Isodose distribution

Revised definition provided by Internationa Organisation for Medical Physicists

Medical Physicists (Revised definition provided by Internationa Organisation for Medical Physicists)
Medical Physicists apply knowledge and methodology of science of physics to all aspects of
medicine, to conduct research, develop or improve theories and address problems related to
diagnosis, treatment, and rehabilitation of human disease. They are directly involved with patients
and people with disabilities.

Tasks include –

1.  Conducting research into human disorders, illnesses and disabilities; investigating biophysical techniques associated with any branch of medicine.
2.  Conducting specialised examinations of patients and the disabled, improving patient careand clinical services, developing innovative imaging and non-imaging diagnostic procedures for specific medical applications.
3.  Developing novel instrumentation and physiological measurement techniques,mathematical analysis and applications of computers in medicine in response to clinical need for patients, and aids to everyday living for the disabled;
4.  Ensuring the quality, safety testing and correct maintenance and operation of treatment machines, x-ray equipment, radiation treatment planning computers; medical uses of ultrasound, MRI, and infrared; and the correct delivery of prescribed radiation doses to patients in radiation therapy;
5.  Ensuring the accuracy of treatment unit parameters and settings used for a patient’s treatment, including correct transfer of parameters between the simulator, treatment plan and the treatment unit, and periodic review of each patient’s chart.
6.  Calculating dose distributions and machine settings; design and fabrication of treatment aids and treatment-beam modifiers for individual patient treatments.
7.  In-vivo measurement to verify the dose delivered to a patient; participation at patient discussion conferences.
8.  Advising and consulting with physicians on the physical and radiobiological aspects of patients’ treatments, and the development of treatment plans in such applications as use of ionising radiation in diagnosis, therapy, treatment planning with externally delivered radiation as well as use of internally implanted radioactive sources given the state of technology
9.  Planning, directing, conducting, and participating in supporting programs and remedial procedures to ensure effective and safe use of ionising and non-ionizing radiation and radio nuclides in human beings by physician specialist
10.  Formulating radiation protection guides and procedures specific to hospital environment and other professional groups and organizations; conducting specialised measurements and producing protocols to minimise radiation exposure of patients, staff and the general public;
11.  Participating in and contributing to the development and implementation of national and international standards, laws and regulations relating to patient safety, particularly to radiation and radioactive materials;
12. Teaching principles of medical physics to physicians, residents, graduate students,medical students, technologists, and other health care professionals by means of lectures,problem solving, and laboratory sessions
13.  Preparing, publishing and presenting scientific papers and reports;
14.  Supervising and managing radiation workers and other health professional workers.
15. Examples of the occupations classified here:Clinical medical physicists, Clinical scientist

Professional Status of Medical Physicists in Nepal

 Professional Status of Medical Physicists in Nepal

                                                                                            

                                                                         

Introduction

Early in the 20^th century, the introduction of ionizing radiation into medicine mainly established the role of physicists in health sectors. Charles Philips, honorary physicist to the Royal Cancer Hospital in London, is regarded as the first medical physicist, although he had no formal qualifications. But in 1913, Sidney Russ became the first medical physicist formally appointed by British hospital, the Middlesex hospital in London. In context of Nepal, the activities of medical physicist began after 1991 AD. Mr.Praduman Prasad Chaurasia is the first medical physicist of Nepal.  He is also founder president of neplese association of medical physicist( NAMP) formally registered in 2009.

Number of medical physicists

There are more than 150 hospitals throughout the country. But very few have radiotherapy department where mainly medical physicists have been appointed.presently 9 physicists are in radiotherapy and only one in diagnostic imaging at TU teaching hospital Mahrajganj, Kathamdu. All are male.Kathmandu Cancer Care and Research Center (KCCRC), Nepal Cancer Hospital  and Research Centre(NCHR) and other are going to start RT service with more advanced technology. Therefore, there are rays of hopes for medical physicists.

Hospital	No.of medical physicist	Department
v  BP Koirala Memerial Cancer Hopital,Chiwan v  Bhaktpur Cancer Hospital,Bhaktpur v  Bir Hospital v  Manipal teahing Hospital v  TU teaching Hospital, kathmandu	v  Four v  Two v  one v  one v  one	v   Radiation oncology(radiotherapy) v  Radiotherapy v  Radiotherapy v  Radiotherapy v  Radio diagnostic

Education and training

To be qualified medical physicists, one should have master degree in physics from a recognized university. Besides this, he/she must have diploma in radiological physics or in aggregate one year training in medical physics or three working experience in a radiotherapy/diagnostic department of a recognized institution/hospital or master/PhD degree  in medical physics. We are proud to mention that we all meet above qualifications and we have gained trainings from internationally recognized institutes like ICTP, IAEA,AIIMS and others. It is very sad to mention, that very few that have PhD study, are not in Nepal because they did not get opportunity according to their knowledge.

