Philip M.K. Leung

Medulloblastoma: Treatment technique and radiation dosimetry

Abstract It is well established that elective cranio-spinal irradiation is beneficial in the treatment of medulloblastoma. However, because of the geometry of the brain and the spinal column, a simple treatment procedure which results in a uniform dose over the entire target volume while maintaining a low dose to other tissues is not easy to perform. This report describes a relatively simple treatment technique which can be administered with radiotherapy apparatus that is available in most treatment centers. Special consideration is given to the dosimetry in the junction regions of the contiguous 60 Co fields. Computer dosage calculations were made to check the effectiveness of this technique. This calculated data was verified experimentally with thermoluminescent dosimetry (TLD) in both a rectangular polystyrene phantom as well as the human like Alderson Rando phantom. The dose to critical organs such as the eyes was found to be less than 15% of the tumor dose, while the male gonadal dose was less than 10%. Detailed consideration is given to the factors affecting the overall variation of dose in the entire target volume. It has been estimated that using this technique, the overall dosage variation to the cranio-spinal axis is within ±10% of the prescribed tumor dose for the average adult, while for the average child it is better than ±5k.

A method of large irregular field compensation

Abstract For large irregular field radiation treatments, the dose distribution is affected by (a) the surface contour, (b) the change in SSD at points away from the central ray of the beam, and (c) the change in scatter conditions due to the irregular shape of the field. These combined effects could produce a dose variation within the irradiated volume up to 30 per cent. In this report a method of compensating for all these effects is described with special consideration being given to a simple method of producing a missing tissue compensator. This method incorporates a simple mechanical device for marking out thin lead sheets to show lines of equal air gap distance.

Role of mitomycin C in the development of late bowel toxicity foloowing chemoradiation for locally advanced carcinoma of the cervix

PURPOSE To determine if the inclusion of mitomycin C (MMC) in chemoradiation protocols for locally advanced cervical cancer (LACC) significantly enhances the development of serious (Grade 3) late bowel toxicity (SLBT). METHODS AND MATERIALS The incidence of SLBT in 154 patients with LACC entered in six consecutive chemoradiotherapy protocols between February 1982 and June 1987 was determined. Fifty-four patients who were treated with MMC, 5-fluorouracil (5-FU), and radiation were compared to 100 patients who received similar treatment without MMC. Univariate and multivariate analyses assessed the effect of the following parameters on the development of SLBT: (a) external beam dose, (b) rectal and rectosigmoid dose, (c) paraaortic radiation, (d) intracavitary dose and dose rate, (e) volume of tissue irradiated to a total dose of 60 Gy, (f) International Federation of Gynecology and Obstetrics stage, (g) age, (h) number of courses of 5-FU, (i) previous abdominopelvic surgery, (j) split versus continuous radiation, and (k) administration of MMC. RESULTS The overall incidence of SLBT was 15.6%: 14 of 54 (26%) versus 10 of 100 (10%) for patients who did or did not receive MMC, respectively (p = 0.009). Multivariate analysis revealed the administration of MMC as the only factor predictive for the development of SLBT (p = 0.012, odds ratio = 3.15; 95% confidence interval 1.3-7.7). A significant reduction in SLBT was observed with the elimination of MMC from the chemoradiation protocols despite dose escalation of both radiation and 5-FU. No increase in overall survival was observed in patients receiving MMC, 5-FU, and radiation compared with 5-FU and radiation alone. CONCLUSION The inclusion of MMC in these chemoradiation protocols for LACC is associated with significant enhancement in serious late bowel toxicity.

COBALT-60 THERAPY UNIT FOR LARGE FIELD IRRADIATION

Abstract A cobalt-60 therapy unit designed and constructed by The Ontario Cancer Institute is described. This unit is capable of treating large fields up to 50 × 160 cm 2 at a conventional source to surface distance of 90 cm by using large collimator openings. Three selectable filters were incorporated. They all act as electron filters to improve the build-up characteristics of these beams. Two of the three filters also act as beam flatteners. The longitudinal collimators are individually driven so that it is possible to produce a vertical beam edge by closing one of the collimators completely. This condition is very useful for matching adjacent fields. The output of this unit, at its operating distance, ranges from 100–200 rad/min depending on which of the filters is chosen, which makes the treatment time shorter than most known facilities for very large field irradiation. This unit has been in routine use since 1977.

