Developing a Compensator using Cerrobend Materials

ABSTRACT

In external beam radiotherapy (EBRT), dose optimization is achieved by conforming the dose distribution to the shape of the intended target whilst minimizing radiation to normal tissues in close proximity to the target. This is achieved by modulating the intensities of the radiation across the radiation portals forming the irradiation geometry used for the treatment. Compensators may be used to achieve the above effect and can be used to approximate the fluence map by appropriate linear attenuation coefficient of individual beamlets making up the original open beam fluence. This may be done with a treatment planning system (TPS) with inverse planning capabilities or with a bolus placed on the surface of the patient at the beam entrance point. This work describe the procedures for designing, constructing and dosimetric considerations of cerrobend compensator for high energy photon beams, using the bolus option on the surface of the phantom planned with Prowess Panther TPS. Also correction factors that account with effects of field size, treatment depth and changes in thickness ratio because of using bolus were introduced. The cerrobend compensator was adjusted to account for beam divergence and reduction in dose contributed by scattered radiation. The correction factors were applied to the thickness ratio for determination of appropriate thickness of cerrobend that mimic bolus. The measurements were done in Theratron Equinox 100 cobalt-60 teletherapy unit using Cerrobend slabs constructed to account for divergence of the beam for the maximum field size considered in this research (30×30 cm²). The narrow and broad beam linear attenuation coefficient for cerrobend were determined using simple attenuation model, varying the field size from 4×4 cm² to 30×30 cm² field sizes in air, and also varying the thickness of cerrobend from 0.5cm to 4.6cm. The value found was 0.4574cm^-1 and also the field size dependence of linear attenuation coefficient were investigated. The scatter produced by cerrobend was accessed and evaluated. The scatter-to-primary ratio dose contribution was found to be negligible for small field size as reported by Dimitriadis (2002), and can cause error in the final dose calculation up to 13.3% for 30×30 cm² and 4.09 cm thickness of cerrobend. The cerrobend compensator was successful designed and constructed. The dosimetric accuracy for constructed cerrobend compensator was found to be deviating with that predicted with Prowess Panther Treatment Planning System with percentage error ranging from 0.365 to 25%, which is associated with limitations in producing precise thickness of cerrobend with the same accuracy of that generated by the equation 3.04 and limitations in generating flat surface topography and also the presence of air bubble in the cerrobend compensator which was not investigated in thiswork.

CHAPTER I

1. INTRODUCTION
   1. BACKGROUND

Cancer is a significant health care problem. On average about half of all cancer patients are treated with radiation therapy worldwide (IAEA, 2004).

Radiotherapy, also referred to as radiation therapy, radiation oncology or therapeutic radiology is one of the three modalities used to treat malignant disease (cancer) the other two being chemotherapy and surgery (Suntharalingamn et al, 2005). Radiotherapy uses ionizing radiation to eradicate cancerous cells with the least possible damage to normal tissues.

The first therapeutic use of ionizing radiation was demonstrated in 1897 by Wilhelm Alexander Freud, a German surgeon before Vienna Medical Society when he demonstrated the disappearance of a hairy mole following treatment with x-ray (Hall, 2000). The first recorded experiment in radiobiology was also performed by Becquerel when he advertently left a radium container in his vest pocket and subsequently described the skin erythema two weeks later (Hall, 2000).

The modalities of radiotherapy are divided into two types, tele-therapy and brachytherapy. Brachytherapy is a method of treatment in which sealed sources are used to deliver radiation at short distances by interstitial, intracavitary or surface application (Khan, 2010). Tele-therapy is a treatment modality in which the source of radiation is at a distance from the patient, also called external beam radiation therapy, it uses photons ranging from kilo voltage to megavoltage photons, and electron beams from linear accelerators or Co-60 tele therapy units. In External Beam Radiation Therapy (EBRT), the methodology of treatment depends on different factors, which may include the shape and size of the tumour to be treated within the patient, sparing of normal tissues within the vicinity of the target from excessive irradiation, financial constraints and the quest of optimization of radiation dose to the target. There are different treatment techniques ranging from 2-D conventional radiotherapy to more advanced Intensity Modulated Radiotherapy (IMRT). IMRT is a treatment planning and delivery technique that can greatly improve the process of conformal radiotherapy which refers to the process of blocking a beam with irregular shaped beam portal so that the dose delivered corresponds more closely to tumour whilst reducing the dose to normal tissue. In developing countries, most of the centrees are restricted to a Co-60 tele therapy unit with basic treatment planning and simulation capabilities.

Patients present irregular surface topographies and tissue heterogeneities. According to Chang (2004), a compensator is a traditional tool for modern application and is an alternative IMRT delivery technique.