Challenges for medical physicist

v   Till now here is no M.Sc. course in medical/ radiation physics.

v  we depends on foreign resources to get expertise in medical physics, which too is rare event.

v  There is not  professional council to certify the medical physicist as other health profession but our society  ‘‘NEPALESE ASSOCIATION OF MEDICAL PHYSICIST(NAMP)” consider who can be qualified medical physicists.

v  We medical physicist have not also legal right to perform RSO/RPO activities independently.

v  Nepal has not yet regulatory body/Nuclear act despite being a member of IAEA .

v   Many Medical colleges have MD radiology/radiotherapy course of three years but hardly one or two have medical physicist faculty like manipal teaching hospital, pokhara.

v  Maximum radiographer course are conducted without radiation physics faculties.

v  There are no single physicists in radio diagnosis department in Nepal for Quality assurance and radiation protection.

Current Needs

Nepal has to implement  a lot to improve the medical physics professional status

v  To start medical physics programs(M.Sc. and PhD)

v  To conduct examination for medical physicist to provide license as other health professional

v   To strat formal awareness programs throughout the nation.

v  To form regulatory body according to nuclear act, which must define role and responsibility of radiation worker like medical physicist, radiation oncologist, radiologists and others.

v  To stop the brain-drain by providing facilities and research environment.

v  To appoint the medical physicist/RSO/RPO with all equipments of quality assurance (QA) in each diagnostic department of hospital/ medical college/ in ministry of health to start the radiation audit program.

v  To implement rules/guidelines of IAEA for all radiation activities.

International institution/ society should  also support by giving more opportunity to Nepalese student/ medical physicist for further study in medical physics like M.Sc and PhD.   NAMP has more responsibility to enhance the status of medical physics profession by spreading knowledge of good and bad effects of radiation on human beings in health sectors, industry, and agriculture etcetera.

Ram Narayan Yadav

                                                                               Medical Physicist

BP Koirala Memorial Cancer Hopital

IMRT Dosimetry Tools

IMRT Dosimetry Tools

Point dosimetery

1) Ionization chambers: Cylindrical ionization chambers are used for point-dose measurements in megavoltage photon radiation therapy because of their excellent stability, linear response to absorbed dose, small directional dependence, beam-quality response independence, and traceability to a primary calibration standard.High-spatial resolution is important for IMRT measurements.

a)Ionization chambers should be used:

 (i) In homogeneous dose regions (ii) To verify monitor unit outputs (iii) To verify critical structure doses.

b)Ionization chambers should not be used:

(i) To measure beam profiles that will be employed to model the IMRT beam penumbra.

(ii) When the leakage current will yield an integrated charge of >5% of the expected radiation-induced charge. Leakage current corrections should be applied if the expected uncorrected error is >2%.

c)Chamber selection

(i) A cylindrical ionization chamber should be used.(ii) The size of the ionization chamber should be appropriate for the task (1) For measuring output factors: The radiation field should be 1.5 cm wider than the effective length of the ionization chamber.(2) For IMRT dose measurements: The ionization chamber size should be small enough to            limit the dose heterogeneity across the chamber active volume to 10% and 5% if the   measurements are being compared against volume-averaged and point doses, respectively.(iii)The ionization chamber electrode should be fabricated out of low-Z materials(e.g.,aluminum). When high-Z electrodes are used, the chamber should be cross-calibrated in conditions that minimize photon spectral differences(e.g., the same depth and minimizing field size differences).

d) Measurement protocols

(i) When ionization chambers are used to validate single-point IMRT absolute doses: (1) The ionization chamber should be placed in a region of the dose distribution where the    expected dose heterogeneity is less than 10% or 5% across the ionization chamber and the expected dose heterogeneity is less than 10% within 2 mm from the intended ionization chamber position, if the measurements are being compared against volume-averaged or point doses, respectively

 (2) While point-dose comparisons are possible with all treatment planning systems,the measured dose should be compared to the calculated doses that have been averaged throughout the active ionization chamber active volume. This is typically done by computing the dose using a CT scan with the ionization chamber in place, contouring the ionization  chamber volume, and querying the treatment planning system dose statistics for the mean dose. This system can also be used to identify the expected dose heterogeneity.

2)solid state detectors: P-type semiconductor diode detectors have some attractive characteristics for megavoltage photon beam dosimetry, especially for small-field measurements. not only is the active volume of diodes much smaller than the smallest ionization chamber but also the sensitivity of these diodes can be 20–100 times greater. In contrast to silicon diode detectors, diamond detectors are almost soft-tissue equivalent in terms of atomic composition (although they have a physical density much greater than water at 3.5 g cm−3). Recently, chemical vapor deposition (CVD) diodes have been investigated for radiation therapy dosimetry. Another type of small-field dosimeter that has been used in IMRT is the thermoluminescence dosimeter (TLD).

