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Craniospinal treatment with the patient supine
Medical Dosimetry, Vol. 28, No. 1, pp. 35–38, 2003 Copyright © 2003 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/03/$–see front matter
doi:10.1016/S0958-3947(02)00239-X
CRANIOSPINAL TREATMENT WITH THE PATIENT SUPINE BRUCE THOMADSEN, PH.D., MINESH MEHTA, M.D., STEVEN HOWARD, M.D., and RUPAK DAS, PH.D. Departments of Human Oncology and Medical Physics, University of Wisconsin, Madison, WI (Accepted 18 April 2002)
Abstract—Radiotherapy of the craniospinal axis in young children is frequently complicated by the need for access to the patient’s airway for sedation and anesthesia delivery or by frequent, unanticipated movement. Positioning the patient supine, instead of in the conventional prone position, allows the use of immobilization facemasks with body molds and more positive patient fixation, and improved airway access. The procedure for establishing the various fields differs from the prone approach. In this paper, we describe the methodology to achieve successful supine positioning. © 2003 American Association of Medical Dosimetrists. Key Words:
Craniospinal irradiation, Radiotherapy dosimetry, Radiotherapy simulation, Pediatric radiotherapy.
slice thicknesses for most standard computed tomography (CT) scanners. Newer technologies that improve the axial resolution for CT units may make the virtual approach more attractive in the future. Given the current state of the art, virtual simulation should be followed by a careful evaluation of the resultant fields, preferably on a conventional simulator. Verification of the simulation on the treatment unit requires clear and distinct images. Fig. 1 illustrates the concepts of the treatment approach. The patient lies on the simulator table in the supine position. A plastic-mesh facemask (Med-Tech, Orange City, IA) and either a foam-based or vacuum bead body mold provide immobilization assistance. For stability, a common bracket fixes both the facemask bracket and the body mold together and to the treatment couch. During the making of the body mold, the therapist checks the alignment of the spinal column under fluoroscopy. Aligning lead shot markers on the bony canthi with horizontal lasers assures proper rotation of the head during fabrication of the facemask. The flexion of the head should be near a neutral position, but slightly extended. Excessive extension shortens the region of the neck available for field junctions. Even in cases where hyperextension does not eliminate the space necessary for junctions, it does make the long procedure more difficult for the patient. After making the facemask, any constraint for the jaw is removed, allowing the passage of tubes if anesthesia will be used, or allowing the patient to swallow if not. Minor movement of the jaw during treatment does not affect patient position as long as the facemask adequately fixes the rest of the head. This usually only requires a close fit at the nasion. The usual procedure for establishing the treatment fields used with the patient prone begins with the posterior spinal field, and matches the cranial fields to the posterior field along the patient’s neck using the projec-
INTRODUCTION Craniospinal irradiation forms an important part of several pediatric cancer protocols.1,2 The conventional treatment places the patient prone in an immobilization mold.3 However, treatment of small children poses particular complexities with the conventional prone patient position. Two particular problems are: 1. access for anesthesia (when used) to the patient’s mouth or face, because the face rests in a supporting device; and 2. providing adequate fixation for fidgety patients without anesthesia. The patient facing downward, an undesirable position from the anesthesiologist’s viewpoint, complicates the problem of access. While devices to hold tubes or masks in place on the patient can be fabricated, they generally tend to compromise patient fixation. Without anesthesia, young patients who don’t understand the importance of holding still often squirm, significantly altering their position. Duplicating the patient’s position from the simulator to the treatment unit often proves challenging, as does repositioning the patient for daily treatment. To overcome these problems, we have developed and implemented a supine craniospinal approach. METHODS Lateral, cranial fields The discussion that follows assumes the use of a conventional simulator. The fields could, of course, be determined by virtual simulation; however, the precision required approximates the limits of resolution given the Reprint requests to: Dr. Bruce Thomadsen, 1530 Medical Sciences Center, University of Wisconsin, Madison, WI 53706. Portions of this article presented at the meeting of the American Association of Medical Dosimetrists, Minneapolis, June 13–17, 1999. 35
36
Medical Dosimetry
Volume 28, Number 1, 2003
that, and sets of 3 markers the most caudal field when used. Posterior spinal fields One important criterion guides the positioning of the spinal field: the cephalad edge must be coplanar with the caudal edge of the cranial fields. Accomplishing this requires angling the beam to make the cephalad edge vertical (nondivergent in the craniocaudal direction). Such an orientation requires rotating the treatment couch 90°, and then rotating the gantry toward the patient’s head through an angle given by Fig. 1. Craniospinal irradiation with the patient supine. The field orientation uses a gantry rotation on the undertable posterior spinal field to produce a vertical superior border to junction with the cranial fields. A facemask and body mold provide immobilization of the patient.
