Myron Wollin

Radiation tolerance of the vaginal mucosa.

Abstract Sixteen patients with cancer of the vagina that were controlled locally for a minimum of eighteen months after teletherapy (T) or brachytherapy (B) or both (T & B), were analyzed for radiation tolerance of the vaginal mucosa. The site of vaginal necrosis did not always coincide with the site of the tumor. The posterior wall appeared more vulnerable than the anterior or lateral walls. For the distal vaginal mucosa, necrosis requiring surgical intervention occurred following combined T & B, if summated rad exceeded 9800. The upper vagina tolerated higher dosages. No patient required surgery for upper vaginal necrosis even though summated (T & B) dosage up to 14,000 rad was applied. Placing radioactive needles on the surface of the vaginal cylinder with or without interstitial perinea) needles should be avoided. Further accumulation of data is needed to define these vaginal mucosa tolerance limits more closely.

The CT scanner as a therapy machine

Many tumors in the brain and in other tissues can be delineated precisely in images obtained with a CT scanner. After the scan is obtained the patient is taken to another room for radiation therapy and is positioned in the beam with the aid of external markers, simulators or stereotactic devices. This procedure is time consuming and subject to error when precise localization of the beam is desired. The CT scanner itself, with the addition of a collimator, is capable of delivering radiation therapy with great precision without the need for external markers. The patient can be scanned and treated on the same table, the isocenter of the beam can be placed precisely in the center of the lesion, the beam can be restricted to just those planes in which the lesion appears several arcs can be obtained by simply tilting the gantry, and the position of the patient in the beam can be monitored continuously during therapy. We describe here the properties of the CTX, the CT scanner modified for therapy.

Single‐ and double‐plane iridium‐192 interstitial implants: Implantation guidelines and dosimetry

Computerized dosimetric studies of single- and double-plane iridium-192 (Ir-192) planar implants were performed. With respect to dose homogeneity, we found that the optimal source and ribbon separation for single-plane implants was 1.0 cm. For double-plane implants, the preferred ribbon and plane separation was 1.5 cm, maintaining a 1-cm separation for the sources. Using these separations, standard dose rate curves for single- and double-plane Ir-192 implants were generated by computer calculations. These standard curves are useful for quickly and fairly accurately estimating the dose from any size planar implant, without requiring more time-consuming individual computer dosimetry. We believe that the curves will prove to be of practical clinical value to physicists and radiotherapists.

Radiation sensitive breast cancer patients

Abstract The age dependence of poor cosmesis from breast irradiation and the frequency of micronuclei (MN) in buccal cells provide indirect evidence for the presence of a relatively radiosensitive subpopulation of ataxia telangiectasia (AT) heterozygotes in breast cancer patients

Optimization of box technique to reduce femur dose in radiation therapy of the pelvis

Abstract Using the four field or “box technique” (opposing AP and lateral fields) in irradiating the pelvis, one can reduce the dose to the bladder and rectum while unavoidably irradiating the femurs. Four methods of applying the box technique with 4 MVP, 60 Co, and 24 MVP were analyzed to discover which technique delivers the lowest dose to the femurs: (1) Source-surface distance (SSD) equal given dose to all fields; (2) Source-axis distance (SAD) equal “air” dose to all fields; (3) SSD equal tumor dose to all fields; (4) SAD equal tumor dose to all fields. The results indicate that technique 1 (SSD equal given dose to all fields) gives the lowest dose to the femurs.

Conversion from Cs-137 to Ir-192 for high dose rate remote afterloading: practical considerations.

High dose rate (HDR) remote afterloading is increasingly being used to replace many conventional low dose rate (LDR) brachytherapy procedures. Implementation of the microSelectron-HDR with Ir-192 at our facility necessitated this study to obtain equivalent dosimetric distributions with those of our LDR Cs-137 techniques using our current treatment planning system. Three anatomical sites are presented: nasopharynx, esophagus, and uterine cervix. Attention must be given to the anisotropy of Cs-137 tubes when converting to Ir-192; for linear geometries, total equivalent activity may be preserved but the shapes of the resulting isodose curves for Ir-192 are longer than those of Cs-137. In the case of Fletcher-Suit intracavitary treatments of the uterine cervix, the longer contours for Ir-192 in the vaginal ovoids results in higher isodose levels reaching the bladder and rectum. Maintaining the traditional dose levels to these organs is accomplished by modifying the loading of the ovoids to approximately 85% of the corresponding Cs-137 activity. Computerized dosimetry is presented, along with a chart we have devised to easily convert a standard LDR treatment to HDR dwell times. Our results are especially suitable to those users who will continue to make use of their present computer treatment planning system.