Diode detectors( i.e. single-point detectors)

a) Diode detectors should be used:

(i) For measuring relative dose distributions, particularly for measuring MLC penumbras used in beam modeling in a treatment planning system (ii) For providing dose measurement points supplemental to ionization chamber measurements

b) Diode detectors should not be used:

(i) To measure absolute doses (ii) As the sole measurement device for measuring beam profiles that will be employed to model the IMRT beam penumbra. This is due to the potential for over-response in the low-dose regions outside the field boundaries. Other detectors with little energy response variations should be compared to diode-measured profiles to assure that the diode profile provides accurate out-of-field results.

(c) Diode selection

(i) Use unshielded diode detectors (ii) Diode detectors designed for in vivo dosimetry should not be used for in-phantom measurements.

(d) Measurement protocols

(i) Diode response varies with orientation, so the relative orientation of the diode to the radiation beam should be carefully considered.

Thermoluminescent dosimeters (chips)

a) TLD detectors should be used:

 (i) When the phantom geometry will not allow ionization chamber measurements.(ii) When multiple simultaneous point measurements are desired.

b)TLD detectors should not be used:

(i)To measure absolute doses if the overall measurement precision needs to be better than 3%.

c) TLD selection

(i)Low-atomic number TLDs (e.g., LiF) should be used.

(d) Measurement protocols

(i) A strict annealing and calibration protocol should be adopted that provides relative response factors for individual chips. (ii) Care should be taken to assure that the user accurately knows the TLD positions with respect to the linear accelerator.

Electrometer

The basic requirements for electrometers are (1) accuracy,(2) linearity, (3) stability, (4) sensitivity, (5)high impedance,and (6) low leakage. These electrometers have much lower leakage currents than most of the older models. Physicists should also assess the performance of electrometers with automatic leakage correction.

2D Dosimetry

The current commercial options for two-dimensional (2D) dosimetry are radiographic film, radiochromic film, computed radiography, diode arrays,and ionization chamber arrays.

Film: Film is an excellent tool for 2D dose mapping due to its extremely high spatial resolution with grain sizes typically having dimensions on the order of microns with appropriate densitometer.

a)Radiographic film

Two commercial radiographic films are most commonly used Kodak EDR2 and XV2. XV2 should no bet used to measure doses greater than 100 cGy and EDR2 should not be used to measure doses greater than 500cGy.

 i Radiographic film should be used:

1)For relative IMRT dose distribution measurements. 2) To measure beam profiles that will be employed to model the IMRT beam penumbra. 3) For measuring relative output factors of small  fields

ii Radiographic film should not be used:

1) For absolute dose measurements 2 )To verify monitor unit outputs

(iii) Measurement protocols

1) A sensitometric curve should be measured for each radiographic film experiment.2) The sensitometric curve films should be selected from the same batch as the measurement films.3) The sensitometric curve films should be processed at the same time as the measurement films.

4 )The recommendations of AAPM Task Group 69 should be observed for radiographic film and densitometry.5) Handle film carefully with clean hands or light cotton gloves.6) Bending, stretching, or scratching films should be avoided.7) For EDR-2, wait at least 1 h after irradiation before processing.

b) Radiochromic film

i Radiochromic film should be used:

1) For measuring relative dose distributions.2) For measuring dose distributions that will be used to model the IMRT beam penumbra.3) For measuring relative output factors of small fields.

4) When a radiographic film processor is not available

ii_Radiochromic film should not be used:

1) For absolute dose measurements.2) To verify monitor unit outputs

iii Film selection

1 EBT-2 is the only film commercially available with appropriate sensitivity

(iv) Measurement protocols

1)A sensitometric curve should be measured for each radiochromic film experiment.2) The sensitometric curve films should be selected from the same batch as the measurement films.3) The optical density distribution should be measured no sooner than 1 h after irradiation.4)The sensitometric curve and measurement films should be irradiated on the same day.5)The sensitometric curve and measurement films should be scanned on the same day.6) The orientation of the sensitometric and measurement films during scanning needs to be consistent with respect to the original orientation.7) Handle film carefully with clean hands or light cotton gloves.8) Bending, stretching, or scratching films should be avoided.

Array detectors

Array detectors calibrated to yield multiple cumulative readings of absorbed dose across a 2D plane represent a recent and popular new addition to the tools available for routine clinical IMRT QA. Commercially available two-dimensional diode array detector utilized n-type diode technology. This device is called the Mapcheck . The Mapcheck contains an array of 445 variably spaced diodes over an area of 22x22 cm2. The diode spacings are 7.07and 14.14 mm in the central 10x10 cm2 and the outer regions, respectively. The diode plane has an effective build-up depth of 2 cm and a backscattering thickness of 2.3 cm. The physical cross section of each diode is 0.8 mm2. The 2D ionization chamber arrays have detector spacingof 1x1 cm2.