tion of light-localizer for the spinal field. With the patient supine, however, because the light field indicating the border of the spinal field cannot be seen on the patient (because of the intervening mold), the setup begins with the cranial fields. The cranial fields, in the simplest setup, have no collimator rotation. The vertical edge of the inferior border of the fields, when matched with the posterior spinal field later, allows sparing of the salivary glands. The height of the center of the field falls coincident with the bony canthi, preventing divergence of the exit passing under a block and through the contralateral eye. To minimize overlap or gaps at the junction with the spinal field, only the superior half of the cranial fields are used (i.e., half-beam blocks cover the caudal half of the fields.) The central plane of the cranial fields (now the inferior edges) is placed just inferior to the submandibular salivary gland. The field edges in the cephalad and posterior directions fall approximately 1 cm beyond the patient. Blocks shield the eyes and oral cavity, including the salivary glands, in the conventional manner. Each set of fields corresponding to a different junction requires its own blocks because of the change in isocenter. Depending on the total dose prescribed, either 2 or 3 sets of junctions are used. The positioning of the cranial field as described above establishes the cephaladmost junction. The second junction falls 1 cm caudal to that, and the third, if applicable, 1 cm caudal to that. For the more caudal fields, the entire field arrangement remains the same, with only the centers of the fields displaced. The inferior edge of the spinal field is realigned with the bottom of the target using an independent collimator. Upon setting the cranial fields, lead-shot markers are placed along the caudal edge of the light field along both sides of the neck: single markers indicating the cephalad-most field, double markers the field caudal to
⫽ tan⫺1
冉
冊
collimator length , 2 SAD
(1)
where collimator length equals the opening size of the collimator in the direction along the patient’s body axis at the source-to-axis distance (SAD). The collimator length selection poses a difficult task a priori because of the divergence of the beam inferiorly along the spinal column. The use of a field length that more than covers the target length when aligned with the cephalad-most junction allows flexibility and assures adequate treatment. After positioning the spinal field, the inferior border is set using independent collimation. Following setting the angle of the spinal field, the superior border is placed under fluoroscopic guidance, aligning the edge of the field with the lead-shot markers placed on the neck during the cranial field setup process. Fig. 1 shows the isocenter placed at the posterior surface, or at the bottom of the mold. If necessary, extended distances can be used. Variations Low-dose cases. For some patients who receive very low doses to the spinal axis (e.g., 6 Gy total), a single junction poses no risk. In this case, a low match line in the neck, corresponding to the lowest junction (were 3 junctions used) is used with the collimator axis vertical for the posterior field, as shown in Fig. 2. This field orientation requires no rotation of the treatment couch. As a disadvantage, the superior border of the posterior field diverges through the mouth, exposing the salivary glands if the match line is not low enough in the neck. The divergence of the posterior field also necessitates a collimator rotation on the cranial fields, equal to in the equation above. Other than these changes, the rest of the setup remains the same. Large cranial fields. While for most pediatric cases the cranial fields fit within the half-blocked field (even with matching at the C2 vertebral body), some adult cases may require more than half the field to cover the target. In these cases, the divergence on the inferior edge of the cranial fields is removed by using a couch angle, as is standard practice. With the setup as in Fig. 1, the
Supine craniospinal ● B. THOMADSEN et al.
