Treatment Planning Study of Craniospinal Irradiation (CSI)

Introduction

Medulloblastoma is a childhood cancer that affects 250 to 500 children per year.1 Along with surgery and chemotherapy, radiation therapy is also a common treatment option for this cancer. Generally, if children are over the age of 3, radiation therapy is given to the entire brain and spine with boosts to specific tumor sites. This planning study will further discuss a radiotherapy technique used to treat a patient with medulloblastoma.

Patient Setup

This patient was simulated in the supine, head-first position. The patient’s arms were positioned at their sides, and a support cushion was given for under the knees. In the head and neck region, the patient’s head was supported using a headrest in a neutral position with an aquaplast mask for immobilization. The goal for patient positioning was to ensure that the patient was comfortable, while maximizing setup reproducibility.

Treatment Planning

Research

Historically at my clinical site, total CSI patients were treated with parallel-opposed whole brain fields, a posterior T-spine field, and a posterior L-spine field. The T and L-spine fields contained segmented migrations to feather the match between fields. This approach was very daunting in the treatment planning process, requiring multiple prescriptions and detailed attention to migrations and field sizes. Recently, my clinical site started to incorporate intensity modulated radiation therapy (IMRT) in the planning process to increase planning efficiency and further reduce dose to critical structures. The method that I chose to use for treatment planning was an intensity modulated radiation therapy (IMRT) approach called a three-isocenter overlap-junction (TIOJ). My planning approach closely follows the article by Wang et al2, with a modification to the brain fields to achieve the treatment planning goals specific to this study.

Field Setup

Traditional field borders were not delineated in this plan due using an IMRT planning approach. However considerations were taken in initial field setup and isocenter placement to allow for optimal planning. An initial measurement of the length of the planning target volume (PTV) as delineated by the radiation oncologist was taken. According to the paper by Wang et al2, if the length is less than 80 cm then isocenters should be placed approximately 25 cm apart from one another. Therefore, 3 isocenters were placed with a 25 cm gap to ensure for proper overlap of the various fields (Figure 1).

Figure 1. Measurement of gap between isocenters maintained at 25 cm

When placing the brain isocenter, care was taken to ensure that the lower field border would not enter through the patient’s shoulders. All 3 isocenters were placed at equal depths with the same left/right measurement for ease of treatment setup and reproducibility (Figure 2).

Figure 2. Coordinate display of isocenters with equal lateral and anterior/posterior directions

Beam Arrangement

Similar treatment angles were maintained as the article with the omission of the AP beam for the brain beam set. This was done to help meet dose constraints for the critical structures near the eye, such as the lenses and optic nerves. Therefore, the beam arrangement for the brain isocenter included gantry angles of 65°, 100°, 123°, 230°, 257°, and 290° (Figure 3).

Figure 3. Beam arrangement for brain isocenter

The beam arrangements for the T and L-spine isocenters both included 3 beams with gantry angles of 145°, 180°, and 215° (Figure 4).

Figure 4. Beam arrangements for T and L-spine isocenters

All field sizes were initially set to maximum length to allow an overlap region between beam sets of 10-15cm (Figure 5).

Figure 5. Measurement of overlap between fields at each isocenter

Finally, an SAD technique was used and once beams were placed, source to skin (SSD) measurements were reviewed to verify plan feasibility.

Prescription

The plan was prescribed to 3600cGy in 20 fractions with normalization to the PTV volume. In this case, the PTVbrain and PTVspine were combined into a PTVall structure for simplicity. Once this structure was created, the left and right optic nerves were subtracted out of the volume for sparing, and the remaining structure (PTVopt) was used for planning (Figure 6).

Figure 6. Contour of PTVopt (light green) with exclusion of optic nerve critical structures

Beam Energy

The beam energies selected included 6 MV and 10 MV. The brain and T-spine isocenters were treated with 6 MV, while the L-spine was treated with 10 MV. Lower beam energies are recommended when treating pediatric patients to avoid neutron contamination with higher energies. Neutron production is said to occur at energies greater than 10 MV, which contributed to the choice of energies in this case.3 Due to the anterior curvature of the L-spine and evaluation of the isodose lines, the 10MV energy was selected to eliminate hotspots in the L-spine region (Figure 7).

