Nagalingam Suntharalingam

Limitations of the minimum peripheral dose as a parameter for dose specification in permanent 125I prostate implants

PURPOSE The objective of this work is to investigate whether the minimum peripheral dose is a practical parameter for dose specification in permanent 125I implants of the prostate. METHODS AND MATERIALS The investigation was carried out by use of a computer model of ellipsoidal 125I implants in which the average dimension and elongation factor were varied to provide a wide range of geometries. Both ideal and nonideal implants were investigated. The 125I seeds were confined to the target volume except for a portion of the study in which the effect of placing seeds outside the target volume was investigated. RESULTS The minimum peripheral dose was found to be very sensitive to the seed placement. The irregularities in the seed spacing that inevitably occur in actual implants tend to lower the minimum peripheral dose. As a result, the minimum peripheral dose is generally significantly less than planned by an amount that is unpredictable, and often exceeds 25%. However, the percentage of the target volume that receives a dose less that the prescribed minimum peripheral dose is generally less than 10%. Implanting seeds outside the target volume improves the dose uniformity, but does not appear to offer any advantage in dose coverage, and increases the volume of normal tissue irradiated. CONCLUSION If a minimum peripheral dose is prescribed for a permanent 125I prostate implant, and the implant is planned using an idealized implant having precisely spaced seeds, the prescribed dose will rarely, if ever, be achieved. Reasonable agreement with the prescribed dose can be achieved only if the requirement for coverage is relaxed from 100 to 90%, or if the total source strength is increased by 20% to compensate for the anticipated imperfections in seed placement.

Precision radiotherapy for cancer of the pancreas: Technique and results

Forty patients with locally extensive, unresectable adenocarcinoma of the pancreas received precision high dose (PHD) radiation therapy with a 45 MeV betatron. PHD radiotherapy was generally well tolerated. During treatment, only 7 patients experienced significant nausea, vomiting, diarrhea or anorexia. Late gastrointestinal radiation reactions were observed in 7 patients. Twelve patients received adjuvant chemotherapy. The projected survival of patients with unresectable pancreatic cancer treated with PHD radiotherapy is comparable to that of patients with resectable disease operated on for cure. The projected one year survival rate is 49%.

Concept of dose nonuniformity in interstitial brachytherapy

PURPOSE Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio. METHODS AND MATERIALS Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. RESULTS For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. CONCLUSION The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

Pancreatic carcinoma treated with high-dose, small-volume irradiation

Eighteen patients with unresectable ductal adenocarcinoma of the pancreas received definitive, high‐dose, small‐volume radiation therapy. All patients had at least one laparotomy, at which time a biopsy was obtained, radio‐opaque clips were placed to define the extent of the gross tumor, and usually some form of bypass procedure was performed. External‐beam irradiation was delivered from a 45‐MV betatron to an area encompassing the clipped tumor volume plus a 1 to 2 cm margin. A three‐field technique, employing opposed lateral 45‐MV photon beams and an anterior “mixed beam”(50% 45‐MV photons and 50% 15‐ to 35‐MeV electrons), was used to treat 13 of the patients. The choice of electron energy used for these patients was based on the depth of the posterior margin of the target volume. Five patients were treated by either four‐field “box” or three‐field, 45‐MV photon techniques. Minimum tumor dose was 6300–6700 rads delivered in 180‐rad fractions in 7–9 weeks. With the three‐field technique, all fields were treated daily; with the four‐field technique, two fields were treated daily. The projected survival rate 12 months post diagnosis is 59%, with the median length of survival, 11.8 months. Seven patients are alive 11.5 to 57 months after diagnosis, all clinically free of disease. Treatment has been generally well tolerated, and there have been no severe late radiation complications. This therapeutic modality appears capable of producing improved palliation, if not offering definitive radiotherapeutic management of localized unresectable carcinoma of the pancreas.

Field size dependence of wedge factors

The radiation output in the presence of wedge filters is characterized by the wedge transmission factor and open beam field size factors. Conventionally, the wedge factor for high-energy photons is measured in a water phantom at depth of maximum dose for a reference field size. Experimental measurements on different wedges indicate that the wedge factors are a function of field size. An analysis of these data show that this is primarily caused by the change in scattered radiation from the treatment head in the presence of wedge filters. The change in phantom scatter and radiation backscattered to the monitor chamber are minimal. For 4- or 6-MV x rays with a 60 degrees wedge, the use of a single wedge factor measured for 10 cm X 10 cm field introduces errors of up to 3.5%, for a 16-cm-wide field. For a 20-cm-wide field with this wedge, the error is 7%. Thinner wedges exhibit less differences.