Fig. 2. Modification of the fields in Fig. 1 for low-dose treatments without the vertical junction between the posterior and lateral fields.
imposition of a couch angle becomes simpler than in typical craniospinal cases because the cranial fields have no collimator rotation, and the couch angle, , simply becomes a form of Eq. (1), ⫽ Tan⫺1(ci/SAD),
(2)
where ci ⫽ the collimator setting from the center of the field to the inferior border (i.e., the setting for the caudal independent jaw). For the technique shown in Fig. 2, the couch angle and the collimator rotation become interdependent, increasing the uncertainty in the settings. For this reason, the use of the technique in Fig. 2 for cases requiring cranial fields larger than that afforded with the half-blocked field is not recommended.
DISCUSSION Positioning the patient supine addresses the problems of airway access and poor immobilization. With the patient supine and the jaw portion of the facemask removed, the anesthesiologist has adequate access to the patient’s airway. For patients not receiving anesthesia, the immobilization aids help minimize both voluntary and involuntary movement. In general, supine positions tend to be more stable than prone, both during a treatment and for repeated positioning over the course of therapy. In the prone position, many unanaesthetized children tend to try to lift their head and look around. The facemask not only prevents head movement, but with the head facing up, the child feels less isolated from the activity in the room. Making the cephalad border of the spinal field vertical, as noted, moves the primary beam below the mouth. This does add some complexity to the setup due to the rotation of the couch. For patients receiving very low doses, this precaution may not be worth the introduction of additional uncertainties due to the manipulations, leading to the approach in Fig. 2. On the other
37
hand, the vertical junction does eliminate the collimator rotation, adding some simplification. The oblique incidence of the spinal field would produce a variation in dose along the spine of approximately 10% if uncorrected. Delivering half the monitor units (MUs) for a treatment using a 15° wedge compensates for this variation. However, most facilities have no wedges large enough to cover a long field, nor do virtual (or dynamic) wedges allow coverage of such fields. The wedge can be simulated with a manually produced virtual wedge, by delivering the MUs to the whole field required to give the upper part of the field the prescribed dose, and then blocking the cephalad two-fifth of the field to deliver an additional exposure of approximately 7% to 10% of the MUs given to the large field (a dose sufficient to complete the prescription to the caudal part of the field). Delivering the supplemental exposure in this manner produces a dose that varies by ⫾ 2% or less over the target. The use of this technique at our facility is limited to patients for whom the entire spinal axis fits within a single posterior field. While possible to junction 2 posterior fields with the source under the table, the methods to assure proper matching consume considerable time and may fail to provide better junctioning than with the patient prone. The likelihood for undetected errors also increases for the use of 2 junctioning posterior fields from under the couch. However, with the large cutout portions available on the couches of modern accelerators and very careful verification of field placement, supine treatment of patients requiring 2 adjoining fields to cover the spinal axis becomes a feasible consideration. Since the development of this approach in 1997, we have used it in the treatment of about half of our pediatric craniospinal patients. In 2 cases, patients originally oriented prone who presented reproducibility problems were changed to the supine position, eliminating the problems. Field adjustment during the course of treatment has been markedly reduced or eliminated with this approach. For the half of the patients who continued to be treated prone, the reason given was that the physician felt that visualization of the match line should produce more reproducible junctions. This premise has not been borne out in practice. The technique has also been used for the treatment of 2 adults with short (less than 40 cm) spinal field lengths. CONCLUSIONS Positioning pediatric craniospinal patients in the supine position often provides better immobilization and improves airway access compared to the conventional prone orientation. REFERENCES 1. Children’s Cancer Group, Clinical Trial CCG-5941: A Pilot Study for the Treatment of Newly-Diagnosed Disseminated Anaplastic
38
Medical Dosimetry
Large Cell Ki-1 Lymphoma and T-Large Cell Lymphoma. Arcadia, CA: National Childhood Cancer Foundation. 2. Children’s Cancer Group, Clinical Trial CCG-A9961: A Phase III Prospective Randomized Study of Craniospinal Radiotherapy Followed by One of Two Adjuvant Chemotherapy Regimens in Chil-
Volume 28, Number 1, 2003 dren with Newly-Diagnosed Average-Risk Medulloblastoma. Arcadia, CA: National Childhood Cancer Foundation. 3. Kun, L.E. Brain tumors in children. In: Perez, C.A.; Brady, L.W., editors. Principles and Practice of Radiation Oncology. Philadelphia: Lippincott Williams & Wilkins; 1998.
