The practical development of a synchronizing instrument and its clinical application for the field shaping withlead shots

Lee, Y H; Yoon, Y K; Nam, J W; Choi, B S

Abstract

For accurate shielding of irregular shaped organ, a synchronizing instrument and it's auxiliary device for thefield shaping with lead shots have been developed, in addition to minimize penumbra region, for Co-60 teletherapy.The contours to be shielded area in the field which was already drown on the roentgenogram taken under theidentical condition to those of the treatment were cut out of a piece of styrofoam at the adequate position byusing the synchronizer. In cutting out of styrofoam, electric current is the most recommendable to obtain a cleanedge. Styrofoam, its cut out cavities were filled with the commercial lead shots, found to be effective for theshielding of desired area. These techniques have been practically applied to eleven patients. Two of them were thecases of the malignant lymphoma which was involved to the cervical lymph nodes bilaterally and nine of them werethe cases of the carcinoma of the breast after the radical or a bilateral simple mastectomy. To settle down thepatient in steady state during the treatment session, also a piece of styrofoam board which was scooped outextremely identical to the curved surface of the back of the body(patient holder) were used. In an experimentalstudy, authors found that was ideal to keep the patient from radiation damage by means of backscatter and orsecondary electrons emitted from the material. And radiation interactions with styrofoam were found to be airequivalent. The special features and advantages of this developmental study would be summarized as follows; 1. Thematerials used in this technique such as lead shots, styrofoam and plexiglass etc. were easy to acquire withreasonable price and tractable. 2. The HVL of the commercial lead shots was 19.2 mm, 10 cm thickness of this wassufficient to shield against the energy of Co-60 radiation for the treatment. 3. It was possible to built thesynchronizer with relatively low cost. 4. Taking into single portal treatment plan with greater field, percentagedepth dose would getting be much higher with greater source-surface-distance. This technique would therefore bepractically useful to obtaine relatively uniform depth dose distribution. 5. Simultaneous treatment with samedoses would be possible to the extensively located lymph nodes. 6. To use patient holder can fully be expectedmuch accurate, stable and comfortable positioning in each treatment session. 7. It was very effective to minimizesecondary penumbra for the protection of healthy tissue from unnecessary ionizing radiation regardless of themagnification to source-surface-distance. 8. At the time to scoop out of styrofoam (patient holder), the backforming was already adapted to make the front surface be flat. It was, therefore, not necessary to compensate thesloping field.