In IMRT, the compensator is used not in the sense of compensating for missing tissue or tissue heterogeneity but as beam intensifier like dynamic wedges and multileaf collimators (MLC). The goal is to achieve dose uniformity throughout the whole target volume and, more importantly to spare critical structures according to the dose and dose volume constraint prescribed by the clinicians for specific patients (Jiango et al, 1998). Therefore, compensators are designed to produce an optimized primary fluency profile at the patient’s surface. This is achieved by modulating the intensities of the radiation across the radiation portals forming the irradiation geometry used for the treatment.

There are various methods by which compensators can be made. According to Williams and Thwaites (2000), the three main types are grid-blocking system, contour system and a system using machined compensator. The first compensators made by Ellis et al. (1958) were constructed by stacking aluminum pillars. Another method reported by Lam et al. (1983) describes the construction of compensators from thin sheets of lead. Today compensators are more commonly made from molds filled with molten alloy or wax. Using molds is advantageous since it results in compensators with smoother surfaces and thus greater accuracy.

To make a compensator for an IMRT practice, it is required to calculate the effective attenuation coefficient () of its materials, which is affected by various factors as field size, depth, off- axis distance, compensator thickness (Haghparast et al, 2013). A number of elements have been used to form compensators which include tungsten-epoxy mixture (Xu et al, 2002), Lucite (Khan et al, 1970), gypsum (Weeks et al, 1988), tin-wax (Van et al, 1995), tin (Chang et al, 2000), cerrobend (Waltz BJ et al, 1973), steel (Van et al, 1995), aluminum (Ellis et al, 1959), brass (Ellis et al, 1959; Tess, 2014), lead (Leung et al, 1974; Cunnighan et al, 1976; Andrew et al, 1982; Spicka et al, 1988), coper (Tess, 2014). In this study, a cerrobend compensator will be constructed using a simple attenuation model to determine its effective attenuation coefficient. Film and an ionization chamber will be used for dosimetric measurements and for verification of measured dose distribution and compared with those calculated with the PROWESS Panther TPS software at Korle-Bu Teaching Hospital.

2. PROBLEM STATEMENT

In external beam radiotherapy (EBRT), dose optimization is achieved by conforming the dose distribution to the shape of the intended target whilst minimizing radiation to normal tissues in close proximity to the target. Most dosimetric measurements are done on flat surface and homogenous medium, however patient’s surface is highly irregular and internal tissues are heterogeneous. The main aim of radiation therapy is to deliver uniform dose distribution within +7 % and –5 % (ICRU report 50, 1993) of the dose prescription without exceeding the tolerance dose of the critical structure around tumor volume. To achieve this goal, the above irregularities should be corrected. Thus different studies suggested and implemented bolus which is a tissue equivalent material placed at the surface of the patient to compensate the missing tissue. However, this technique doesn’t spare the skin beneath the bolus. This is because, the buildup region is in the bolus and D_max (depth of maximum dose) will be at skin surface. To solve such complications compensators have been introduced by different people on different approaches to correct both surface irregularity and tissue heterogeneity which is now done by using MLC based IMRT.

Advanced technological innovations in anatomic and functional imaging modalities (CT, MRI, PET, and US) have led to improved visualization and the delineation of tumour. Radiation treatment planning and delivered techniques have also seen a marked improvement. Intensity modulated radiotherapy (IMRT) provides a high degree of dose conformity to the planning target volume (PTV) and the conformal avoidance of organs at risk. Therefore radiation field is not only geometrically shaped to conform to the outline of the planning target volume at the beams eye view, but is also intensity modulated.

The National Centre for Radiotherapy and Nuclear Medicine of Korle-Bu Teaching Hospital (KBTH) presently uses paraffin wax for construction of a compensator and cerrobend for shielding blocks, but there is a need to implement physical compensator based IMRT using materials which are available in the Centre and is inexpensive. This research will focus on design and construction and dosimetric considerations of cerrobend compensators to modulate the intensities of the radiation across the radiation portals forming the irradiation geometry used for the treatment.

3. OBJECTIVES
   1. GENERAL OBJECTIVE

The general objective of this work is designing and constructing a compensator using cerrobend materials.

2. SPECIFIC OBJECTIVES

• To clarify the effect of scattered photons generated within the compensator on head scatter factor.
• To evaluate dosimetric accuracy and dose coverage.
• To compare and evaluate measured and predicted data.
• To evaluate the variation of dose distribution by the compensator.