doi:10.1016/S0958-3947(02)00239-X
CRANIOSPINAL TREATMENT WITH THE PATIENT SUPINE BRUCE THOMADSEN, PH.D., MINESH MEHTA, M.D., STEVEN HOWARD, M.D., and RUPAK DAS, PH.D. Departments of Human Oncology and Medical Physics, University of Wisconsin, Madison, WI (Accepted 18 April 2002)
Abstract—Radiotherapy of the craniospinal axis in young children is frequently complicated by the need for access to the patient’s airway for sedation and anesthesia delivery or by frequent, unanticipated movement. Positioning the patient supine, instead of in the conventional prone position, allows the use of immobilization facemasks with body molds and more positive patient fixation, and improved airway access. The procedure for establishing the various fields differs from the prone approach. In this paper, we describe the methodology to achieve successful supine positioning. © 2003 American Association of Medical Dosimetrists. Key Words:
Craniospinal irradiation, Radiotherapy dosimetry, Radiotherapy simulation, Pediatric radiotherapy.
slice thicknesses for most standard computed tomography (CT) scanners. Newer technologies that improve the axial resolution for CT units may make the virtual approach more attractive in the future. Given the current state of the art, virtual simulation should be followed by a careful evaluation of the resultant fields, preferably on a conventional simulator. Verification of the simulation on the treatment unit requires clear and distinct images. Fig. 1 illustrates the concepts of the treatment approach. The patient lies on the simulator table in the supine position. A plastic-mesh facemask (Med-Tech, Orange City, IA) and either a foam-based or vacuum bead body mold provide immobilization assistance. For stability, a common bracket fixes both the facemask bracket and the body mold together and to the treatment couch. During the making of the body mold, the therapist checks the alignment of the spinal column under fluoroscopy. Aligning lead shot markers on the bony canthi with horizontal lasers assures proper rotation of the head during fabrication of the facemask. The flexion of the head should be near a neutral position, but slightly extended. Excessive extension shortens the region of the neck available for field junctions. Even in cases where hyperextension does not eliminate the space necessary for junctions, it does make the long procedure more difficult for the patient. After making the facemask, any constraint for the jaw is removed, allowing the passage of tubes if anesthesia will be used, or allowing the patient to swallow if not. Minor movement of the jaw during treatment does not affect patient position as long as the facemask adequately fixes the rest of the head. This usually only requires a close fit at the nasion. The usual procedure for establishing the treatment fields used with the patient prone begins with the posterior spinal field, and matches the cranial fields to the posterior field along the patient’s neck using the projec-
INTRODUCTION Craniospinal irradiation forms an important part of several pediatric cancer protocols.1,2 The conventional treatment places the patient prone in an immobilization mold.3 However, treatment of small children poses particular complexities with the conventional prone patient position. Two particular problems are: 1. access for anesthesia (when used) to the patient’s mouth or face, because the face rests in a supporting device; and 2. providing adequate fixation for fidgety patients without anesthesia. The patient facing downward, an undesirable position from the anesthesiologist’s viewpoint, complicates the problem of access. While devices to hold tubes or masks in place on the patient can be fabricated, they generally tend to compromise patient fixation. Without anesthesia, young patients who don’t understand the importance of holding still often squirm, significantly altering their position. Duplicating the patient’s position from the simulator to the treatment unit often proves challenging, as does repositioning the patient for daily treatment. To overcome these problems, we have developed and implemented a supine craniospinal approach. METHODS Lateral, cranial fields The discussion that follows assumes the use of a conventional simulator. The fields could, of course, be determined by virtual simulation; however, the precision required approximates the limits of resolution given the Reprint requests to: Dr. Bruce Thomadsen, 1530 Medical Sciences Center, University of Wisconsin, Madison, WI 53706. Portions of this article presented at the meeting of the American Association of Medical Dosimetrists, Minneapolis, June 13–17, 1999. 35
36
Medical Dosimetry
Volume 28, Number 1, 2003
that, and sets of 3 markers the most caudal field when used. Posterior spinal fields One important criterion guides the positioning of the spinal field: the cephalad edge must be coplanar with the caudal edge of the cranial fields. Accomplishing this requires angling the beam to make the cephalad edge vertical (nondivergent in the craniocaudal direction). Such an orientation requires rotating the treatment couch 90°, and then rotating the gantry toward the patient’s head through an angle given by Fig. 1. Craniospinal irradiation with the patient supine. The field orientation uses a gantry rotation on the undertable posterior spinal field to produce a vertical superior border to junction with the cranial fields. A facemask and body mold provide immobilization of the patient.
