The use of tertiary collimation for spinal irradiation with extended SSD electron fields.

PURPOSE: The spine can be treated with an electron beam when its maximum posterior depth is within the therapeutic range of electrons. Electron fields treated at extended source-to-surface distances (SSDs), however, have larger penumbras and narrower therapeutic isodose widths relative to those at the standard SSD of 100 cm. We investigated the use of tertiary collimation close to the patient surface for these fields to sharpen the penumbra, minimizing dose to normal tissue and maximizing target coverage. METHODS AND MATERIALS: Using film dosimetry in a polystyrene phantom, we measured the dose distribution for electron fields at extended SSD under varying collimation conditions. Beam penumbra and therapeutic width as a function of depth, SSD, applicator insert size, and tertiary collimator opening were determined. We also measured the dose distributions in the junction region for various gaps between x-ray fields and an electron field as used for craniospinal irradiation. RESULTS: Measurements show that tertiary collimation close to the skin surface reduces penumbra width (lateral distance between the 90 and 20% isodose lines) by 56% and increases therapeutic isodose width (lateral width of the 90% isodose curve) by 25% at a depth of dmax relative to standard collimation. These numbers change to 23 and 13%, respectively, at an average depth of the spine. When lateral brain and posterior spine fields are used to irradiate the entire craniospinal axis, tertiary collimation aids in reducing the volume of the hot spot in the junction region by as much as 10% without compromising target coverage. CONCLUSIONS: Tertiary collimation for extended SSD electron fields is preferable to standard collimation in order to minimize dose to normal tissue and increase target coverage. This technique can be applied to both spinal and craniospinal irradiation. Support structures for the tertiary blocking are needed because the weight of the lead is usually too great for placement on the skin.

Collimator (head) scatter at extended distances in linear accelerator-generated photon beams.

PURPOSE: A calculation formalism is proposed to predict variation of head scatter as a function of field size and treatment distance. METHODS AND MATERIALS: Assuming that the head scatter for the linear accelerator studied was contributed predominantly by the flattening filter, a formalism was devised to predict beam intensity as a function of distance from the target position. The method used the concept of an equivalent collimator field in which a given field at any distance can be equated to a field at the isocenter such that the extent of the flattening filter seen at the two positions is the same. RESULTS: The equation derived from the concept of equivalent collimator field size predicated change in head scatter with distance to within 0.5% for collimator field sizes ranging from 8 x 8 to 40 x 40 cm and distances up to 300 cm from the target. CONCLUSIONS: Considering flattening filter to be the main source of head scatter, the observed deviation from inverse square law for extended treatment distances can be accounted for by an equivalent collimator field size, which sees the same extent of the flattening filter at the isocenter as the field at the given distance.

Evaluation of electronic portal imaging device for missing tissue compensator design and verification.

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.

Maintaining accuracy in stereotactic radiosurgery.

PURPOSE: To provide the manufacture's specification for the base phantom of a commercially available stereotactic radiosurgery system so that its accuracy can be confirmed, and to describe a calibration device that allows the accuracy of the base phantom to be verified quickly and on a routine basis. Modifications to the target pointer system that make matching the pointer tips easier and less likely to damage the pointer tips are also described. METHODS AND MATERIALS: In stereotactic radiosurgery, spatial accuracy is the key factor for successful dose delivery. With some commercially available systems, this accuracy depends on the accuracy of the base phantom coordinate system, how closely the tip of the target pointer can be matched to the tip of the base phantom pointer, and how accurately the coordinates set on the isocentric subsystem match those set on the base phantom. Two major problems, usually overlooked when evaluating system accuracy are, first, the base phantom, which establishes the stereotactic coordinate system, is assumed to be completely accurate. This is a dangerous assumption because the base phantom is used frequently for routine patient treatments and for standard quality assurance tests. To exacerbate the problem, no independent device is provided with stereotactic systems to check the accuracy of the base phantom. Second, the accuracy of the isocenter coordinates set on the head support stand depends upon how closely the target pointer and the base phantom pointer can be aligned. The hardware provided with the system is difficult to use and easily leads to damage of the pointer tips. RESULTS: In this work, we provide the manufacturer's specifications for a popular stereotactic system, describe a device that can be used to check quickly and easily the accuracy of the base phantom, and describe a modification to the transfer pointer system that allows the pointer tips to be more easily aligned with reduced possibility of damage to the pointer tips. CONCLUSION: The methods and apparatus described in this paper should be useful to anyone using a base phantom for testing radiosurgery accuracy.

Radical vulvectomy with postoperative irradiation for vulvar cancer: therapeutic implications of a central block.

PURPOSE/OBJECTIVE: To report the long-term results of vulvectomy, node dissection, and postoperative nodal irradiation using a midline vulvar block in patients with node positive vulvar cancer. METHODS AND MATERIALS: From 1971 through 1992, 27 patients with carcinoma of the vulva and histologically involved inguinal lymph nodes were treated postoperatively with radiation therapy after radical vulvectomy and bilateral lymphadenectomy (n = 25), radical vulvectomy and unilateral lymphadenectomy (n = 1), or hemivulvectomy and bilateral lymphadenectomy (n = 1). Federation Internationale de Gynecologic et d'Obstetrique stages were III (n = 14), IVA (n = 8), and IVB (n = 5) squamous cell carcinoma. Inguinal lymph nodes were involved with tumor in all patients (average number positive = 4, range 1-15). Postoperative irradiation was directed at the bilateral groin and pelvic nodes (n = 19), unilateral groin and pelvic nodes (n = 6), or unilateral groin only (n = 1). These 26 patients had the midline blocked. In addition, one patient received irradiation to the entire pelvis and perineum. Doses ranged from 10.8 to 50.7 Gy (median 45.5) with all patients except 1 receiving > or = 42.0 Gy. RESULTS: Actuarial 5-year overall survival and disease-free survival estimates were 40% and 35%, respectively. Recurrences developed in 63% (17/27) of the patients at a median of 9 months from surgery (range 3 months to 6 years) and 15 of these have died; two patients with recurrences are surviving at 24 and 96 months after further surgery and radiation therapy. Central recurrences (under the midline block) were present in 13 of these 17 patients (76%), either as central only (n = 8), central and regional (n = 4), or central and distant (n = 1). Additionally, three patients developed regional recurrences and one patient developed a concurrent regional and distant relapse. One patient developed a squamous cell cancer of the anus under the midline block 54 months after the initial vulvar cancer and an additional patient developed transitional cell carcinoma of the ureter (outside the radiation field) 12 months after diagnosis. Factors associated with a decreased relapse-free survival included increasing Federation Internationale de Gynecologic et d'Obstetrique stage (p = 0.01) and invasion of the tumor into the subcutaneous (SC) fat or deep soft tissue (p = 0.05). Chronic lower extremity edema developed in four patients, but there have been no other complications. CONCLUSIONS: Radical vulvectomy has often been considered sufficient central treatment for vulvar carcinoma, with postoperative irradiation directed only to the nodes. Although designed to protect the radiosensitive vulva, use of a midline block in this series resulted in a 48% (13/27) central recurrence rate, much higher than the 8.5% rate previously reported with this technique. Routine use of the midline block should be abandoned and, instead, postoperative irradiation volumes should be tailored to the individual patient.