Amjad Hussain

Studying wedge factors and beam profiles for physical and enhanced dynamic wedges

This study was designed to investigate variation in Varians Physical and Enhanced Dynamic Wedge Factors (WF) as a function of depth and field size. The profiles for physical wedges (PWs) and enhanced dynamic wedges (EDWs) were also measured using LDA-99 array and compared for confirmation of EDW angles at different depths and field sizes. WF measurements were performed in water phantom using cylindrical 0.66 cc ionization chamber. WF was measured by taking the ratio of wedge and open field ionization data. A normalized wedge factor (NWF) was introduced to circumvent large differences between wedge factors for different wedge angles. A strong linear dependence of PW Factor (PWF) with depth was observed. Maximum variation of 8.9% and 4.1% was observed for 60° PW with depth at 6 and 15 MV beams respectively. The variation in EDW Factor (EDWF) with depth was almost negligible and less than two per cent. The highest variation in PWF as a function of field size was 4.1% and 3.4% for thicker wedge (60°) at 6 and 15 MV beams respectively and decreases with decreasing wedge angle. EDWF shows strong field size dependence and significant variation was observed for all wedges at both photon energies. Differences in profiles between PW and EDW were observed on toe and heel sides. These differences were dominant for larger fields, shallow depths, thicker wedges and low energy beam. The study indicated that ignoring depth and field size dependence of WF may result in under/over dose to the patient especially doing manual point dose calculation.

A Novel Translational Total Body Irradiation Technique

A novel translating bed total body irradiation treatment delivery technique that employs dynamically shaped beams is presented. The patient is translated along the floor on a moving bed through a stationary radiation beam and the shape of the radiation beam is changed dynamically as the patient is moved through it, enabling compensation for local variations in patient thickness and tissue density. We demonstrate that the use of dynamically shaped beams results in greatly improved dose homogeneity compared with standard techniques, which use a single static beam shape. Along a representative dose profile through the lungs of a mock-human body, the maximum range of dose deviation from the average is 5.6% (from +2.7% to -2.9%) for the dynamic beam technique compared with 12.8% (from +3.6% to -9.2%) for the static beam technique. A novel, dual-interlock system that prevents bed motion when the radiation beam is stopped and stops the radiation beam when the bed motor is stopped has also been developed. The dual-interlock not only enhances the safety of the treatment but also ensures accuracy in the delivery of the treatment.

Implementation of quality medical physics training in a low‐middle income country — sharing experience from a tertiary care JCIA‐accredited university hospital

To the Editor: Our team wishes to share with the readers of a medical physics journal our experience of establishment of a two-year radiation oncology physics postgraduate training program. Our purpose of sharing this experience is to encourage medical physics colleagues to come up with innovative ideas and to take up this challenge of improving the quality of medical physics training in order to improve the overall health-care quality of professionals involved in the radiation oncology services of cancer patients in countries like Pakistan. The Aga Khan University (AKUH), Pakistan’s first private international university, is committed to the provision of education, research, and health care of international standard relevant to Pakistan and the region. In line with this vision, the University Hospital has established a state-of-the-art Radiation Oncology facility. AKUH is the only hospital in Pakistan, and one of the few teaching hospitals in the world, to be awarded Joint Commission International (JCI) accreditation for achieving and maintaining highest international quality standards in health care. AKUH has also received the ISO 9001:2008 certification. In the specialty of radiation oncology, medical physicists play an imperative role. They are skilled professionals with multidisciplinary responsibilities including, but not limited to, radiation safety, treatment planning, treatment delivery, and quality assurance in radiation therapy. A number of new modern radiotherapy units, such as Gamma Knife, Cyber Knife, Varian Trilogy, Elekta Synergy, have recently been installed in Pakistan and especially in Karachi. This makes the deficiency of skilled medical physicists even more critical in private sector. The AKUH has taken the initiative of ‘capacity building’ locally, with the help of international expertise, by establishing a two-year Certificate Program in Medical Physics specializing in radiation therapy to meet the emerging needs of quality radiation therapy.