tion of light-localizer for the spinal field. With the patient supine, however, because the light field indicating the border of the spinal field cannot be seen on the patient (because of the intervening mold), the setup begins with the cranial fields. The cranial fields, in the simplest setup, have no collimator rotation. The vertical edge of the inferior border of the fields, when matched with the posterior spinal field later, allows sparing of the salivary glands. The height of the center of the field falls coincident with the bony canthi, preventing divergence of the exit passing under a block and through the contralateral eye. To minimize overlap or gaps at the junction with the spinal field, only the superior half of the cranial fields are used (i.e., half-beam blocks cover the caudal half of the fields.) The central plane of the cranial fields (now the inferior edges) is placed just inferior to the submandibular salivary gland. The field edges in the cephalad and posterior directions fall approximately 1 cm beyond the patient. Blocks shield the eyes and oral cavity, including the salivary glands, in the conventional manner. Each set of fields corresponding to a different junction requires its own blocks because of the change in isocenter. Depending on the total dose prescribed, either 2 or 3 sets of junctions are used. The positioning of the cranial field as described above establishes the cephaladmost junction. The second junction falls 1 cm caudal to that, and the third, if applicable, 1 cm caudal to that. For the more caudal fields, the entire field arrangement remains the same, with only the centers of the fields displaced. The inferior edge of the spinal field is realigned with the bottom of the target using an independent collimator. Upon setting the cranial fields, lead-shot markers are placed along the caudal edge of the light field along both sides of the neck: single markers indicating the cephalad-most field, double markers the field caudal to
⫽ tan⫺1
冉
冊
collimator length , 2 SAD
(1)
where collimator length equals the opening size of the collimator in the direction along the patient’s body axis at the source-to-axis distance (SAD). The collimator length selection poses a difficult task a priori because of the divergence of the beam inferiorly along the spinal column. The use of a field length that more than covers the target length when aligned with the cephalad-most junction allows flexibility and assures adequate treatment. After positioning the spinal field, the inferior border is set using independent collimation. Following setting the angle of the spinal field, the superior border is placed under fluoroscopic guidance, aligning the edge of the field with the lead-shot markers placed on the neck during the cranial field setup process. Fig. 1 shows the isocenter placed at the posterior surface, or at the bottom of the mold. If necessary, extended distances can be used. Variations Low-dose cases. For some patients who receive very low doses to the spinal axis (e.g., 6 Gy total), a single junction poses no risk. In this case, a low match line in the neck, corresponding to the lowest junction (were 3 junctions used) is used with the collimator axis vertical for the posterior field, as shown in Fig. 2. This field orientation requires no rotation of the treatment couch. As a disadvantage, the superior border of the posterior field diverges through the mouth, exposing the salivary glands if the match line is not low enough in the neck. The divergence of the posterior field also necessitates a collimator rotation on the cranial fields, equal to in the equation above. Other than these changes, the rest of the setup remains the same. Large cranial fields. While for most pediatric cases the cranial fields fit within the half-blocked field (even with matching at the C2 vertebral body), some adult cases may require more than half the field to cover the target. In these cases, the divergence on the inferior edge of the cranial fields is removed by using a couch angle, as is standard practice. With the setup as in Fig. 1, the
Supine craniospinal ● B. THOMADSEN et al.
