Year : 2006 | Volume
: 16 | Issue : 4 | Page : 875--877
Radiation dose monitoring in a lung cancer patient with a pacemaker - a case report
D Mitra, K Ghosh, P Gupta, J Jayanti, AR Dev, PK Sur
Department Of Radiotherapy, Medical College, Kolkata, 88, College Street, Kolkata 700 073, India
Department Of Radiotherapy, Medical College, Kolkata, 88, College Street, Kolkata 700 073
|How to cite this article:|
Mitra D, Ghosh K, Gupta P, Jayanti J, Dev A R, Sur P K. Radiation dose monitoring in a lung cancer patient with a pacemaker - a case report.Indian J Radiol Imaging 2006;16:875-877
|How to cite this URL:|
Mitra D, Ghosh K, Gupta P, Jayanti J, Dev A R, Sur P K. Radiation dose monitoring in a lung cancer patient with a pacemaker - a case report. Indian J Radiol Imaging [serial online] 2006 [cited 2020 Aug 14 ];16:875-877
Available from: http://www.ijri.org/text.asp?2006/16/4/875/32372
The number of cardiac pacemakers inserted annually is on the rise. The percentage of patients who have pacemaker and developing malignancy is also increasing with increasing longevity, better diagnosis and management of both cardiac and malignant disease . Patients of carcinoma of breast, lung and mediastinal lymphomas are candidates who are mainly treated by radiation therapy where pacemaker might come in close proximity to the radiation field.
Before 1970s pacemakers employed conventional bipolar semiconductors and were relatively resistant to damage by ionizing radiation. At present pacemakers use Complimentary Metal Oxide Semiconductor (CMOS) units and silicone dioxide based Integrated Circuits (IC), which are particularly susceptible to damage by ionizing radiation but the specific nature and type of the damage and the safety dose limit is yet to be defined ,. The following case, which we have treated at our center, has a few unique features, which we like to share.
Materials and Methods
A 76 years old gentleman presented to the radiotherapy department with severe respiratory distress, puffiness of face and engorgement of veins. A diagnosis of Superior Vena Caval Syndrome (S.V.C.S.) was made and after a quick and thorough examination that included CT scan and CT guided Fine Needle Aspiration Cytology (FNAC), the cause was found to be due to carcinoma lung - non small cell type. Radiation therapy to the chest wall overlying the tumour and superior mediastinum was planned. He had a cardiac pacemaker implanted seven years ago. The pacemaker came at the edge of the tumour causing problem in planning radiation field (Fig-1). As the patient was in severe respiratory distress it was decided to administer inj. cyclophosphamide 1 gm I.V. on an emergency basis. Patient's condition improved to some extent and then pacemaker manufacturing company and the cardiologist was consulted regarding the possibility of relocation of pacemaker to opposite side of chest wall. But as his condition was not stable the idea was abandoned and radiotherapy was decided taking carefully into account the dose to the pacemaker. Total dose delivered was 4000 cGy in conventional fractionation in two phases - a) 1000 cGy by anterior field only with SSD technique, b) 3000 cGy by anterior and posterior fields with SAD technique. Lead shields were used at the edge of the beam overlying the pacemaker to reduce the dose. The radiotherapy planning was done using ASHA 3D Treatment Planning System (TPS). The CT scan was fed in the planning system and probable dose to pacemaker was evaluated (Fig.-2). The details of the treatment given with techniques and evaluated doses are given in Table - 1.
TABLE - 1
Evaluation of dose to pace maker
Monitoring of the patient was carried out daily. It included detailed attention regarding field setting and immobilization, ECG to check the function of the pacemaker and noting down his symptoms and signs. Weekly blood tests were also carried out. A backup temporary pacemaker was kept available under a cardiologist who was consulted regarding the patient's degree of pacemaker dependence and the likely cardiac complications. Patient completed the radiotherapy well. Daily E.C.G. did not show any abnormality. He is kept on chemotherapy regime and has been found well after 1 year.
As the number of patients with cardiac pacemaker requiring radiation therapy to the chest wall is on the rise, it is imperative that the physicist and radiation oncologist should be aware of the potential risk to the pacemaker and its prevention.