4. SCOPE AND LIMITATION

The scope of this thesis is in the area of the IMRT by means of physical compensators specifically using cerrobend which are manually fabricated. In most centres which are practicing IMRT, the construction of the compensator to provide the needed modulation is done by generating a fluency map of the radiation portal needed. This is done with a treatment planning system (TPS) with inverse planning capabilities or with a bolus placed on the surface of the patient at the beam entrance point. The bolus option will be used in this research as currently there is no TPS in the country that can do inverse planning. In this case, the cerrobend compensator will be used to replicate dosimetric effects of the bolus placed on the surface of the patient. According to Jiang et al (1998), the calculation of compensator thickness profile (an optimized primary fluency profile) is straightforward typically using the exponentially attenuation model. With reference to this, the shape of the compensator will be adjusted to account for beam divergence and reduction in dose contributed by scattered radiation. Thus the dosimetric considerations is part of the scope of this research. The measurement will be made from a Co-60 tele therapy machine at Korle Bu Teaching Hospital (KBTH).

5. RELEVANCE OF THE PROJECT WORK

In radiation oncology, a patient should get the best treatment option as much as possible in order to improve quality of patient care. So the expected results such as correction factors to account for reduction in scatter for using the cerrobend compensator to mimic bolus would have immense contribution to scientific and technical knowledge. From this work, it will be possible to implement IMRT delivering technique at National Centre for Radiotherapy and Nuclear medicine of Korle-Bu Teaching Hospital. The clinical implementation of IMRT technique requires at least two systems (Khan, 2010), which are: treatment planning computer system that can calculate non-uniform fluence maps for multiple beams directed from different directions to maximize dose to target while minimizing dose to critical normal structures. This may be done with a treatment planning system (TPS) with inverse planning capabilities or with a bolus placed on the surface of the patient at the beam entrance point. The second one, is a system delivering a non-uniform fluence as planned, so each of these systems must be appropriately tested and commissioned before the actual clinical use. The bolus option will be used in this research as currently there is no TPS in the country that can do inverse planning. The cerrobend compensator will be used to replicate dosimetric effects of the bolus placed on the surface of the patient. Similar research was done using different materials by Teclehaimanot (2014) in which the results were not in the clinically acceptable levels, so with this work we are expecting to reach such clinical levels with deviation less than 5%.

Intensity modulated radiotherapy (IMRT) is widely used in clinical applications in developed countries, for the treatment of malignant and non-malignant diseases. This technique uses multiple radiation beams of non-uniform intensities. The beams are modulated to the required intensity maps for delivering highly conformal doses of radiation to the treatment targets, while sparing the adjacent normal tissue structures. This treatment technique has superior dosimetric advantages over 2-dimensional (2D) and conventional 3-dimensional conformal radiotherapy (3DCRT) treatments. It can potentially benefit the patient in three ways. Firstly, by improving conformity with target dose, it can reduce the probability of in-field recurrence. Secondly, by reducing irradiation of normal tissue, it can minimize the degree of morbidity associated with treatment. Finally, by facilitating escalation of dose, it can improve local control (Cheung, 2006).

Compensator based IMRT has a lot of advantages over MLC, many literature reported by Taherkhani (2010), report that the penumbra regions created by MLCs are larger than those generated by cerrobend blocks. Compensators provide more consistent dose, impose no limitations on the dose delivery rate, reduce skin surface doses, and because of the high density of the cerrobend allows improved skin sparing with low production rate of secondary electrons (Gray, 1979; Hine, 1951) reported by Shery (1987). It gives continuous intensity modulation, high spatial resolution, gives room to treat large field size, easy quality assurance (QA), shorter treatment time delivery with some drawbacks which are lack of automation (Chang, 2004), but there are some disadvantages like the therapist having to go to the treatment room to change the compensator in multiple fields and production cost, being labor intensive and time consuming. But now these drawbacks have been fixed in many developed countries by introducing a milling machine which is incorporated with the Treatment Planning System (TPS), and an automated compensator-IMRT technique (Javedan et al. 2008).

Other main advantage of using cerrobend in this research are: its low melting point of 158⁰F (70⁰C) which makes it easy to be recycled. It is readily available, inexpensive, high density (9.8g/cm³) and is used as shielding blocks in EBRT where doses are reduced by 95% or 99% of their initial value.

As a material for compensation with high energy photons, cerrobend provides several advantages over tissue equivalent material (Shery, 1987). In the past, Cerrobend had not been considered as an excellent compensator material despite its high density. Recently Chang et al (2004) found that there are cerrobend filling techniques that produce smooth and accurate compensators with consistent density. Solidified Cerrobend in the compensator mold becomes one of the top choices of compensator material. And it can be easily shaped to the intended form with uniform density using the technique described by (Chang, 2004).

Chang et al (2004) showed that compensator-IMRT technique has several benefits for delivering continuous intensity modulation and have shown that the finer resolution compensator-IMRT technique can also produce dosimetry that is closer to the ideal IMRT treatment (without any delivery limitation) compared with the segmental MLC IMRT technique. From this work the patients treated at the National Centre of Radiotherapy and Nuclear Medicine will benefit from all the advantages of IMRT techniques mentioned above. Consequently patients will also get a better and inexpensive treatment option.