Dosimetric considerations of very large cobalt-60 fields

Abstract Because of the growing trend toward the use of larger radiation fields, a special cobalt-60 unit has been constructed at the Princess Margaret Hospital which provides field sizes up to 160 × 50 cm at 90 cm from the source. Conventional percentage depth doses and tissue-air ratios have been published up to 35 × 35 cm fields. This report considers measurements that were performed to extend these data up to an equivalent field of 75 × 75 cm. Various problems associated with very large field dosimetry are considered. Effects of radiation backscattered from the floor are not negligible. The inverse-square law shows much larger deviations than at more conventional field sizes. Build-up characteristics were also found to be much worse than for smaller fields; however, appropriate electron filters have improved this. Usually, tissue-air ratio and percentage depth dose measurements are obtained under infinite phantom conditions. However, patients are not infinite phantoms. The difference between infinite and non-infinite phantoms has been found to be significant.

Utilization of a High Energy Photon Beam for Whole Body Irradiation

Use of a high energy linear accelerator (giving high dose rates) for whole body irradiation is recommended because (a) there is a shorter treatment time compared with other techniques using orthovoltage or 60Co irradiation; and (b) there is more comfort for the patient, who does not have to turn over to receive an acceptably uniform dose. Dosimetry indicates that the usual data is not applicable for such treatments given at large distances from the source. A method of dosage calculation at large distances was checked by using TLD dosimetry in a Rando phantom.

Use of electron filters to improve the buildup characteristics of large fields from Cobalt‐60 beams

It has been shown that for Cobalt-60 units capable of producing large fields (up to 40 X 40 cm2)at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1-2 mm instead of 5 mm; and second, the maximum does in some extreme cases is 15%-20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%-20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.

Application of small 60Co beams in the treatment of malignant melanoma at the optic disc.

An easy and convenient method for the treatment of a malignant melanoma located near the optic disc by means of small /sup 60/Co beams is presented. With an existing treatment unit, a small beam is produced by a secondary collimator 7 cm above the patients skin surface. A uniform dose (+-5 percent) over one third of the volume of the eye can be obtained by a special arrangement of a pair of oblique beams. The disadvantages shared by all the implantation techniques are not encountered in our method. The uncertainty inherent in the large dose gradient and high contact dose obtained when one uses an implant have been eliminated. It is shown that the dose to the patients skin and lens can be kept well below the limits of normal tolerance.

Eye sparing in high energy X ray beams

Treatment of cancer of the antrum and nasopharynx often includes the radiation of tissues close to an uninvolved eye. One treatment method consists of using an anterior high energy X ray beam directed to the tumor through the eye. To maintain a high dose adjacent to and behind the eye while reducing the entrance dose to the eye, build-up material is placed on the skin and a tunnel cut through to the eye. When the build-up material is tissue-like, the tunnel can be several centimeters in height and scattered radiation from the tunnel walls will largely offset the build-up properties of the beam. Using higher density build-up material, the dose to the superficial layers of the eye can be reduced almost to the limit set by the open beam characteristics. This technique has been used successfully for 8 years.

Dosimetry under pencil eye shields for Cobalt-60 radiation

The use of pencil eye shields to reduce the dose to the anterior chamber of the eye during radiotherapy with Cobalt-60 beams has been evaluated. It was found that the optimum shield placement is about 1 cm from the surface. This keeps the size of the penumbra and the effect of electron contamination at a minimum. The dose under such shields is mainly produced by the transmission through the shield and the scattered radiation both from within the phantom and from the collimator. While the scattered component is a function of the beam size and otherwise cannot be altered, the transmission can easily be reduced to a negligible level.