Improved dose homogeneity in the head and neck using computer controlled radiation therapy.

Computer-controlled radiation therapy techniques are demonstrated which improve dose homogeneity throughout the nasopharynx when compared to conventional treatment techniques. The typical approach using a heavily weighted anterior field and opposed wedged lateral fields results in a dose gradient from 95% to 110% or greater. All three of the computer-controlled techniques investigated improved the dose uniformity to a range from 95% to 105% or less. Multiple overlapping fields are used to compensate for patient anatomy and treatment beam characteristics. Treatment planning and monitor unit calculations are quite time-consuming at this stage of development. Actual treatment time is not unreasonably long and can be improved in future releases of the therapy machine control software.

Custom beam profiles in computer-controlled radiation therapy.

A computer-controlled radiation therapy technique is demonstrated which uses multiple concurrent boost fields to modify the beam profile of a conventional treatment beam. A principal field, identical to that of a corresponding conventional treatment plan, delivers the major component of the prescribed dose. Dose increments given from boost fields placed within this principal field compensate for variations in patient anatomy, for variations in target volume shape, and/or for imperfect beam characteristics, such as excessive off-axis dose or inadequate beam wedge angle. This concurrent boost field technique is demonstrated for several treatment sites. It produces significant improvement in uniformity of dose delivered to the target compared to conventional treatment. Implementation of these treatments requires a computer-controlled linear accelerator with independently-movable collimator jaws, an automatic beam set-up procedure, and a patient prescription database. Since all fields are delivered under computer control, concurrent boost technique treatment times are not much longer than those of conventional treatments.

Long term variation in beam symmetry as a function of gantry angle for a computer-controlled linear accelerator.

Testing computer-controlled linear accelerators for patient safety and proper patient dose delivery requires that certain beam characteristics be monitored over an extended period of time. Computer-controlled conformal radiation therapy using asymmetric collimator jaw settings necessitates stable symmetric treatment beams. Long term beam symmetry measurements have been performed on a Philips SL20 dual energy computer-controlled linear accelerator. Symmetry in both the radial and transverse axis of each x-ray beam was monitored for eight gantry positions. These measurements were undertaken to determine the effectiveness of the SL20 beam steering system during dose delivery of 50 monitor units (MU) per field. Evaluation of the data shows that careful beam steering setup procedures result in x-ray beams in which fluctuations in symmetry as a function of gantry angle are within +/- 1.5%. Day to day instabilities produce a total overall variation in beam symmetry on the order of +/- 2.0%. Results suggest the measurement of symmetry as a function of gantry position become a routine quality assurance procedure for this accelerator.

Thermoforming plastic in lead shield construction.

Radiation treatments using low energy X-rays or electrons frequently require a final field defining shield to be placed on the patient's skin. A custom made lead cut-out is used to provide a close fit to a particular patient's surface contours. We have developed a procedure which utilizes POLYFORM thermoplastic to obtain a negative mold of the patient instead of the traditional plaster bandage or dental impression gel. The Polyform is softened in warm water, molded carefully over the patient's surface, and is removed when "set" or hardened, usually within five minutes. Then lead sheet cut-outs can be formed within this negative. For shielding cut-outs requiring thicker lead sheet, a positive is made from dental stone using this Polyform negative. We have found this procedure to be neat, fast and comfortable for both patient and the dosimetrist.

Total excision of mucinous biliary cystadenoma.

In the past year, total excision of mucinous biliary cystadenomas was carried out in three patients by shelling these lesions out from the liver substances. Formal hepatic resection was unnecessary. As any previous surgical procedure on the cyst will make subsequent excision technically more difficult, or even impossible, it is believed that excision should be carried out at the initial operation. Thus, a correct pre-operative or intra-operative diagnosis is important. This requires a high index of clinical suspicion, an adequate pre-operative work-up and an appropriate use of intra-operative ultrasound and frozen section to differentiate these lesions from other cystic lesions of the liver.

Leiomyomatosis peritonealis disseminata.

A 49 year old woman presented with leiomyomatosis peritonealis disseminata (LPD) which is a benign and very rare disease entity. This patient is particularly interesting because she presented with many atypical features. She is the second Oriental case reported, and the follow-up period was long. The disease was symptomatic and progressive despite bilateral salpingo-oophorectomy, hysterectomy and repeated excision of all the recurrent nodules. She had documented recurrences of LPD in three episodes at 4 years apart. There was no obvious systemic hormonal abnormality including pregnancy or oestrinizing granulosa tumour, and she had never taken any contraceptive drugs. The importance of differentiation of this condition from generalized metastatic intra-abdominal malignancy is discussed as LPD has a very good prognosis.