Predicting normal tissue injury in radiation therapy

We tested three radiobiologic models, the nominal standard dose (NSD), the biologic index of reaction (BIR), and the linear quadratic (LQ) models to determine which best predicts normal tissue injury in radiation therapy. Clinical data for radiation myelopathy, rib fracture and pericardial effusion were used for all three models to predict injury. We assumed that on the average injuries occurred at higher equivalent doses of radiation than were received by patients who were not injured. We used a t-test to determine whether there were in fact significant differences in the mean values of the equivalent doses among the injured and non-injured. The means were calculated for the four sets of injury by the three models. For the LQ model it was necessary to choose a value for the parameter alpha/beta; the results were not sensitive to the choice over the range of 1/2 to 12 Gy. None of these models showed a significant difference between injured and non-injured patients for all four sets of data. The BIR model showed significant differences in three sets, the LQ model was significant in two and marginally significant in one set, and the NSD was significant in two sets. This analysis illustrates therefore, that the linear quadratic model can be adopted for analysis of clinical data with results that are no worse and possibly better than the NSD model.

Unequal weighting of given doses in opposed fields in treatment of cancer of the tonsillar region using 60Co, 4-, 8-, 15-, 24-MVp photons.

Weighting of given doses from parallel opposed fields is a widespread practice in radiation therapy of cancer in the tonsillar region. In order to determine when weighting is useful, dose distributions on the central axis were calculated for various energies (60Co, 4-, 8-, 15-, and MVp photons), weighting factors, and field separations. Criteria for judging the usefulness of weighting were (i) that variation in the dose across the tumor volume not exceed 10%, and (ii) that the dose to critical tissues outside the target volume be at least 20% less than target volume dose. The clinical situation that met these criteria was a lateral lesion that extended to less than one-third of the transverse diameter of the head, treated with either 60Co, 8, or 4 MVp with 4:1 weighting. Under these conditions, weighting was considered to be justified.

Application of the ICRU Report 38 reference volume concept to the radiotherapeutic management of recurrent endometrial and cervical carcinoma

Radiation therapy was given to 25 patients presenting with pelvic recurrence of endometrial (14) and cervical (11) cancer. Of these patients, all but one had undergone hysterectomy following their original diagnosis. Two endometrial patients received preoperative intracavitary irradiation. The recurrence-free interval ranged from 5 to 71 months (mean = 21 months). External beam radiation therapy for pelvic recurrence ranged from 3000 to 5000 cGy. Additional central radiation was given to 18 patients with either external beam, intracavitary, interstitial, or transvaginal technique. Dose and dose rates from brachytherapy are documented with maximum values, along with the location of these dose points. Such specification is essential in obtaining a more accurate impression of the total dose delivered to the patient, especially when different techniques are employed to increase the dose to the center of the pelvis. Reference volume dimensions, similar to those specified by ICRU Report 38 for intracavitary treatments, are presented. Mean follow-up from completion to radiotherapy is 22 months; 16 patients are dead, 2 are alive with disease, and 7 are alive with no evidence of disease.

The need for individualized dosimetry for tangential breast treatment

We have examined the isodose distributions of 119 intact breast patients treated on a 6 MV linac to determine if a library of treatment plans could be used instead of individualized computer plans for patient treatments without compromising the quality of those treatments. The parameters studied were: field width, baseline separation, central axis separation, wedge angle, and isodose coverage. At least two wedges were used in the computer plans for each patient and the best plan was then chosen. In order to construct a library of plans, the choice of wedge, treatment isodose, and dose uniformity should be predictable. Our results show that for 90 out of 119 plans (76%), the 30 degrees wedge was best. In the other 29 cases, either the 15 degrees or the 45 degrees wedge yielded better plans. On average, the improvement in dose homogeneity due to choice of wedge was about 2% (range 0-7%) for these cases. Although grouping like-patient parameters generally restricted the isodose variation to +/- 2.5%, there were five patients for which up to a 7% underdosage would not have been predicted. For the set of plans using a 30 degrees wedge, a significant correlation was found for the ratio of the baseline to central axis separation vs. treatment isodose. The average isodose which covered the target area was 97% (range 90-100%) and 102 out of 107 patient plans using the 30 degrees wedge fell between 94 and 100%. We conclude from these results that the variation in dose distribution found with seemingly similar sized breasts is due to the variation in breast shape and symmetry. The use of a library plan with a single wedge and a standardized isodose line for tangential field treatment of intact breast could cause up to a 7% dose difference compared to the actual dosimetry for that patient.