Fig. 2. Modification of the fields in Fig. 1 for low-dose treatments without the vertical junction between the posterior and lateral fields.
imposition of a couch angle becomes simpler than in typical craniospinal cases because the cranial fields have no collimator rotation, and the couch angle, , simply becomes a form of Eq. (1), ⫽ Tan⫺1(ci/SAD),
(2)
where ci ⫽ the collimator setting from the center of the field to the inferior border (i.e., the setting for the caudal independent jaw). For the technique shown in Fig. 2, the couch angle and the collimator rotation become interdependent, increasing the uncertainty in the settings. For this reason, the use of the technique in Fig. 2 for cases requiring cranial fields larger than that afforded with the half-blocked field is not recommended.
DISCUSSION Positioning the patient supine addresses the problems of airway access and poor immobilization. With the patient supine and the jaw portion of the facemask removed, the anesthesiologist has adequate access to the patient’s airway. For patients not receiving anesthesia, the immobilization aids help minimize both voluntary and involuntary movement. In general, supine positions tend to be more stable than prone, both during a treatment and for repeated positioning over the course of therapy. In the prone position, many unanaesthetized children tend to try to lift their head and look around. The facemask not only prevents head movement, but with the head facing up, the child feels less isolated from the activity in the room. Making the cephalad border of the spinal field vertical, as noted, moves the primary beam below the mouth. This does add some complexity to the setup due to the rotation of the couch. For patients receiving very low doses, this precaution may not be worth the introduction of additional uncertainties due to the manipulations, leading to the approach in Fig. 2. On the other
37
hand, the vertical junction does eliminate the collimator rotation, adding some simplification. The oblique incidence of the spinal field would produce a variation in dose along the spine of approximately 10% if uncorrected. Delivering half the monitor units (MUs) for a treatment using a 15° wedge compensates for this variation. However, most facilities have no wedges large enough to cover a long field, nor do virtual (or dynamic) wedges allow coverage of such fields. The wedge can be simulated with a manually produced virtual wedge, by delivering the MUs to the whole field required to give the upper part of the field the prescribed dose, and then blocking the cephalad two-fifth of the field to deliver an additional exposure of approximately 7% to 10% of the MUs given to the large field (a dose sufficient to complete the prescription to the caudal part of the field). Delivering the supplemental exposure in this manner produces a dose that varies by ⫾ 2% or less over the target. The use of this technique at our facility is limited to patients for whom the entire spinal axis fits within a single posterior field. While possible to junction 2 posterior fields with the source under the table, the methods to assure proper matching consume considerable time and may fail to provide better junctioning than with the patient prone. The likelihood for undetected errors also increases for the use of 2 junctioning posterior fields from under the couch. However, with the large cutout portions available on the couches of modern accelerators and very careful verification of field placement, supine treatment of patients requiring 2 adjoining fields to cover the spinal axis becomes a feasible consideration. Since the development of this approach in 1997, we have used it in the treatment of about half of our pediatric craniospinal patients. In 2 cases, patients originally oriented prone who presented reproducibility problems were changed to the supine position, eliminating the problems. Field adjustment during the course of treatment has been markedly reduced or eliminated with this approach. For the half of the patients who continued to be treated prone, the reason given was that the physician felt that visualization of the match line should produce more reproducible junctions. This premise has not been borne out in practice. The technique has also been used for the treatment of 2 adults with short (less than 40 cm) spinal field lengths. CONCLUSIONS Positioning pediatric craniospinal patients in the supine position often provides better immobilization and improves airway access compared to the conventional prone orientation. REFERENCES 1. Children’s Cancer Group, Clinical Trial CCG-5941: A Pilot Study for the Treatment of Newly-Diagnosed Disseminated Anaplastic
38
Medical Dosimetry
Large Cell Ki-1 Lymphoma and T-Large Cell Lymphoma. Arcadia, CA: National Childhood Cancer Foundation. 2. Children’s Cancer Group, Clinical Trial CCG-A9961: A Phase III Prospective Randomized Study of Craniospinal Radiotherapy Followed by One of Two Adjuvant Chemotherapy Regimens in Chil-
Volume 28, Number 1, 2003 dren with Newly-Diagnosed Average-Risk Medulloblastoma. Arcadia, CA: National Childhood Cancer Foundation. 3. Kun, L.E. Brain tumors in children. In: Perez, C.A.; Brady, L.W., editors. Principles and Practice of Radiation Oncology. Philadelphia: Lippincott Williams & Wilkins; 1998.