SU‐E‐T‐270: Optimized Shielding Calculations for Medical Linear Accelerators (LINACs)

PURPOSE The purpose of radiation shielding is to reduce the effective equivalent dose from a medical linear accelerator (LINAC) to a point outside the room to a level determined by individual state/international regulations. The study was performed to design LINACs room for newly planned radiotherapy centers. METHODS Optimized shielding calculations were performed for LINACs having maximum photon energy of 20 MV based on NCRP 151. The maximum permissible dose limits were kept 0.04 mSv/week and 0.002 mSv/week for controlled and uncontrolled areas respectively by following ALARA principle. The planned LINACs room was compared to the already constructed (non-optimized) LINACs room to evaluate the shielding costs and the other facilities those are directly related to the room design. RESULTS In the evaluation process it was noted that the non-optimized room size (i.e., 610 × 610 cm2 or 20 feet × 20 feet) is not suitable for total body irradiation (TBI) although the machine installed inside was having not only the facility of TBI but the license was acquired. By keeping this point in view, the optimized INACs room size was kept 762 × 762 cm 2. Although, the area of the optimized rooms was greater than the non-planned room (i.e., 762 × 762 cm 2 instead of 610 × 610 cm 2), the shielding cost for the optimized LINACs rooms was reduced by 15%. When optimized shielding calculations were re-performed for non-optimized shielding room (i.e., keeping room size, occupancy factors, workload etc. same), it was found that the shielding cost may be lower to 41 %. CONCLUSIONS In conclusion, non- optimized LINACs room can not only put extra financial burden on the hospital but also can cause of some serious issues related to providing health care facilities for patients.

Treatment Planning in Radiation Therapy

Radiation therapy is the clinical use of ionizing radiation as part of a comprehensive cancer treatment to eradicate malignant/cancerous cells. It works by damaging the DNA of cancerous cells, which is the primary cause of cell death. Normal cells are also damaged by ionizing radiation; however, they generally have a better recovery mechanism than the cancerous cells.

Basic Concepts in Radiation Dosimetry

Directly and indirectly ionizing radiations deposit their energy in a medium while passing through it. Radiation dosimetry is a procedure that deals with the methods for quantitative determination of that deposited energy. To be more specific, quantitative determination of energy absorbed in a given medium by directly or indirectly ionizing radiations is called radiation dosimetry. It plays a crucial role in radiation therapy, nuclear medicine and radiation protection. Due to its significance, accurate determination of the deposited energy (often termed a radiation absorbed dose) at the point of interest in the medium (i.e., human body or phantom) is needed. A number of quantities and units have been defined for describing the radiation beam, which ultimately leads to the determination of radiation absorbed dose to the medium by incident radiation. These quantities and units are explained in this chapter. Furthermore it covers the fundamental ideas and principles involved in radiation dosimetry.

Acquisition of small field electron dosimetry data using ion chamber & Gafchromic® EBT3 film and development of GUI for treatment parameters calculation

Small electron field is a preferential choice of the oncologists in case of the treatment of small superficial lesions (scars). Small field electron dosimetry is necessary to perform prior to the treatment to assure the accuracy of the dose delivery. The main objectives of this work is to acquire small field electron dosimetric data with narrow circular cutouts and to develop the graphical user interface on the basis of this acquired data to extract desired field parameters from the data. Results show that as the cutout size become smaller, percentage depth dose shifts towards surface due to the lack of lateral scatter equilibrium. Thus surface dose is increased i.e. the depth of maximum dose (dmax) is reduced. Similarly the depth of 90% isodose level is decreased as well as the range (i.e. R50 and Rp). Whereas the x-ray contamination is increased with decrease in cutout diameter, field size coverage for 90% isodose curve and the relative output factor is decreased. Results of the radiochromic films were not up to expectations because of the existence of air pockets as the acquisition was performed with solid water phantom slabs. It is, therefore, suggested to use these films in real water. Based on experimental data, GUI enables us to find treatment parameters i.e. energy, cutout dimension, margins, bolus thickness, monitor units etc.

SU-F-T-266: Dynalogs Based Evaluation of Different Dose Rate IMRT Using DVH and Gamma Index

PURPOSE This work investigates the impact of low and high dose rate on IMRT through Dynalogs by evaluating Gamma Index and Dose Volume Histogram. METHODS The Eclipse™ treatment planning software was used to generate plans on prostate and head and neck sites. A range of dose rates 300 MU/min and 600 MU/min were applied to each plan in order to investigate their effect on the beam ON time, efficiency and accuracy. Each plan had distinct monitor units per fraction, delivery time, mean dose rate and leaf speed. The DVH data was used in the assessment of the conformity and plan quality.The treatments were delivered on Varian™ Clinac 2100C accelerator equipped with 120 leaf millennium MLC. Dynalogs of each plan were analyzed by MATLAB™ program. Fluence measurements were performed using the Sun Nuclear™ 2D diode array and results were assessed, based on Gamma analysis of dose fluence maps, beam delivery statistics and Dynalogs data. RESULTS Minor differences found by adjusted R-squared analysis of DVHs for all the plans with different dose rates. It has been also found that more and larger fields have greater time reduction at high dose rate and there was a sharp decrease in number of control points observed in dynalog files by switching dose rate from 300 MU/min to 600 MU/min. Gamma Analysis of all plans passes the confidence limit of ≥95% with greater number of passing points in 300 MU/min dose rate plans. CONCLUSION The dynalog files are compatible tool for software based IMRT QA. It can work perfectly parallel to measurement based QA setup and stand-by procedure for pre and post delivery of treatment plan.

