3 Radiation Protection in Brachytherapy

Radiation Protection in Brachytherapy

7

THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part I: The basics of Brachytherapy Version 1 - 01/12/2014

so that during HDR treatments, patient and treatment proce- dure can be constantly monitored and a verbal communication channel is always available between the patient and medical staff members. Other requirements are the direct availability of emergency equipment such as a cutting device, long forceps and a storage container in case of a source obstruction during treatment. Writ- ten emergency procedures must be available on site. Emergency response practice should be conducted periodically to ensure emergency preparedness of the staff members. It is quite common for a conventional external beam treatment room to be used as a HDR room, especially for clinics where HDR procedures are relatively rare and/or budget/space is limit- ed. If a conventional external beam treatment room is used as a HDR treatment room, the room shielding is normally adequate although a careful radiation survey is highly recommended and the room may need to be modified to be equipped with all the above-mentioned safety and interlock features. When time and distance are not adequate to ensure acceptable levels of protection, recourse to the third option for radiation protection in practice, shielding, has to be made. This can be simply described as the process where one needs to calculate the thickness of a particular material that must be interposed in a radiation field, to achieve a reduction of the expected exposure to a shielding design goal exposure at a point of interest (POI). The shielding design goal exposure at a POI cannot be greater than the regulatory dose limits for occupational and public ex- posure expressed in mSv per year (see Table 3.1). As an example, UK recommendations for controlled and public areas are 6 mSv and 0.3 mSv, respectively (11). Other countries may have defined different shielding design goals (33, 34). With the assumption of a 50 week working year and a brachytherapy facility workload that is evenly distributed in time, weekly derivative shielding de- sign goals are traditionally used (i.e. 0.12 mSv/week and 6 μSv/ week in controlled and public areas, respectively, in the UK). Given the source strength and the treatment duration, the air kerma can used for the calculation of the shielding requirements at the POI. The expected exposure at a reference point, common- ly referred to as the facility workload, needs to be determined in units of air kerma per week of operation. In order to estimate the workload, one needs the reference air kerma rate ( RAKR ) of the source(s) in units of Gym 2 h -1 , as well as an estimate of the number of patients treated per week and the average treatment time, so that the product will yield the workload in units of Gy per week at 1 m from the source(s). The estimate of the num- ber of patients should be based on reliable data (such as average national figures available) and as a minimum meet the viability standards set by the hospital management or national regula- tions. For example, the NHS in UK suggests that a brachytherapy centre patient throughput should, at minimum, be 50 patients per year overall, further elaborated to at least 10 intrauterine insertions, 10 of each of low throughput treatment sites (head and neck interstitial, bronchial and oesophageal intraluminal, 4. BRACHYTHERAPY FACILITY SHIELDING

breast and rectal interstitial and cervical applicator insertions), or 25 permanent prostate interstitial implants, as applicable (31). High-volume departments should use their own, higher, esti- mates. Treatment time is of course site and technique specific. It should be noted that the workload estimate should also include periodic quality control procedures and measurements. The workload should be appropriately weighted for the distance from the source to the POI using the inverse square law. This distance should be the shortest one (if applicable) and the POI is usually taken at 30 cm behind any physical barrier/wall. Since we are not seeking to protect the area but the most exposed indi- vidual occupying it, the workload must also be weighted by the appropriate occupancy factor which corresponds to the average fraction of working time during which the POI is occupied by the single person who spends the most time there. The occu- pancy factor is usually determined by facility employees and as- sumes a value of 1 for controlled areas (for a list of indicative oc- cupancy factors for shielding calculations refer to (11) or (35)). In summary, the reduction factor, R , by which the expected ex- posure at a POI must be reduced to comply with a shielding de- sign goal exposure, is given by: where: • P is the shielding design goal in units of effective or equivalent dose per week (Sv per week) for the POI, • W is the facility workload in units of air kerma per week at 1 m from the source(s) (Gym 2 per week where air kerma can be conservatively assumed equal to effective or equivalent dose, i.e. Gy≈Sv), • F is the occupancy factor at the POI (usually denoted by the capital letter T , not used here to avoid confusion with the trans- mission factor, T = R -1 , defined in the following), • d is the distance from the source(s) to the POI (m), The weekly workload is obtained from the multiplication of the source air kerma strength RAKR (in Gym 2 h -1 ), the number of sources or source positions used in procedures and the dwell time t (in h) summed over all used sources or dwell positions, and the estimate of the number of treatments per week. For exceedingly low workloads the assumption made above of a workload evenly distributed in time does not apply. Shielding calculations using Eq. 3.1 might therefore prove inadequate due to high instantaneous dose rates. For example, due to the time necessary for catheter placement and the limited availability of operating rooms, brachytherapy treatment rooms are not used as frequently as those for external beam treatments, and while the average workload might be low, the dose rate will be zero most of the time and high during treatment sessions. In order to accomodate such scenarios, additional regulatory shielding de- sign goals may also have been set for the instantaneous dose rate ( IDR ), the dose per hour over a given time interval. Having calculated the reduction factor, R , or equivalently the transmission factor, T = R -1 , defined as the ratio of air kerma rate at a POI with and without the interposition of a shielding barrier, this transmission must be translated into a shielding thickness for a given material. Care must be taken to use the transmission R = Pd 2 WF (3.1)