(1)Useful for efficient routine QA of a precommissioned IMRT technique. Initial commissioning should be performed with a system with higher spatial resolution(e.g., film).(2) For calibration and all measurements with the device, the linear accelerator dose repetition rate should be the

same as for the clinical treatment.(3) The device calibration should be checked monthly, or as specified by the manufacturer or published literature.(4) Careful consideration should be given to the development of pass/fail acceptance criteria for the evaluation of the results from an array detector. For example, AAPM Task Group 119  demonstrated pass rates of >90% of the evaluated points when using 3 mm/3% distance-to-agreement (DTA) and dose-difference criteria,respectively, when reporting institution’s planar diode detector measurement QA results. Each physicist should determine acceptance criteria that are appropriate for the treatment site, the treatment objectives, and the clinic’s policies.

Computed radiography

Computed radiography (CR) has been available for more than 20 years, but is gaining interest in radiation oncology as a dosimeter because of the removal of film processors from radiation oncology departments.CR techniques have been successfully used for megavoltage beam relative dosimetry by employing low-energy filters, but care should be taken if CR is used for IMRT QA because the photon spectrum varies widely across IMRT fields.

3D dosimetry/ gel-dosimetry

Truly comprehensive dosimetric verification of IMRT requires a dosimetry system with full capability in 3D. A goal of 3D dosimetry has been formulated in the resolution-time-accuracy-precision (RTAP) criteria. The RTAP ideal functionality incorporates a spatial resolution of 1 mm3, short read-out time of <<1 h, accuracy within 3% of true value, and a noise level within 1%. The three principal categories of 3D dosimeters are polyacrylamide (PAG) gels, Fricke gels, and radiochromic plastics.

The original PAG gel consisted of bis (3%), acrylamide (3%), nitrogen, and gelatin (5%) by weight. PAGs utilize the mechanism of radiation-induced polymerization of monomers, where small monomer molecules join together under the influence of ionizing radiation. The resultant polymer microparticles are fixed in the gelatin lattice, yielding a stable impression of the dose. High resolution 3D maps of dose have been achieved both by MR scanning and optical-CT scanning.

Fricke and radiochromic gels and plastics: A limitation of gelatin Fricke dosimeters is that the radiation-induced ferric ions diffuse through the gel matrix, leading to degradation and eventual loss of the recorded dose distribution with time. The practical implication is that Fricke gel dosimeters must be scanned within 2 h postirradiation, presenting logistical difficulties in a clinical setting. Recently, a promising new radiochromic material, PRESAGE, has been introduced and detailed studies of the basic dosimetric properties have confirmed its promise for 3D dosimetry.

PHANTOMS

Verification processes for IMRT vary significantly in their phantom requirements. Phantoms are typically constructed using either water or water-equivalent plastic. Open water phantoms can be used when the beam is perpendicular to the phantom surface, and where great flexibility in detector positioning is desired. With the proper procedures and design, water-equivalent plastic phantoms can support multiple detectors, radiographic film, and rapid and efficient setup reproducibility. Such phantoms can also include the substitution or addition of heterogeneous materials. To conduct an overall evaluation of an IMRT delivery system, anthropomorphic phantoms are useful in conjunction with other phantoms.

Geometric phantoms

Simple geometric phantoms that can accommodate ionization chambers and film are used for measuring single point and planar doses. Cubic phantoms, comprised of slabs, are easy and accurate to set up and allow for measurements at multiple depths. A NOMOS phantom was modified to accommodate TLDs and multiple ionization chamber positions by using different spacers of water-equivalent material. Rectangular phantoms are useful for measuring single field or composite dose distributions. Cylindrical phantoms have a convenient geometry for coplanar composite IMRT delivery verification, while allowing for multiple ionization chamber positions. A novel cylindrical phantom that places radiographic film in a spiral slot and an ionization chamber at the phantom center was developed for tomotherapy verification.

Anthropomorphic phantoms

Anthropomorphic phantoms are fabricated in the shape of a human. anthropomorphic phantoms have been effectively used for limited measurements to evaluate the process of patient treatment planning and delivery and to identify treatment planning or dose delivery problems that are not evident in simple homogeneous geometric phantoms.While anthropomorphic phantoms are good for assessing the overall IMRT planning and delivery process, many commercial phantoms are composed of thick transverse slices, which limit the flexibility in film and point-dosimeter placement. The sources of this article is AAMP reports.

Ram Narayan Yadav

                                                                                                                           Medical Physicist

                                                                                                                         Department of radiation oncology

BP Koirala Memorial Cancer Hospital

                                                                                                                       Bharatpur,Chitwan.