Figure 7. Plan using 6 MV energy showing hot spots (red lines) of 110% of prescription dose on left compared to 10 MV energy on the right

Plan Evaluation

During optimization, the PTVopt structure was used to achieve the dose goal of 95% coverage to the PTVbrain and PTVspine. Maximum dose structures were also used for the PTVbrain and PTV spine to limit hotspots. An overlap structure was also created where the fields intersect (i.e. brain overlap with T-spine, T-spine overlap with L-spine) and assigned a minimum dose of 3600cGy to reduce cold spots between fields. Organ at risk (OAR) structures that were pushed during the optimization process included the kidneys, the lenses, the optic nerves and the parotids. Once achieving OAR tolerances, the plan was then tweaked by contouring cold spots within the PTVbrain and contouring any hotspots above 3960cGy.

Upon plan evaluation, some multileaf collimator (MLC) leaves were manually adjusted in the brain segments to further reduce hotspots to an acceptable level (Figure 8).

Figure 8. Contour (purple colorwash) used to delineate 110% isodose line (red line) for manual leaf adjustment

The final hotspot for this plan was 110.5% (3978cGy) located in the PTVbrain. All PTV dose objectives were met with 95% PTV coverage achieved for both the PTVbrain and PTVspine (Figure 9).

Figure 9. Dose-volume histogram of PTV coverage with corresponding isodose line display

There were limited hotspots in this plan with 0.01% of the PTVspine volume receiving 3960cGy, and 0.03% of the PTVbrain volume receiving 3960cGy. The OAR structures also met the assigned constraints with the right optic nerve just slightly over the ideal metric (3403.7cGy).

One struggle with completing this planning study was normalizing the plan for submission into the ProKnow plan evaluation system. Although some OAR structures were met in Pinnacle, the transfer to ProKnow showed results of a hotter plan (Figure 10-11).

14

Figure 10. Scorecard results from Pinnacle versus ProKnow results in red.

Figure 11. Score sheet for objectives in ProKnow

Therefore, a balance between the planning system’s dose-volume histogram evaluations versus ProKnow was made before accepting a final plan (Figure 12-15).

Figure 12. Dose-volume histogram of targets and critical structures in Pinnacle

Figure 13. Dose-volume histogram of critical structures in Pinnacle

Figure 14. Dose-volume histogram of critical structures in Pinnacle

Figure 15. Dose-volume histogram of critical structures in Pinnacle

Conclusion

Overall this planning study was challenging as the traditional planning methods for CSI that I was used to, did not fit the constraints for this pediatric case. It was a great chance to research cases performed at my other clinical site that more commonly treats pediatrics. In addition, I was able to confirm these planning methods with literature to come up with a planning solution. This case was a great opportunity to pull information from multiple resources in order to obtain a new planning perspective.

Craniospinal treatments are very complex in nature and trying to limit unnecessary hotspots and dose to normal tissues can be challenging. There are also many considerations for treatment such as the number of beam angles, treatment time, setup complexity, and the possible need for anesthesia that can all influence planning decisions. Some areas that I would work to improve on for future cases would be to focus on improving the amount of low dose to organs, as I did not try to further optimize OAR that were already meeting the constraints of this study. It would be interesting to see the potential of lowering dose to these structures further. Also, I would be curious to try different beam arrangements to limit the amount of treatment time for the patient.

References:

1. Medulloblastoma - Childhood - Treatment Options. Cancer.Net. https://www.cancer.net/cancer-types/medulloblastoma-childhood/treatment-options. Published January 31, 2018. Accessed October 5, 2018.

2. Wang Z, Jiang W, Feng Y, et al. A simple approach of three-isocenter IMRT planning for craniospinal irradiation. Rad Onc. 2013; 8:217. http://dx.doi.org/10.1186/1748-717X-8-217

3. Rahman AR, Wang J, Huang Z, Montebello J. To reduce hot dose spots in craniospinal irradiation: An IMRT approach with matching beam divergence. Med Phys. 2009; 36(6): 2658-2658. http://dx.doi.org/10.1118/1.3182079