2 , re...

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