Reference dose rates for single‐ and double‐plane 192Ir implants

The proper selection of the reference dose rate is critical in the irradiation of a tumor or tumor bed using interstitial implant. The selected reference dose rate should result in the delivery of highly homogeneous dose throughout the target volume, adequately cover the target volume, and minimize irradiation of the surrounding normal tissues. In this paper, the influence of the reference dose rate on the adequacy of the irradiation of idealized target volumes using single- and double-plane 192Ir implants was studied in terms of three volumetric irradiation indices. The results show that maximum relative dose homogeneity can be attained if the proper reference dose rate is chosen. This isodose rate contour exhibits a width larger than the thickness of the target volume in the central plane. The lowest dose rate within the idealized target volume is not recommended as the reference dose rate to avoid a large volume of the surrounding tissues from receiving dose rates equal to or greater than this dose rate, and also the inhomogeneous irradiation of the target volume.

Dosimetric characteristics of a 6 MV photon beam from a linear accelerator with asymmetric collimator jaws

Dosimetric measurements have been made of a 6 MV photon beam from a linear accelerator equipped with asymmetric jaws. The field size factors for asymmetrically set fields are compared to those for symmetrically set fields. The change of beam quality has been measured as a function of off-axis position of the asymmetric fields to assess its effect on depth dose. Additional measurements include beam penumbra and shape of isodose curves for open and wedge fields as the field opening is moved asymmetrically from the central ray.

Spatially fractionated (GRID) radiation for palliative treatment of advanced cancer

To evaluate the use of single large doses spatially fractionated radiation (GRID) therapy either alone or in combination with conventional radiation in the palliative treatment of advanced cancer. Sixty-one patients with advanced cancer who had exhausted conventional approaches to palliative treatment with surgery, chemotherapy and/or radiotherapy were treated with high dose GRID therapy using high energy photons. Seventy-two symptomatic areas of disease were irradiated. A 50:50 GRID (open to closed areas) was utilized and a single fraction of 1,000–2,000 cGy to dmax was delivered to the open areas using a single field with either 6 MV or 25 MV photons. Follow-up ranged from 2 weeks to 28 months. Short follow-up times in some patients was due to their end stage disease. Patients were analyzed for palliation of symptoms and normal tissue morbidity. Sixty-four of 72 treatments were evaluable for palliative response. The results of treatment indicated a 27% (17/64) complete palliative response. A partial response was obtained in 64% (41/64). Overall response rate was 91%. Pain was the primary reason for treatment. Twenty-eight percent complete pain relief and 62% partial pain relief was achieved with GRID therapy. Symptoms related to large tumor masses were completely relieved in 19% and partially relieved in 71%. No acute morbidity was observed in spite of the large single doses delivered. Eight patients who were followed 12 to 28 months have shown no late morbidity. The use of spatially fractionated GRID therapy to deliver large single doses of radiation for palliative treatment has proven effective, especially for patients with short life expectancies. Further optimization of the GRID radiation distribution and maximum tolerable doses need to be established. Radiat Oncol Invest 1996;4:41–47.

Coordinate transformations and calculation of the angular and depth parameters for a stereotactic system

Stereotactic systems have been used to assist in the precise implantation of radioactive sources in selected brain tumors. Use of such systems requires an algorithm that transforms spatial points in computed tomography coordinates into stereotactic frame coordinates. A simple algorithm performing the coordinate transformations, intended for inclusion in treatment-planning software packages for interstitial brain implants, has been developed. This algorithm was formulated using the geometrical configurations of the Brown-Roberts-Wells (BRW) stereotactic system. After the transformations, the BRW angular coordinates and depth specifying the probe direction, defined from the entry point to the target point, are determined from their respective cartesian coordinates. These angular coordinates and depth on the BRW stereotactic system allow accurate neurosurgical implantations of catheters into the brain, and thereafter the insertion of radioactive sources.

DOSIMETRIC CONSIDERATIONS OF STEREOTACTIC BRAIN IMPLANTS

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.