The pacemaker can be damaged by any of the three ways: -
1) Damage to the CMOS chip secondary to depolarization brought about by ionizing radiation.
2) Damage to the memory bits which interferes with the activity of the pacemaker software.
3) If the radiation is given in linear accelerator the electro-magnetic field is another potential source of damage.
The damage may be minor posing little risk to the patient or it may be a major one. Minor malfunction may manifest as transient or prolonged change to interference or safety mode pacing, increases in pulse width, changes in paced rate and programming . These changes may be seen at as low as 200 cGy. The major malfunction includes extreme fixed rate output, prolonged pacemaker inhibition or total shutdown. These types of malfunction require immediate replacement of the pacemaker. Due to the complex circuit of modern pacemakers, it is difficult to predict the nature and extent of pacemaker damage . There is no consistent way to predict how and at what radiation dose a device will fail. Ideally the manufacturer should provide specific dosage guideline but in many cases including our, they failed to provide any specific dosage recommendations.
Several case reports of radiation therapy in pacemaker-implanted patients exist in the literature . These patients were mostly treated by Linear Accelerator (Linac). In many of them the pacemaker had to be replaced after damage was detected after giving 1500 to 3600 cGy thus interrupting radiation therapy and imparting heavy cost to the patient. In some cases pacemaker was reallocated to a different location prior to radiotherapy. From a survey of literature it is seen that although minor pacemaker malfunction have been reported with doses as low as 200 cGy, significant failure has been reported at around 1000 cGy or higher. Nibhanupudy described the successful treatment of one breast patient with pacemaker receiving radiation therapy . The pacemaker was reallocated about 5 cm away from the radiation field. The dose to the pacemaker was measured to be 170 cGy.
There are few interesting and important observations in this particular case. We could not reallocate the pacemaker due to poor general condition of the patient. But it also avoided the delay in initiation of treatment and saved treatment related cost. Second important point was the administration of chemotherapy, which not only helped to improve the patient's condition but also helped the physician in field placement, by shrinking the tumour. We were able to place field properly protecting the pacemaker. Thirdly lead shields were used over the pacemaker up to the edge of the field thus minimizing the scatter dose considerably. American Association of Physicists in Medicine (AAPM) has issued several guidelines for successful management of these patients and all the recommendations were followed in this case . The pacemaker was regarded as a critical structure with tolerance of 200 cGy. Great importance was given to the scatter factor. The total dose was tried to be kept ideally within 200 cGy and certainly within 1000 cGy. The dose to the pacemaker was evaluated by measurement in a phantom of stacked perspex sheets. The point of measurement was 7.5 cm from the central axis of the beam and at a depth of 2 cm that corresponds to the position of the pacemaker in the patient's body. The measured dose was 166.25 cGy with lead shields as used during treatment which is well below the prescribed tolerance of 200 cGy. If shield was not used the dose would have been 209.64 cGy. But the dose evaluated from TPS with shield was 73.25 cGy. This emphasizes the role of scatter that is often ignored and affirms the need to shield the pacemaker upto the beam edge to prevent scatter. In this regard our observations are:
1) No lead shield was used in the posterior field, as the depth of the pacemaker was larger from the posterior surface (16 cm). But it was observed that contribution of dose from posterior field was much more compared to the shielded anterior field, which is due to the divergence of the beam. So we recommend use of shield for posterior field as well.
2) The dose evaluated from the TPS is less than the measured dose. It may be due to the fact that dose contribution outside the beam due to the collimator scatter is not considered in the algorithm used in TPS.
The tolerance dose for the different types of pacemaker is to be evaluated like other critical organs during radiotherapy. The pace maker should be shielded properly. In this case as per our observation it is necessary to use shield in both anterior and posterior fields, which will further reduce the dose to the pacemaker. For evaluation of dose, measurement in phantom is a better guide and is recommended. It is to be checked by dosimetry in phantom whether the TPS algorithm incorporates the evaluation of scatter dose beyond geometrical edge of the beam.
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