Translating bed total body irradiation lung shielding and dose optimization using asymmetric MLC apertures

A revised translating bed total body irradiation (TBI) technique is developed for shielding organs at risk (lungs) to tolerance dose limits, and optimizing dose distribution in three dimensions (3D) using an asymmetrically‐adjusted, dynamic multileaf collimator. We present a dosimetric comparison of this technique with a previously developed symmetric MLC‐based TBI technique. An anthropomorphic RANDO phantom is CT scanned with 3 mm slice thickness. Radiological depths (RD) are calculated on individual CT slices along the divergent ray lines. Asymmetric MLC apertures are defined every 9 mm over the phantom length in the craniocaudal direction. Individual asymmetric MLC leaf positions are optimized based on RD values of all slices for uniform dose distributions. Dose calculations are performed in the Eclipse treatment planning system over these optimized MLC apertures. Dose uniformity along midline of the RANDO phantom is within the confidence limit (CL) of 2.1% (with a confidence probability p=0.065). The issue of over‐ and underdose at the interfaces that is observed when symmetric MLC apertures are used is reduced from more than ±4% to less than ±1.5% with asymmetric MLC apertures. Lungs are shielded by 20%, 30%, and 40% of the prescribed dose by adjusting the MLC apertures. Dose‐volume histogram analysis confirms that the revised technique provides effective lung shielding, as well as a homogeneous dose coverage to the whole body. The asymmetric technique also reduces hot and cold spots at lung‐tissue interfaces compared to previous symmetric MLC‐based TBI technique. MLC‐based shielding of OARs eliminates the need to fabricate and setup cumbersome patient‐specific physical blocks. PACS number(s): 87.55.‐x, 87.55.de, 87.55.D‐A revised translating bed total body irradiation (TBI) technique is developed for shielding organs at risk (lungs) to tolerance dose limits, and optimizing dose distribution in three dimensions (3D) using an asymmetrically-adjusted, dynamic multileaf collimator. We present a dosimetric comparison of this technique with a previously developed symmetric MLC-based TBI technique. An anthropomorphic RANDO phantom is CT scanned with 3 mm slice thickness. Radiological depths (RD) are calculated on individual CT slices along the divergent ray lines. Asymmetric MLC apertures are defined every 9 mm over the phantom length in the craniocaudal direction. Individual asymmetric MLC leaf positions are optimized based on RD values of all slices for uniform dose distributions. Dose calculations are performed in the Eclipse treatment planning system over these optimized MLC apertures. Dose uniformity along midline of the RANDO phantom is within the confidence limit (CL) of 2.1% (with a confidence probability p=0.065). The issue of over- and underdose at the interfaces that is observed when symmetric MLC apertures are used is reduced from more than ±4% to less than ±1.5% with asymmetric MLC apertures. Lungs are shielded by 20%, 30%, and 40% of the prescribed dose by adjusting the MLC apertures. Dose-volume histogram analysis confirms that the revised technique provides effective lung shielding, as well as a homogeneous dose coverage to the whole body. The asymmetric technique also reduces hot and cold spots at lung-tissue interfaces compared to previous symmetric MLC-based TBI technique. MLC-based shielding of OARs eliminates the need to fabricate and setup cumbersome patient-specific physical blocks. PACS number(s): 87.55.-x, 87.55.de, 87.55.D.

SU‐E‐J‐81: Adaptive Radiotherapy for IMRT Head & Neck Patient in AKUH

Purpose: In this study we proposed Adaptive radiotherapy for IMRT patients which will brought an additional dimension to the management of patients with H&N cancer in Aga Khan University Hospital. Methods: In this study 5 Head and Neck (H&N) patients plan where selected, who’s Re-CT were done during the course of their treatment, they were simulated with IMRT technique to learn the consequence of anatomical changes that may occur during the treatment, as they are more dramatic changes can occur as compare to conventional treatment. All the organ at risk were drawn according RTOG guidelines and doses were checked as per NCCN guidelines. Results: The reduction in size of Planning target volume (PTV) is more than 20% in all the cases which leads to 3 to 5 % overdose to normal tissues and Organ at Risk. Conclusion: Through this study we would like to emphasis the importance of Adaptive Radiotherapy practice in all IMRT (H&N) patients, although prospective studies are required with larger sample sizes to address the safety and the clinical effect of such approaches on patient outcome, also one need to develop protocols before implementation of this technique in practice.